Life Unit 3

meiosis overview

- INTERPHASE: = G1 = S (only between interphase and meiosis I; not between meiosis I and meiosis II) = G2 - MEIOSIS I: = prophase I - nuclear envelope dissolves - DNA begins to condense; chromosomes (2 sister chromatids + centromere) become visible as 'threads' - DNA has already been replicated - synapsis occurs - crossing over occurs - interphase microtubule structure dissolves; turns into spindle fibers = metaphase I - paired homologues are "locked together" (sister chromatid cohesion/crossing over) - chromosome structure captures spindle fibers, is moved to center of cell - homologous chromosome pairs are "unlocked" and aligned (one on either side of the metaphase plate) - kinetochores of sister chromatids work together to 'catch' microtubules from one pole of the cell and attach them to the homologue/centromere, rather than to chromatid A and chromatid B. - scientists have hypothesized that the centromere-kinetochore complex compacts during meiosis I, which allows all three elements to function as a single unit. Now we can pull chromosomes apart, rather than chromatids! - this attachment then helps pull both paired homologues to the equator of the cell (metaphase plate) - orientation of homologue pairs is randomized/not based on gender (there is no designated 'side' for maternal or paternal chromosomes; either type can go towards either pole). Leads to independent assortment. = anaphase I - microtubules w/in spindle fibers begin to shorter. Due to stress, the chiasmata (points w/in the centromere that act as a psychical 'link' between chromosomes) are broken. Individual chromosomes can now be pulled towards opposite poles of the cell. - sister chromatid cohesion ceases; centromere binding sister chromosomes stays intact (cohesin proteins in both; destroyed between non-sister chromatids, but centromere prevents cohesin protein destruction, continuing to bind sister chromatids together) = telophase I - chromosomes are segregated at both poles of the cell - nuclear membrane begins to reform around both 'sets' - sister chromatids are no longer identical bc crossing over has occurred (prophase I) - cytokinesis may occur during or after telophase - MEIOSIS II: = prophase II = metaphase II = anaphase II = telophase II # interval between meiosis I and meiosis II; length can vary (not immediate, but other than that, it's up in the air)

genetic recombination + Creighton and McClintock

- In Creighton and McClintock's experiments, they first chose a type of chromosome in corn that had distinct and visible characteristics- a knob on one end, and an extension on the other. They chose these chromosomes to study because they wanted to see if the knobs/extensions would stay on the chromosomes, or if chromosomes without knobs/extensions would cross with knobbed/extended chromosomes. - in addition to their unique psychical properties, these chromosomes also carried genes that determined kernel color (colored/colorless) and texture (waxy/starchy). They already knew that the long, knobbed chromosome carried the dominant allele for colored kernels (C), and the recessive allele for waxy texture (wx). - heterozygotes with this chromosome (C and wx) were crossed with 'normal' chromosomes (colorless c and starchy texture Wx). The resulting F1 generation heterozygous, colored (C), and starchy (Wx). They were also heterozygous for both chromosome types (one "normal", one knobbed/extended). - this F1 generation was then crossed with colorless (cc), waxy plants (wx). These plants had chromosomes that appeared normal. - these offspring were then analyzed for both psychical recombination (any knobs/extensions?), and genetic recombination (phenotypes). The results showed that all of the genetically recombinant progeny (colored starchy or colorless waxy) only had one of the distinct chromosomal "markers" (knob *or* extension). We could actually see that trait combination led to psychical exchange of chromosome material (chromosomal characteristics were exchanged).

epistasis

- Mendel also thought that the products of genes did not interact with one another, but they do, in fact, have the ability to behave inter-connectedly, changing the phenotypic ratio expected from independent assortment - things in our bodies interact with, are affected by, and depend on each other all the time. It's not that surprising if you think about it. - it is not always possible to identify genotype through phenotype, because genetic interaction can make multiple genotypes look the same - for example, zea mays, a particular of corn, has one variety that has a purple pigment. In 1918, geneticist R.A. Emerson crossed 2 varieties of zea mays that did not present purple pigments, and got offspring that *did* present purple pigment. - next, when these purple-pigmented plants were crossed with one another, 56% of the offspring were pigmented, and 44% were not (almost half-and-half). Because of these results, Emerson concluded that multiple genes were responsible for producing a purple pigment in corn (something of a dihybrid cross). Mendel got it right that alleles within a dihybrid cross could combine in 16 different ways, so Emerson had to figure out a special 16 ratio that would result in 2 different phenotypes. - to do this, Emerson multiplied (.56)16 = 9, and (.44)16 = 7, getting the ratio 9:7 (pigmented to non-pigmented) - we can understand how this works by considering the products that these genes produce. When gene products act sequentially (one must cause an effect before another gene can begin causing an effect), any allele/allele combination that results in a defective enzyme stops those genes from 'finishing' the pathway. When one gene can interfere with a later gene in such a manner, it is called *epitasis*. - in order for zea mays to produce purple pigment, it must go through two steps, and possess at least one functional copy of each enzyme-producing gene). The dominant allele encodes for functional enzymes, and the recessive for nonfunctional. Out of 16 combinations, only 9 contained at least one dominant allele in each allele 'pair', so they produced purple corn. The other 7 (3 + 3 + 1 = 7; a "modified" 9:3:3:1 ratio) were all completely recessive in at least one of their allele pairs, producing colorless corn

phenotypes can be affected by the environment

- Mendel did not know that the environment could effect the relationship between genotype and phenotype (genotype + environment = phenotype) - genes in some animals encode heat-sensitive products that are affected by temperature differences in different parts of their bodies. The ch allele in Siamese cats, for example, encodes the heat-sensitive enzyme tyrosinase (produces pigments- lack of tyrosinase leads to albinism). In higher-temperature regions (torso, head), the enzyme is 'shut down', which produces white fur. In lower temperature regions (ears, face, tail), tyrosinase activates, producing melanin which turns the surrounding fur dark and absorbs more light, thereby heating up colder areas of the body.

Mendelian laws + crossing over + meiosis

- Mendelian traits are determined by alleles, which are carried by genes on chromosomes - the independent assortment that Mendel observed during his experiments (different traits could mix regardless of their alignments within the paternal generation) reflected the re-combination of maternal and paternal chromosomes during meiosis - however, independant assortment of traits is also due to crossing over. This allows alleles that are both on different chromosomes, and on the *same* chromosome, to recombine within the genome. Alleles on the same chromosome can't do independent assortment, but they can do crossing over. - organisms have more independently assorting genes than they do chromosomes- making it extremely unlikely that independent assortment is the only mechanism responsible for trait recombination - during prophase I of meiosis, homologues cross over, psychically exchanging DNA. Because chromosomes are organized into homologue pairs (X X) rather than chromatid pairs (X), chromosomes, rather than chromatids, separate at anaphase I.

genetic recombination + Thomas Hunt Morgan

- Thomas Hunt Morgan was the first to provide evidence for recombination. When studying three different genes all found on the X chromosomes of fruit flies, he found that offspring inherited the same traits, but in different combinations, from their parents. Morgan hypothesized that this was because these traits (genes) were all on the same chromosome (all inherited together, or *coinherited*), which was inherited by all fruit flies. - Morgan was also the first to suggest that this 'mixing up' of coinherited traits (recombinant genotypes) was caused by homologues crossing over during meiosis. - later, experiments performed by Barbara McClintock and Harriet Creighton (corn), and by Curt Stern (fruit flies) provided evidence for Morgan's hypothesis of crossing over.

recombination + Alfred Sturtevant

- We now know that certain genes are located in certain places on the chromosome. The discovery of this fact began with Thomas Hunt Morgan, who suggested that the frequency with which a 'group' of recombinant traits appeared in offspring was due to the location of each gene on a chromosome (close together = traits often get "swapped" together). - Alfred Sturtevant, and undergraduate in Morgan's laboratory, tested this. Sturtevant's hypothesis was that the frequency of recombinations observed in crosses could be used as a measure of genetic distance (the further genes are from one another on a chromosome, the less likely they are to recombine). If his hypothesis were true, then the frequency of recombinant gametes would 'determine' their chromosomal distance. - in order to easily measure recombination frequency, investigators began test-crossing fruit flies (test-cross allows offspring to reflect the gametes given to them by a doubly heterozygous parent). While studying recombination, this meant that offspring that appeared like their parents had not undergone crossover, while offspring that appeared recombinant had undergone crossover. - genes that are close together on a chromosome are *linked* genes. Linkage is genetically defined as an "excess of parental genotypes over recombinant genotypes". - frequency of recombination = # of recombinant offspring/total offspring (%). (Measured in 1% increments; each increment is a centimorgan (cM; named after Thomas Hunt Morgan)/map unit (m.u.).) - one can construct a genetic map by performing test-crosses with doubly heterozygous individuals and then using the frequency of recombination equation on their offspring. - EX: a. fluit flies homozygous for two mutations (vestigial wings vg and black body b) are crossed with wild-type ("natural"; that was supposed to happen) homozygous individuals (vg+ and b+). b. F1 offspring yielded; then testcrossed with homozygous recessive individuals (vg b). F2 offspring are counted. c. RESULTS (1000 offspring): - vestigial wings, black body (vg b; parental) = 405 - long wings, grey body (vg+ b+, parental) = 415 - vestigial wings, grey body (vg+ b+; recombinant) = 92 - long wings, black body (vg+ b+; recombinant) = 88 d. recombinant offspring (92 + 88)/ total offspring (1000) = recombination frequency (.18). 18 cM of distance between body color + wing type loci.

dihybrid cross

- a cross (parent + parent ---> offspring) following multiple traits - Mendel understood the behaviors of single traits, but he wondered if these singular traits behaved completely independently, or if they could be influenced by other traits - to attempt to answer this question, Mendel established 2 true-breeding lines of peas that each differed in two of the seven characteristics he'd been studying (color and texture). He then crossed these 2 lines to create offspring that were doubly heterozygous (had genes for both green/yellow color, and for wrinkled/smooth texture), or *dihybrid*. - Mendel then self-crossed these dihybrid, F1 plants to produce an F2 generation (could see their genotypes), and recorded his results - Mendel's results showed that traits in a (pea plant) dihybrid cross behave independently (different combinations of traits do not 'alter' ratios- dominant/recessive ratios always stay the same)

monohybrid cross

- a cross that follows *two* variations of a single trait (as opposed to three or more) - all seven of the pea plant characteristics that Mendel studies were monohybrid crosses (each trait had only 2 forms)

binary fission

- a form of cell division occurring in bacteria. The chromosome is replicated, and each chromosome is pushed to opposite ends of the cell. These steps are right next to each other (unlike in eukaryotes, where replication is early, and separation doesn't occur until later) - most bacterial genomes (collection of all DNA) are made up of one circular DNA molecule. The circular shape of DNA allows the bacteria to save space within the cell ("coils"). - bacterial DNA is found in the *nucleoid* (not nucleus) - to compact and organize DNA within the nucleoid, we use SMC (structural maintenance of chromosome) proteins. (eukaryotes also use SMC proteins- cohesin and condensin.)

pleiotropy

- a genetic circumstance in which a single gene can affect multiple traits (an allele that affects multiple phenotypes is *pleiotropic*) - first discovered by French geneticist Lucien Cuenot. He studied fur colors in mice, and found that yellow (dominant allele) fur color could only be achieved heterozygously- having two dominant, yellow alleles (pure bred) led to yellow coat color, but it also led to a lethal developmental defect - a pleiotropic allele may be dominant in regards to one trait (yellow fur), and recessive in regards to another (developmental defect) - pleiotropic effects can be hard to predict because we don't know everything that a given gene does - pleiotropic diseases include cystic fibrosis and sickle cell anemia. Multiple symptoms (phenotype defects) can be traced back to a singular defective gene

gametes + types of division

- after undergoing meiosis (chromosome # under control; can now fuse to create a zygote), gametes can divide mitotically in order to produce more gametes (occurs plants, fungi, and protists) - some plants and insects perform mitosis with reproductive cells in order to produce individuals with varied numbers of chromosomes

X-chromosome inactivation

- all mammals undergo X-chromosome inactivation in order to produce dosage compensation - females that are heterozygous for X-chromosome alleles will have different traits in different cells, depending on which chromosome is randomly condensed (non-condensed chromosome expresses trait) - female calicos, for example, have both brown/black and orange fur. This is due to heterozygosity for an X-chromosome gene that determines fur color. One allele results in dark fur (brown/black), the other in orange. Because X-chromosomes are deactivated at random (both dark and orange end up being deactivated), the female calico will show both dark *and* orange colors. - white fur color and patchy fur are both due to a separate gene that it epistatic to the gene responsible for fur color. This gene produces pigment in some areas ('canceling' out dark and orange fur colors), but not others (does not 'cancel out' other fur colors), leading to splotchy white coloring

crossing + probability

- as long as a cross has some degree of independence/variability, we can calculate probabilities to try and predict the outcome (but if traits only ever paired in certain ways, it would be useless to calculate outcome, because it would be the same every time) - impossible = 0 (0%), certain = 1 (100%). Flipping a coins to get heads (1 outcome out of 2) = 1/2, or .5. - in the case of a heterozygous pea plant (Pp), the plant can pass on either gamete. If you want to get that P gamate, that's one outcome out of a possible 2, so the probability is 1/2, or .5. - *mutually exclusive* events are events that cannot happen at the same time (such as flipping heads or flipping tails- the coin can only land on one side, so you only get one at a time). - if you flipped a coin two times and tried to get heads each time, each coin flip would be an independent event, because the outcome of one flip does not influence the outcome of another.

bacterial cell division

- bacteria are not sexual; therefore, they do not/cannot use 2 different 'sets' of DNA to produce offspring. Instead, they only need/only use their own, and divide themselves as a way of reproduction. - clonal- each 'offspring' produced by cell division is an identical copy of the parent cell

plant cells + cell plates

- because plant walls are too rigid to be 'pinched off, plant cells assemble cell wall components within their interiors, at right angles to the spindle apparatus. The cell membrane then develops and expands until it reaches the plasma membrane on both sides. This separates the cells from one another. - cellulose is then combined with the cell membranes to create two more cell walls for each daughter cell - the 'original' wall is then used as a 'compartment' for storing sugars, and is now referred to as the *middle lamella*

prometaphase

- begins after the breakdown + absorption of the nuclear envelope - spindle fibers attach to chromosomal kinetochores, linking each spindle fiber to one corresponding sister chromatid ("paired") - a second group of microtubules begin to grow from the poles of the cell and towards the centromeres of the chromosomes. These microtubules then bond to the kinetochores of the sister chromatids. This results in each of the two 'copies' being pulled to different, opposite ends of the cell (after cell division is over, 1 copy will be in each. This bipolar attachment is a very important step in the process of cell division- if done improperly, one cell could end up with too few chromosomes, one too many, both of which can be fatal) - the chromosomes are then moved to the equator of the cell - two mechanisms for chromosomal movement (usually work in unison): 1) when microtubules link to kinetochores, it kinda pushes the chromosomes 2) motor proteins within the kinetochores and the poles of the cell pull on the microtubules in order to provide a 'tug of war' type mechanism for positioning chromosomes

checkpoints of the cell cycle

- checkpoints allow the cell to stop 'cycling' for as long as it wants, allowing the cell to go and do other stuff - Cdk enzymes tell cell division to stop or start up again - checkpoints exist because, once the cell decides to go ahead with any divisive process, it can't "undo" it. Checkpoints make sure that division is really what we want to do. THE G1/S CHECKPOINT: - the cell "decides" whether or not to divide - decision influenced by external factors (does the environment around the cell make division a good idea? eg., starvation), growth factors, and damage to DNA - the G1/S checkpoint in yeast is called START. In animals, it is called the restriction point, or *R point*. - initiates mitosis/meiosis once the cell begins preparing to replicate DNA THE G2/M CHECKPOINT: - stimulates mitosis - checkpoint 'managed' by Cdks - this checkpoint looks at how the cell has replicated its DNA, and stalls the cell cycle if the cell has replicated it wrong, so that the cell itself can have time to fix it - if DNA is damaged at this point, the cell cycle will also stall THE SPINDLE CHECKPOINT: - happens during late metaphase - makes sure chromosomes are attached to spindle fibers so that they can be pulled apart during anaphase; makes sure chromosomes are aligned correctly on the metaphase plate - cannot be reversed once anaphase begins (chromosomes begin to pull apart)

chloroplast genes

- chloroplasts are mainly maternally inherited, but some species can exhibit biparental chloroplast inheritance as well - in 1909, Carl Correns was the first to hypothesize that chloroplasts were responsible for inheritance of a type of leaf color within four o-clock, or Mirabilis jalapa, plant. Regardless of the male's phenotype, the offspring always exhibited the phenotype of the female parent. - additionally, in Sager's work on green algae, resistance to a certain kind of antibiotic (strepomycin) was transmitted through chloroplastal DNA, which was only passed on by the mt+ mating type. The mt- mating type did not contribute chloroplasts/chloroplastal DNA (mt+ and mt- gametes fuse to form a zygote).

anaphase

- cohesion proteins holding sister chromatids together are removed (in centromere); sister chromatids split apart - sister chromatids are pulled to opposite poles of the cell - anaphase A and anaphase B are both forms of chromosomal movement driven by microtubules within the cell - ANAPHASE A: = microtubules shorten; kinetochores are pulled toward the poles, bringing the chromatids closer as well = microtubules are shortened by the removal of tubulin subunits (microtubule monomers) near the kinetochores - ANAPHASE B: = spindle fibers 'leave' center of the cell- left-side spindle fibers transfer themselves to the right side of the cell, and right-side spindle fibers transfer themselves to the left side of the cell = this pushes the poles of the cell apart, which in turn pushes kinetochores/sister chromatids connected to the spindle fibers apart = spindle fiber 'pushing' may elongate the cell if it has a flexible outer membrane - anaphase ends when sister chromatids are separated When the sister chromatids separate in anaphase (we can now ensure each cell will have the correct number of chromosomes)

genes can have multiple alleles

- diploidic cells (can hold 2 sets of chromosomes) can have genes can have 2+ alleles (the gene for blood type, for example, has 3 alleles, A, B, and O) - just like in genes that have two alleles, dominance relationships between multiple alleles can be understood by combining phenotypes together/'mapping' the results

cytokinesis in fungi/protists

- during cellular division in fungi (and some protists), the nuclear membrane does not dissolve. Because of this, mitosis must be contained entirely within the nucleus - when mitosis is done, the organism divides the nucleus in two, then goes into cytokinesis to separate each nucleus into one new daughter cell - kind of trial and error because there is no mechanism in this type of division to make sure that the nuclei divide evenly, or that the cells divide the nuclei evenly. Fortunately, fungi are pretty good with taking chromosomal errors in stride. - similarly, there is no mechanism in this type of division that makes sure the needed amount/type of organelles are distributed into both daughter cells. Luckily, the fungi is also pretty ok with this- it only needs one organelle to be able to make copies, and then it can produce all of that organelle type that it needs

homologous chromosomes + meiosis

- during prophase I, homologous chromosomes are paired (synapsis) - homologue = 2 paired sister chromatids THE SYNAPYTONEMAL COMPLEX: - in order for homologous chromosomes to "find" each other during pairing, a structure called the *synaptonemal complex* is created to psychically joined paired homologues. - this complex consists of a main 'piece' that connects to two other sideways pieces via filaments - the synaptonemal complex seems to be structured fairly similarly across all biological systems CROSSING OVER: - also occurs in prophase 1 - genetic recombination; homologues exchange chromosomal material by swapping alleles - the sites where crossing over occurs are called chiasmata (singular chiasma) - chromosomes interact with each other in this way until anaphase I, where they are pulled apart. Being psychically linked in this way helps homologues hold together MITOSIS V. MEIOSIS: - METAPHASE 1: = paired homologues move to the metaphase plate. each homologue is attached to a spindle fiber, both fibers going to opposite sides of the cell (poles) = one 'type' of chromosome (maternal, paternal) cannot have both sides (maternal, maternal); it must 'choose' one (maternal, paternal or paternal, maternal) = mitotic homologues have no 'pairs'; behave independently of one another bc it's all the same genetic material - ANAPHASE I: = in meiosis, homologues are pulled to opposite poles of the cell = in mitosis, sister chromatids ("half" a homologue) are pulled to opposite poles of the cell

prophase

- first stage of mitosis - chromosomes are now visible due to G2 condensation - chromosomes continue condensing through prophase; they "bulk up" - once the chromosome bearing rRNA (ribosomal RNA) genes is condensed, ribosomal RNA synthesis stops - spindle apparatus begins to appear; normal microtubule structure within cell is replaced by spindle. - in animal cells, the two centrioles formed during G2 will begin to move apart; as they move apart, spindle fibers are formed. Once they have reached opposite sides of the cell, they have formed a 'bridge' of microtubules referred to as a *spindle apparatus*. - in animal cells, centrioles will extend many microtubules toward the plasma membrane once they have reached the poles of the cell. This plasma membrane + microtubule combo is called an *aster*. Scientists think it holds the centioles in place and keeps them attached to the spindle during spindle retraction. - while the spindle apparatus forms, the nuclear envelope breaks down and is absorbed by the endoplasmic reticulum. By this point, the microtubule fibers have extended completely across the cell. This allows the cell to divide down the middle in later stages.

characteristics of growth factors, PDGF, and the G0 phase

- growth factors trigger signaling receptors within the cell in order to initiate division. - fibroblasts (cells used to build supporting organic structures/heal wounds) have intramembrane receptors for the growth factor PDGF. The PDGF receptor is a receptor tyrosine kinase that initiates a MAP cascade in order to trigger cell division - fibroblasts will grow and divide only when blood serum is present. Serum is a liquid within clotted blood, and, because fibroblasts grow in blood plasma, fibroblasts are triggered when external injuries occur. As a response, blood clotting begins, which lets the fibroblasts know they need to activate in order to promote wound healing. - researchers hypothesized that the platelets within these blood clots were releasing something into the serum in order to trigger fibroblast growth. This 'something' turned out to be the growth factor PDGF. - PDGF has the ability to override division inhibition because one has the ability to become injured at any time, and injuries must be healed (and, because blood clots in response to injury, it always triggers the release of PDGF, causing cells near the wound to divide) - only a small amount of the growth factor PDGF is needed in order to trigger division in cells with PDGF receptors CHARACTERISTICS OF GROWTH FACTORS: - proteins - specific cell surface receptors are 'matched' with specific growth factors - many growth factor receptors often initiate MAP kinase cascades, which results in a kinase (enzyme) being transported to the cell's nucleus, where it uses phosphorylation to activate transcription factors (in this case, we're transcribing the genes that stimulate the production of G1 cyclins/proteins involved in cell cycle progression) - cells select what growth factors they do/don't respond to by only carrying specific types of receptors - some factors, like PDGF, affect a broad range of cell types. Others, such as nerve growth factors (NGFs) can only promote growth in specific classes of neurons. - most animal cells need the input of multiple growth factors before they 'can' trigger cell division THE G0 PHASE: - if a cell does not have the growth factor(s) it would need to begin dividing, it halts at the G1 checkpoint of the cell cycle and remains in the dormant *G0 phase* until the correct growth factors can be acquired. - the ability to 'pause' cell division is what allows different types of cells to vary so widely in cell cycle lengths

dihybrid cross probabilities are based on monohybrid cross probabilities

- here's an example: if we cross two heterozygous purple plants (Pp and Pp), the possible outcomes will be PP, pP, Pp, and pp. The probability of any one of their offspring showing a dominant phenotype is 3/4 (PP, Pp x2), and the probability of any of their offspring showing a recessive phenotype is 1/4 (pp). - due to independent assortment, we could calculate the probability of an organism possessing multiple different traits (wrinkled texture, green seed, for example) by finding the probability of achieving one trait, and multiplying it by the probability of achieving the other. The probability of achieving a green seed is 1/4, and the probability of achieving a wrinkled see is 1/4, so the probability of achieving a wrinkled, green seed is 1/16. - independant assortment allows dihybrid crosses to = independent monohybrid crosses - you can calculate the 'amount' of a given genotype/phenotype either by using probability, or with a Punnett square

segregation - crossing over

- homologous chromosome pairs are held together by chiasmata and sister chromatid cohesion. This allows all 2 chromosomes/4 sister chromatids to be moved to the metaphase plate together - however, in fruit fly males, something different happens. Meiosis proceeds without recombination (achiasmate segregation, or "without chiasmata segregation"). Rather than using crossing over to 'bind' homologous chromosomes/sister chromatids, they use an alternate joining mechanism, possibly telomeres or other heterochromatic sequences. - the majority of species use the formation of chiasmata (allows for crossing over) and sister chromatid cohesion to 'glue' homologues together in a way that will lead to genetic recombination when separated

sex chromosomes in humans

- humans have 46 chromosomes (23 pairs), half donated by one parent, half donated by the other. - 44 of these chromosomes (22 pairs) 'match' in both males and females- they are called *autosomes*. The remaining 2 that don't (1 pair) are the non-matching sex chromosomes- XX in female, XY in male. - the Y chromosome only expresses a few select genes. Therefore, any recessive traits on the X chromosome will not end up 'countered' by the Y chromosome, leading to the male being recessive for that trait. - all human beings begin development as females- then, if the individual has a Y chromosome, the SRY gene triggers penal formation and production of secondary sex organs - individuals with at least one Y chromosome are normally male, but trans and intersex people exist, so. For example, part of a Y chromosome can attach itself to one X in a pair of X chromosomes, causing a 'female coded' individual to develop as male. Additionally, XY individuals can have a disorder in which their body fails to respond to sex hormone signals, causing them to develop as female. SRY mutations can cause the same effect. - this form of sex determination is shared among mammals. In fish and reptiles, for example, the environment can change how sex-determining genes are expressed, which changes the sex of the individual.

types of genetic dominance

- if having 2 recessive alleles causes an enzyme to lose functioning and produce a certain trait, then why would a heterozygote have the same appearance as a dominant homozygote, instead of being somewhere in the middle? This is because enzymes usually work in pathways, rather than by themselves, so other components of the pathway can sometimes pick up the slack if a piece is missing. - if an enzyme pathway can do this (cause the same results unless it's a homozygous recessive), dominant alleles that trigger that enzyme are completely dominant INCOMPLETE DOMINANCE: - what we thought blended inheritance was (little bit of both in regards to a singular trait; when AA and aa produce Aa, Aa is halfway between both of them). neither allele is dominant. - Japanese four o'clocks (red + white = pink) - only 'shows' in heterozygotes; all genotypes can be distinguished through distinct genotypes CODOMINANCE - each allele within a gene produces its own effect (alleles themselves are *codominant*) - heterozygotes show multiple 'effects'/some aspect of each homozygous phenotype, but these don't blend together like incompletely dominant heterozygotes - codominant alleles found in ABO blood groups (different phenotypes cause different responses when the proteins they 'code' on the surfaces of red blood cells interact with the immune system. Homozygotes have only one type of these proteins, while heterozygotes have two)

anueploid gametes

- if meiosis is performed incorrectly, it will result in gametes without the correct number of chromosomes - nondisjunction, for example, is when one pole gets two copies of a chromosome, and another gets none. Any gamete with an improper number of chromosomes is called an *aneuploid gamete*. - aneuploid gametes are the most common cause of spontaneous abortion

genetic recombination

- in a Mendelian, dihybrid cross, two true-breeding (RRBB and rrbb, for example) parents will cross to produce doubly heterozygous F1 offspring - if the genes for both of these traits were on a single chromosome, then meiosis would simply 'switch' alleles from one chromosome to another, and all the F1 offspring would end up having the exact same genotype/phenotype as one of their parents. However, because of crossing over, genes can exchange alleles before splitting, causing the F1 offspring to have alleles from both parents, which would allow them to present different genotypes/phenotypes than their parent generation. Gametes that undergo this process of crossing over are called *recombinant gametes* (initially formed by recombination of parental alleles).

germ-line cells

- in animals, zygotic cells (formed by contribution of maternal/paternal chromosomes that then undergo meiosis) go into mitosis, eventually forming bodies - the cells within the body that will eventually undergo meiosis in order to produce gametes are called *germ-line cells*, and are set aside from other somatic cells early on in the developmental period. - both somatic and germline cells are diploid, but somatic cells must always remain diploid (can only undergo mitosis), while germline cells can 'choose' to undergo meiosis and become halpoidic/gametes

animal cells + actin

- in eukaryotes that lack cell walls, the cell can be split in two by using a belt of actin filaments - these filaments slide past one another light a belt tightening, causing the cell to 'pinch' (creating a *cleavage furrow*) and slicing the cell in half

gene for eye color in flies

- in fruit flies, sex is determined by the amount of x chromosomes a particular individual has. Female flies have two x chromosomes, and male flies have one x and one y chromosome. - during meiosis, females produces two x gametes, and males produce one x and one y gamete. If an x sperm fertilizes an x egg, it will become female, but, if a y sperm fertilizes an x egg, the egg will become female. - in fruit flies, the trait for white eyes resides only on the x chromosome (because the male only has one x-chromosome, any white-eyed alleles do not have to 'compete' with potential, dominant red-eye alleles, making it 'easier' for males to have white eyes than females, who must inherit two recessive alleles rather than just one). - this experiment was important because it was the first experiment to show that genes determined traits, and that traits were located on chromosomes

sister kinetochores + meiosis I

- in order to be able to cosegregate sister chromatids (move both chromatids to the same pole via the centromere), the kineochores of sister chromatids must be attached to the same centromere. - separation of homologues occurs in meiosis I. Separation of chromosomes occurs in both mitosis and meiosis II (kinetochores, not centromeres, attach to poles, pulling individual chromatids apart) - during meiosis I, kinetochores protrude from the chromosomes, which makes it easier for monopolar attachment of spindle fibers to occur. In contrast, kinetochores recede during mitosis, which makes it easier for bipolar attachment of spindle fibers.

punnet squares

- in order to test his ideas regarding heredity, Mendel created a symbolic model that he could use to interpret the results of his experiments. - uppercase allele is always the dominant one; lowercase allele is always recessive - true breeding = all uppercase or all lowercase (RR or rr) - heterozygotic organisms = one uppercase, one lowercase (Rr) - Mendel's crossed-pure breeding (self-fertilized so he could see if traits ever varied within that organism, then only used non-varied ones) purple and white-flowered plants (PP x pp) to create generation F1. A PP x pp cross produced only heterozygous (Pp) plants, which could not express recessive traits (all had purple flowers). - F1 individuals were allowed to self-fertilize, meaning that both P and p gametes could randomly combine (resulting in F2 individuals). - the Punnett square was created by English geneticists R.C. Punnett. - Mendel's predictions for the F2 generation, also confirmed by Punnett square, stated that the F2 generation would consist of a 3/4 purple-flowed plants (RR, Rr, Rr) and 1/4 white-flowered plants (rr)

test-crossing

- invented by Gregor Mendel - in a test-cross, an individual with an unknown genotype (dominant phenotype, RR or Rr) is crossed with a homozygous recessive type (rr). Because we know the alleles on one side, the phenotypes of the offspring that are produced will tell us what the other parent's genotype is (/alleles on the other side). - for example, we may not know whether a purple-flowered pea plant is homozygous dominant or heterozygous. However, by crossing it with a white pea plant and predicting for both options (RR + rr or Rr + rr), we will be able to see what genotype it is by looking at the ratios present in the offspring. (RR + rr = all purple; heterozygous (Rr). Rr + rr = half purple (heterozygous dominant; Rr), half white (homozygous recessive; rr).) - if any offspring of a dominant + non-dominant phenotype has a non-dominant phenotype as well, we know the dominant-phenotyped plant is heterozygous, and carrying one recessive allele. - test-crossing can also be used to identify the alleles of individuals within dihybrid crosses - Mendel often performed test-crosses to identify whether plants were heterozygous dominant or homozygous dominant. - a dihybrid individual exhibiting both dominant traits could have all of the following genotypes: AABB, AABb, AaBB, or AaBb. By crossing this individual with a homozygous recessive, you can determine whether or not either of that individual's traits are true-bred (homozygous dominant rather than heterozygous dominant)

chromosome karyotypes

- karyotype: the numbers/appearances of the chromosomes found within various species - chromosomes vary in size, color, centromere location, arm length, and the positions of their constricted regions ("center" of the x) - to determine the number of chromosomes within a given species, we count the number of haploid chromosomes (n; chromosome pairs). For many other species, the total amount of chromosomes is 2 x n, and referred to as the *diploid number*. - for humans, the *haploid number* is 23, and the diploid number is 46. - each parent contributes a haploid number of chromosomes to their offspring, which is how we end up with the total, diploid number of chromosomes. - 1 mother's chromosome + 1 father's chromosome = a *homologous* chromosome pair. Any individual chromosome picked from that pair is a *homologue*.

chromosome replication

- karyotypic chromosomes are only seen during cell division. - Prior to replication, chromosomes are only composed of one 'set' of DNA, arranged into one long, compacted fiber. After replication, each chromosome contains 2 copies of the same sets of DNA, and is held together by complexes of proteins referred to as cohesins. - as chromosomes are further condensed/arranged via the protein scaffold, they begin to appear as two strands held together by a centromere. Each 'half' of the chromosome is called a chromatid. - replicated products must be held together in order to divide properly. DNA must also be replicated to ensure that each cell can receive the full number of chromosomes present in the original cell. In order to avoid having to tell different chromosomes apart, the replications + parent chromosomes are glued together as sets (chromatid pairs) at the centromere. Mitosis then separates these copies, ensuring that parent cells keep all their chromosomes, and that all needed chromosomes are distributed to daughter cells.

sexual life cycle

- meiosis + fertilization = sexual reproduction - diploid cell: = cell w/ two sets of chromosomes = non-somatic (sex cells; gametes) - halpoid cell: = cell w/ one set of chromosomes = somatic - every human inherits 23 maternal chromosomes (maternal homologues) and 23 paternal chromosomes (paternal homologues) for a total of 46 chromosomes, the same number of chromosomes found in all somatic cells - the life cycle of every sexually reproducing organism alternates between diploid and haploid chromosome numbers. Different organisms can spend different lengths of time in either category (algae are mainly haploidic, we are mainly diploidic, some plants/algae alternate between both). - most animals are diplodic. After a zygote is formed via meiosis, it begins dividing mitotically, producing diploid (chromosome "pair" cells). Later in life, some of these diploid cells undergo meiosis in order to produce gametes for potential future reproduction.

telophase

- microtubules broken down into tubulin monomers; spindle apparatus 'dissolves'. Tubulin monomers are then used to build cytoskeletons for new daughter cells. - nuclear envelope reforms around each set of chromatids, which now designates them as chromosomes (sister chromatids not attached via a centromere) - chromosomes begin to uncoil, allowing for the process of gene expression to begin (need to be able to reach all the genes before you can decide which ones to turn on) - rRNA genes expressed early on; nucleolus reforms (rRNA is a ribozyme (RNA enzyme) that catalyzes protein synthesis) - ends w/ cell division - the cell can now 'end' the cycle of division and enter interphase - during interphase, other important organelles (mitochondria, chloroplasts, etc) are also moved to opposite poles of the cell in order to be 'divided into' new daughter cells - cytokinesis, not telophase, is the 'end' of the cell cycle. We have begun to split apart, but not finished. Cytokinesis divides the cell by cleaving it.

replication is suppressed during meiosis

- mitosis copies its DNA so that, when the cell divides, both parent and daughter cell will have the correct number of chromosomes. however, because meiosis is about *reducing* the number of chromosomes with a given cell, reproduction never occurs. - scientists think that meiosis suppresses this reproduction by keeping cyclins (specifically, cyclin B) around as a replication inhibitor- unlike in mitosis, which completely destroys them - during mitosis, specific cyclins are destroyed so that they will no longer be 'preventing' division, and the cell will begin to divide - by mantaining cyclin B levels, meiotic cells cannot form the 'parts' to replicate DNA, preventing DNA replication

meiosis produces non-identical cells

- mitosis produces daughter cells that are identical to the parent cell. Meiosis produces daughter cells unique to both their parent cells and to each other (due to random assortment and crossing over). - mitotic cells only carry one 'type' of chromosomes (maternal or paternal). While fully divided meiotic cells have the same number of chromosomes as mitotic cells, they have a mix of maternal and paternal DNA. - due to the random assortment/crossing over that occurs during meiosis, sexually reproducing organisms have much greater overall genetic variation than asexually producing organisms (who 'reproduce' mitotically).

different species have different #s of chromosomes

- most eukaryotes have between 10 and 50 chromosomes (per each body (non-sperm/egg)) cell - human cells have 46 chromosomes each (23 pairs). - not having the "correct" amount of chromosomes for your species, usually means death

polygenic inheritance

- most phenotypes/genotypes for singular traits are made up of multiple genes (qualities; color, texture, luster) with multiple alleles ("options"; red/grey/brown, smooth/wavy/curly, shiny/dull) - most phenotypes have more than 2 "options" (like hair color, height, etc.) - when you inherit a genotype for a trait that's made up of multiple genes, it is called *polygenic inheritance* - if multiple genes are "fused" together in order to influence something's appearance, those traits will be very similar between organisms of the same type (little variation). - however, when genes develop independently of one another, there can be more of a 'gradient' between organisms (more variation; *continuous variation*) - traits that contribute to continuous variation are called *quantitative traits* - the more independent genes that there are to influence a certain trait (less fusing), the more 'versions' of that trait there will be - often, trait variations can be grouped into categories (height, hair color, skin color, etc). The number of people belonging to each group can be recorded using a histogram - we do not actually know if brown eyes 'overrule' (are dominant to) blue eyes because eye color is controlled by multiple genes, so something else could be 'suppressing' the blue. By seeing which alleles dominate which in traits with multiple genes, we can attempt to product unknown characteristics based on known characteristics (aka externally visible characteristics, or EVCs)

homologous pairing in meiosis

- occurs during prophase I - current research suggests that homologues find each other through mechanisms in their telomeres - sister chromatid cohesion (meiosis) and mitotic cohesion both use cohesin proteins, but the meiotic process seems to have evolved a specific 'sub-type' of cohesin protein used only for meiosis - cohesin proteins are destroyed during anaphase in order to allow chromosomes/chromatids to be pulled to opposite poles of the cell. - use of the synaptonemal complex organizes chromosome pairs to be recombine/fused (crossing over) during meiosis I in eukaryotes - in meiosis, the process of crossing over is always initiated by the synaptonemal complex causing a double-strand break in one homologue pair. - double-strand breakage first evolved as a repair mechanism for DNA breakages, then was adapted for breaking apart homologous pairs. As a result of this, the processes of both meiotic recombination and DNA reparation are quite similar. - recombination aids in proper homologue separation. Studies show that organisms that have lost function/recombination proteins have higher levels of meoitic nondisjunction (homologues/chromatids do not split apart) than those that have retained their function/recombination proteins

the anaphase promoting complex

- once all of the chromosomes within a cell have aligned at the metaphase plate and the microtubules are ready to pull them apart, a signal is transmitted to the *anaphase-promoting complex/cyclosome, or APC/C.* - the APC/C then triggers anaphase. Because sister chromatids are still held together by their centromeres by cohesin at this phase, the APC produces a protein called securin. This protein inhibits another protein called separase. Separese is located within the cohesion complex, and, once it is inhibited, it begins to destroy cohesin proteins. - in budding yeast, separase degrades a specific component of cohesin referred to as Scc1. When Scc1 is degraded, sister chromatids are suddenly released, at which point they are 'pulled forward' by attached microtubules (anaphase) - in vertebrates, cohesin is removed from chromatids during chromosome condensation, when cohesin is replaced by condensin. The majority of what little cohesin remains is 'stored' in the centromere. APC/C ROLES IN MITOSIS: 1) activates the protease (enzyme that breaks down proteins) responsible for removing cohesins from centromeres/separating chromosomes 2) destroys mitotic cyclins (proteins associated w/ mitosis) so that cells will stop dividing. Mitotic cells are destroyed by proteosome, an organelle within the cell that is responsible for degrading proteins. In order to signal this organelle, we use a signaling molecule called ubiquitin, which the APC/C transports to the proteosome.

mitochondrial genes

- organelles are typically inherited from only one parent, generally the mother. When two sex cells fuse, the product receives nuclear DNA from both parents, but gets all of its mitochondrial DNA from the egg cell of the female parent (as egg cells have more cytoplasm than sperm, and, therefore, more organelles such as mitochondria). Then, when the product divides to prepare to form a zygote, mitochondria are randomly partitioned/divided as well. - this particular mode of uniparental (one-parent) inheritance is called *maternal inheritance*. - in human, LHON disease (Leber's hereditary optic neuropathy) shows maternal inheritance. This disease is caused by the mutation of an allele that produces NADH dehydrogenase. Mutant NADH hydrogenase reduces the efficiency of the electron transport chain, which reduces ATP production. As a result, cells that need higher levels of ATP (such as optic system cells) begin to degenerate. - because this disease is located in the mitochondria (where ATP is produced) a mother with LHON disease will pass it on to her offspring, while a father with the disease will not. Females and males can inherit maternally inherited diseases equally; it's just that males can't pass them on. (Only females pass it on to the next generation, which is made up of both sexes.)

MPF's role w/in the cell cycle

- participates in the G2/M checkpoint - MPF is sensitive to disrupted/delayed replication, and damaged DNA - although M phase cyclins (proteins aiding in cell division) are needed in order for MPF to function properly, MPF function is controlled by inhibitory phosphorylation of Cdc2. - by using phosphatase (protein) to remove an inhibitory phosphate group, MPF can begin to make phosphates in order to further phosphorylate its own active site (feedback loop). - at this checkpoint, we see how many inhibitory phosphate kinases (stops phosphate production) and phosphatase proteins (starts phosphate production) we have. - if DNA is damaged, the phosphorylation of MPF is inhibited

human traits with dominant/recessive inheritance

- researchers can't perform hereditary experiments on humans because that would be f#cked up- instead, they have to study dominant/recessive traits through family trees - to visualize these inheritance-based family trees, we use pedigrees, which represents matings and the offspring produced from them over multiple generations. Pedigrees always focus on one particular trait. - by studying pedigrees, we can understand how traits are inherited HEREDITARY JUVENILE GLAUCOMA: - although rare, dominant disease-causing alleles do exist within the human genome (not all diseases are recessive) - in this case, blindness (specifically, hereditary juvenile glaucoma) is a dominant trait - the pedigree we used to study hereditary juvenile glaucoma comes from a French bloodline that spanned over three centuries. We could tell that this trait was dominant because it was present in every generation ALBINISM: - albinism is a recessively inherited trait - multiple genes contribute to albinism, which is why it's a rarer condition than hereditary juvenile glaucoma (individual must be homozygous recessive for all 'required' genes) - in albinism due to nonfunctional tyrosinase (enzyme which produce skin pigment; two nonfunctional alleles (rr) leads to completely nonfunctional enzymes/no pigment in the skin), males and females are affected equally. Additionally, most affected individuals have unaffected parents, pairings with a single affected parent usually do not produce affected offspring, and affected offspring are more frequent when parents are genetically related

centromeres in meiosis

- segregation of homologues (NOT chromosomes) occurs during anaphase - separation occurs when the centromeres of sister chromatids cosegretate (sister chromatids are both pulled to the same pole) instead of splitting each chromatid to different poles. In order to do this, cohesin proteins must first be removed from chromosome arms (separating homologues), then, in meiosis II, from sister centromeres (separating into chromatids). - homologues are joined by chiasmata (crossing over points) + sister chromatid cohesion. By destroying the Rec8 proteins on chromosome arms, the chiasmata is broken, and homologues can be pulled apart during anaphase. - sister chromatid cohesion is maintained by the centromere throughout all of meiosis I, while cohesion between homologues is cut off during anaphase I. This is able to occur because of shugoshin proteins, which protect cohesin proteins in the centromeres during meiosis I. - mice, for example, have 2 types of shugoshins: Sgo-1 and Sgo-2. If there is a lack of Sgo-2 within the cell, sister chromatids will seperate early. Some scientists have suggested that Sgo-2 only "appears" during meiosis II because the tension produced by anaphase II causes Sgo-2 to migrate from chromosomal centromeres (split homologues) to kinetochores (split chromosomes).

non-Mendelian inheritance

- some genes are inherited through other ways than chromosomes. In addition to chromosomes, DNA is also located in organelles (specifically, the mitochondria and chloroplasts). - Ruth Sager was the first to study this type of inheritance, termed *Non-Mendelian inheritance*. She constructed the first-ever map of chloroplast genes using a subspecies of green algae in the 1960's/1970's. - mitochondrial/chloroplast genes are not divided during meiosis like the nuclear genome is

dosage compensation

- someone with two X chromosomes will only have one X chromosomes that can produce the 'required' amount of proteins that it needs to. Someone with XY chromosomes will also have only one X chromosome working to produce proteins, so proteins are being produced in equal amounts in both males and females, regardless of chromosomal organization. - after sex is determined in an XX organism, one of the chromosomes is rendered inactive (unable to produce proteins). This is called *dosage compensation*, and it ensures an equal level of gene expression between organisms of different sexes - X chromosomes are inactivated by condensing. They are inactivated at random- if a woman inherits an X chromosome from both her mother and her father, one of her cells may have a condensed paternal chromosome and the other, a condensed maternal chromosome. - If a woman is heterozygous for a sex-linked trait, some of her cells may express one trait, and some another, depending on which chromosome is condensed

ABO blood groups

- the blood type gene, gene I, 'decides' whether or not to add sugar molecules to the proteins on the surface of red blood cells, and, if so, what kind of sugar (encodes an enzyme). These sugars then give different kinds of directions to the immune system. - gene I alleles: IA- adds galactosamine IB- adds galactose i- does not add sugar - gene I alleles can be combine to produce six distinct genotypes: A individuals: either IAIA or IAi, adds galactosamine (multiple genotypes) B individuals: either IBIB or IBi, adds only galactose (multiple genotypes) AB individuals: IAIB, produce both galactosamine and galactose O individuals: ii, add no sugars to blood cell proteins - both AI and AB are dominant, whereas i is recessive - if a person receives a blood type that has different sugars than theirs, the immune system will consider the new blood a virus and attack it, causing it to clump. Because the O blood type has no sugars, anyone can receive it (universal donor). Conversely, AB individuals have both of these 2 types of sugars, making them able to receive blood of any type (universal recipient).

cell cycle overview

- the cell cycle has two main points after which it cannot be reversed; one is the replication of DNA, and the other is the separation of the sister chromatids - the cell cycle can be put on hold at specific places in the cell cycle called *checkpoints*. At these checkpoints, the division process is 'checked' for any errors and halted if fixing needs to occur, insuring a very low rate of error in cell division as a process - at checkpoints, the cell can 'pause' cell division in order to manage other processes within itself (takes care of itself, sends any received signals)

cyclin-dependent kinases drive the cell cycle

- the cell cycle is controlled through phosphorylation (adds phosphate groups to the amino groups within proteins) - the enzymes that accomplish phosphorylation within the cell cycle are Cdks. One type of cell cycle kinase is called Cdc2, but Cdk enzymes partner with all kinds of different cyclins (proteins aiding in the cell cycle) depending on their positions within the cell cycle. - Cdc2's job is to phosphorylate proteins- however, they kinase itself is controlled by phosphorylation as well (phosphorylation at one site switches it "off", while phosphorylation at another site switches it "on"). - in order to activate Cdc2 kinase, it must both bond with a cyclic and be phosphorylated at the correct site - at the G1/S checkpoint, the organism begins to accumulate G1 cyclins, which bonds to Cdc2 in order to activate it. Once activated, this complex phosphorylates different proteins in order to increase enzyme activity during DNA replication.

interphase

- the centromere is a point on the chromosome that contains DNA sequences with the programming to bind specific proteins together - these proteins make up disc-like structures called kinetochores, which microtubules attach to before extending to separate chromosomes during cell division - after the S phase, the sister chromatids are bound together by the centromere. However, because they are both fully replicated, they do not actually share any portions of the centromere- they are bound to cohesin proteins, which are in turn bound to the centromere. - in multicellular animals, the cohesions that hold sister chromatids together after being replicated are then replaced by condensins, allowing chromosomes to condense in preparation to be pulled apart. This leaves the chromosomes loose and vunerable. - during G1 and G2, cells grow, synthesizing proteins and producing organelles. - after chromosomes are replicated (s phase), they are fully extended and uncoiled. Then, in G2, then begin condensation, which is aided by motor proteins. During G2, cells also begin assembling the structures they will need to move sister chromatids to opposite poles of the cell. In animal cells, *centrioles* are used to organize microtubules. During division, they replicate to produce one centriole for each cell pole. - chromosomes are completely condensed by early mitosis. - the protein that forms microtubules is called tubulin

meiosis = two divisions, no replication

- the first division of meiosis is called "reduction division" because the resulting daughter cells each take only half of the genetic material originally presented to them (2 x1 chromosomes rather than 1 x2) - the second meiotic division does not reduce chromosomes; it separates chromosomes into sister chromatids via another division (1 chromatid each)

multiple crossovers can yield independent assortment results

- the further genetic loci are from one another, the more likely it is that they will be combined during meiosis - when multiple recombinations occur at the same time (two homologues both switch 2+ genes), it actually restored parental combinations (XX and YY. If one Y switches with one X, it becomes, XY XY. However, if *both* switch at the same time, it just turns back in XX and YY, only on opposite sides). - as a result of this phenomenon, the relationship between gene distance and recombination frequency is not linear. As distance increases, recombination frequency also increases, but, after a bit more distance, it "flattens" out at a recombination frequency of .5. - at long distances, "mixing" between gene locations (loci) become frequent. If chromosomes cross over an odd number of times, they will produce recombinant gametes. If they do not cross over, or cross over an even amount of times, they will (re)produce paternal gametes. - at large enough distances, chromosomes cross over evenly as much as they do oddly. This results in the number of recombinant gametes being equal to the number of parental gametes (1/2).

principle of segregation

- the main conclusion of Mendel's model was that there are 'sets' of alleles (not just x, but xx) that segregate from/recombine with one another during the creation of new offspring (and those alleles do the same thing when *that* offspring reproduces) - this concept is called *Mendel's first law of heredity, or the Principle of Segregation*. The Principle of Segregation states that alleles for a gene separate during gamete formation and rejoin in random combinations during fertilization - allele segregation happens due to the behavior of chromosomes during meiosis (homologues separate during anaphase I, then chromosomes separate to create chromatids during anaphase II. This produces gametes that will have the correct amount of chromosomes after fusing (only one 'set', made up of both maternal/paternal DNA)). - explains behavior of alleles/traits in monohybrid (1 trait) crosses (NOT dihybrid (2+ trait) crosses)

F2 generation (dihybrid) results; Mendel's second law (Principle of Independent Assortment)

- the parents of the F2 generation had two different phenotypes: smooth, yellow plants and wrinkled, green plants. If the traits behave independently of each other (not "grouped together"- color and texture are different components), then we should have offspring that are smooth/green and wrinkled/yellow - The parents of generation F2 were both heterozygous in both traits. This can be represented as RY + ry and Ry + rY (assuming that these traits behave independently). By combining these traits, it will produce RY, Ry, rY, and ry, which can then be combined at random to produce the F2 generation. - When crossing RY, Ry, rY, and ry phenotypes, it results in 1 RRYY (yellow smooth), 2 RRYy (yellow smooth), 2 rrYy (yellow wrinkle), 2 Rryy (green smooth), 2 RrYY (yellow smooth), 4 RrYy (yellow smooth), 1 RRyy (green smooth), 1 rrYY (yellow wrinkled) and 1 rryy (green smooth). y = color (green = recessive yy) and r = texture (wrinkled = recessive rr). This produces 9 smooth yellow, 9 wrinkled yellow, 3 smooth green, and 1 green wrinkled (9:3:3:1 phenotypic ratio) - the above is a simplified model of the phenomenon Mendel observed. He obtained 556 F2 seeds, 315 of which were smooth yellow (RY), 108 smooth green (Ryy), 101 wrinkled yellow (rrY), and 32 wrinkled green (rryy). These results were very close to Mendel's predicted 9:3:3:1 ratio (9:3:3:1/556 = 313:104:104:35). - Mendel referred to the concept of traits not being "grouped" together as *independent assortment*. The *Principle of Independent Assortment* is *Mendel's second law*. - additionally, because these traits occurred independently of one another, we still achieved 3:1 ratios when looking at them individually (smooth v. wrinkled = 3:1, yellow v. green = 3:1). - Mendel's second law: the segregation of alleles pairs is independant (alleles for different traits are not "encouraged" or "discouraged" from pairing with one another). This type of independent assortment is due to the behavior of chromosomes during meiosis. Chromatid re-pairing is what causes this "mixing" of alleles, create an allele pair within the offspring that may be different that either of their parents

meiosis in diploidic organisms

- the process of meiosis can vary between eukaryotes, especially during chromosomal separation - meiosis in diploidic organisms consists of: = meiosis I: prophase I metaphase I anaphase I telophase I = meiosis II: prophase II metaphase II anaphase II telophase II

MPF = cyclin + cdc2

- the protein that the cdc2 gene encodes is a protein kinase (regulates activity of other proteins via de/phosphorylation) - MPF is literally has both cyclins and protein kinases within it - the protein kinase within MPF is a cdc2 protein - the cdc2 protein is the first in a group of proteins called *cyclin-dependent kinases (or Cdk)*. These proteins act as enzymes that de/activate other proteins through de/phosphorylation, and are only active when cyclin is added to the mix. . - Cdk enzymes drive cell division

metaphase

- trigger by chromosomes aligning with the equator of the cell - aligned "equator style"; all in the exact middle of the cell, but at different 'depths' to prevent confusion. This "equator" is referred to as the *metaphase plate*. The metaphase plate is not an actual cell structure- it is the line that can be drawn through the arranged chromosomes; the 'center' of chromosomal organization at this point during cell division. The metaphase plate is also an indicator of where the cell will divide in the future. - at this point, the centromere of each chromosome is equidistant from either pole of the cell. - a "transitional" phase- preparing the cells to divide, but division has not actually begun

human genetic disorders + sex linkage

- usually affects males to a greater degree than females because males do not have another X chromosome to 'stop' any recessive X-linked traits (red-green color blindness) - hemophilia = a sex-linked trait that affects the proteins involved in blood clotting, rendering the affected individual unable to stop bleeding = caused by an X-linked recessive allele (heterozygotes (women) are asymptomatic carriers) = exhibited by various European royal families (who could afford to keep records, which meant we could study their pedigrees) = the Romanov royal family (Russian) inherited this trait through Alexandra Feodorovna (member of the British royal family), who married Czar Nicholas II and produced a son, Alexis, with the condition = does not affect the current British royal family; King Edward VII, who was part of the royal family (not exiled, just dead) did not receive a hemophilic allele

duration of cell cycle depends on cell type

1) (animal) embryo cells can complete a cell cycle in 20 minutes or less (depending on animal). Because these types of cells do not need to grow (yet), they skip G1 and G2. They don't need to 'wait around' to divide, making division times much shorter (only S and M phases). 2) mature cells, however, do need to grow, making their cell cycles longer. It takes a mature mammalian cell about 24 hours to go through the cell cycle, but some cells have much longer growth, and therefore division, periods. 3) cells also grow during S phase 4) M phase (mitosis + cytokinesis) takes very little time 5) cell cycle length is primarily determined by the G1 phase. Cells pause between G1 and the start of DNA replication (G0 phase), and can remain in this phase for any length of time before 'resuming' cell division. Almost all cells within an animal organism are in the G0 phase at any given time. Some cells, like muscle and nerve cells, never replicate while out of utero (in G0 until death). Most cells, however, go into G1 in response to injury.

Gregor Mendel, super genius

1) Austrian, born in 1822, studied science and mathematics but he failed so he got a job at a monastery instead. However, Mendel was still a scientist at heart, and began doing experiments on plant hybridization in the monastery's garden. 2) MENDEL CHOSE THE PEA PLANT BECAUSE... - it had been used in many previous hereditary investigations- he knew that any offspring they produced would exhibit trait segregation. - of the plethora of 'pure' varieties to choose from. Mendel chose 'pure' varieties that had easily distinguishable traits (round v. wrinkled pods, yellow v. green color, et.c). - pea plants are small (saves room) and easy to grow, and, like most plants, they produce a lot of offspring. This allowed for Mendel to experiment on many plants at a time, and for him to quickly accumulate multiple generations, leading to faster results. - pea plants have both male and female sex organs enclosed within their flowers, so they can self-fertilize in order to produce offspring. If a flower is left to its own devices, it will produce offspring in this way rather than through cross-fertilization. Additionally, one can prevent self-fertilization in pea plants by removing male sex organs before it can occur. To cross-fertilize and produce genetically varied offspring, one would then have to add pollen from a different plant (cross-pollination). 3) MENDEL'S EXPERIMENTAL DESIGN - only focused on a few, clearly observable differences. objective (color rather than leaf shape) - differences chosen were comparable ('green v. yellow color' rather than 'seed shape v. plant height') - *stages of Mendel's experiments:* 1. Mendel allowed plants to self-fertilize for multiple generations in order to get a clear understanding of the traits that that variety possessed 2. performed crosses between similar varieties of pea plant that showed different traits, and between different varieties of pea plant (*reciprocal crosses*). 3. Mendel then allowed hybrid offspring to self-fertilize for several generations, so we could see what happened after all those traits got 'mixed up' - Mendel also counted the number of offspring that exhibited each chosen trait in each generation

meiosis + chromosome reduction

1) Belgian scientist Edouard van Beneden discovered that the roundworm Ascaris had different numbers of cellular chromosomes in different types of cells. The gamates (egg/sperm cells) only contained two chromosomes each, while somatic cells (non-reproductive cells) contained four chromosomes each. 2) because of this, Beneden proposed that two gametes (2 X 1/2) fused to form one zygote (1) (a process called fertilization/syngamy). He also proposed that this zygote contained two copies (one maternal, one paternal) of each 'type' of chromosome. 3) gamete formation had to have some mechanism that reduced the number of chromosomes within gametes to half (the # found in somatic cells). If it didn't, every new generation of organisms would have twice as many chromosomes as the last, and, eventually, that number of chromosomes would become just too large to sustain. 4) this mechanism is meiosis. Meiosis occurs during gamete formation in order to produce cells with half as many chromosomes as somatic cells. Therefore, when gametes fuse to form a zygote, that zygote will have a normal number of chromosomes, and all the cells that divide from it will have normal numbers, too.

Walther Flemming

1) German embryologist 2) examining dividing cells of salamander larvae; saw threads within the nuclei that then divided the cell lengthwise 3) Flemming coined this event mitosis; Greek "mitos" = thread

distinct features of meiosis

1) HOMOLOGOUS PAIRING/CROSSING OVER - joins maternal + paternal homologues - non-sister chromatids "switch" alleles, altering their genes - occurs during meiosis I 2) SISTER CHROMATIDS - connected at the centromere during all of meiosis I - move in 'pairs' during anaphase I (division of chromosomes) 3) KINETOCHORES - kinetochores of sister chromatids are attached to the same pole in meiosis I - kinetochores of sister chromatids are attached to opposite poles during mitosis/meiosis II 4) DNA REPLICATION does not occur during meiotic division

plant biology pre-Mendel

1) Josef Klreuter pioneered in experiments involving hybridization in 1760 (or, at least, successful hybridization, which is what made him the big name). He cross-fertilized different strains of tobacco, creating hybrid offspring. Most of these offspring differed in appearance from both of their parents and from their 'siblings', but some offspring *did* resemble one/both of their parents, and a few even resembled their "grandparents". 2) this observation directly contradicted the idea of direct transmission (the same traits unfailingly passed down the genetic line). 3) later (1823) an English landholder named T.A. Knight crossed two different varieties of garden peas. One was green, one was yellow, and both varieties were true-breeding (self-fertilized, making exact copies). When these two varieties were crossed, all offspring had yellow seeds, but some of the offspring from *those* offspring produced green seeds as well as yellow seeds. 4) because of this, scientists began to realize that traits *weren't* just uniformly passed down to offspring. There's some mixing up going on in there! A modern geneticist would say that this happened because there can be multiple forms of one trait- some offspring got the yellow 'form', and some got the green. 5) we knew that mixing up was happening (traits were being "segregated" in some way), but we didn't know how until Gregor Mendel discovered the concept of heredity.

probability rules

1) RULE OF ADDITION - if you roll a six-sided die, only one outcome is possible at any time (mutually exclusive). The probability of rolling any one of the six numbers on this die is 1/6. - rule of addition: the probability of two different outcomes is the sum of the individual probabilities of both outcomes - let's say we have two heterozygous purple plants (Pp and Pp). If we cross them, we get four possible outcomes: PP, Pp, pP, and pp. The probability of their offspring being heterozygous as well is 1/2 (1/4 (Pp) + 1/4 (pP)) 2) RULE OF MULTIPLICATION - deals with the outcome of multiple independent events - states that the probability of multiple *independant* events all occurring is the product (multiplication) of the probabilities of each event - an example of this is the offspring of a monohybrid cross. Whichever gamete one parent gives the offspring, it will not affect the outcome of which gamete the other parent gives. Therefore, this outcome has two independent events: which maternal gamete the offspring will receive, and which paternal gamete it will receive. - If two heterozygotes mate (Pp + Pp), the probability that the offspring will be pp is the probability of receiving a recessive allele from one parent (1/2), then another (x 1/2), or 1/4.

monohybrid observations ---> early principles of segregation

1) crossing plants with different phenotypes did not result in blended inheritance (purple + white did not = lilac). Instead, each plant was either entirely purple, or entirely white- they had to 'choose' one 2) if a trait is not being expressed, that does not mean it is gone. Recessive traits are latent- they do not just appear/disappear at random 3) alternative forms of traits are 'divided' amongst offspring. Some individuals exhibit one trait, some exhibit the other. 4) alternative traits were expressed in the F2 generation as a 3:1 dominant:recessive ratio, also referred to as the *Mendelian ratio* within monohybrid crossing.

pre-20th century view of heredity

1) heredity occurs within species (no "cross-heredity"; humans can't inherit clownfish traits) 2) traits are transmitted directly from parent to offspring (no dominant/recessive- everyone has them all) 3) traits are fixed (always passed on every time) and unchanging (new stuff is never added) 4) traits were thought to be inherited through blood; blood blended together to create offspring w/ traits from both sides ("bloodlines") 5) but not all of these assumptions can be true at the same time. If variation from mutations (adding nothing new) and dominant/recessive traits (variation) did not exist, eventually, all members of our species would look the same. Additionally, characteristics are not transferred statically from generation to generation (not the same phenotype every time).

stages of M phase

1) mitosis 2) cytokinesis

flaws in the Mendellian model

1) not every trait is controlled by a single gene with only two alternative alleles ("either A or B") 2) the environment around the developing fetus/zygote can influence which traits are selected, and which are not 3) genes produce their own results independently of other genes (one gene does not 'finish' up earlier and start influencing other developing genes) 4) dominance/recessiveness can be partial

Mendel's five-element model

1) parents do not transmit traits directly to their offspring (no blended inheritance; psychical traits are hidden/expressed from generation to generation). Rather, parents transmit information that allows for potential variation between themselves/other offspring ("factors"/genes) 2) all offspring receive one copy of each gene (1 chromosome) from each parent (2 copies of each gene; 2 'sets' of chromosomes merge to create new, varied offspring). 3) not all copies of a gene are identical. A phenotype can be presented homozygously or heterozygously (RR/rr or Rr). 4) alleles will always remain their own things- they will not blend together to create one allele, or turn each other into different things (R cannot turn r into another R). Alleles segregate in gametes randomly. 5) just because an allele is there, doesn't mean that allele's "assigned" trait will be expressed. Heterozygous individuals (Rr) express whatever allele is dominant. - allele + allele = genotype (blueprint) - what psychical trait those alleles 'cause' = phenotype (building) - mono-hybrid phenotypic ratio: 3(d):1(r) - mono-hybrid genotypic ratio: 1:2:1 (homozygous dominant(RR):heterozygous dominant(Rr):homozygous recessive(rr))

stages of mitosis

1) prophase 2) prometaphase 3) metaphase 4) anaphase 5) telophase

Thomas Hunt Morgan

1) while studying the fruit fly, he discovered a male fly with mutated eye color (white instead of red). Morgan wanted to discover if this trait would be inherited in a Mendelian fashion, so he crossed a mutant male with a normal, red-eyed female to see whether white eyes were a dominant or recessive trait. All of his F1 generation had red eyes; Morgan concluded that red was dominant over white. 2) Morgan then crossed the F1 generation red-eyed flies with one other. Out of 4252 flies, 782 (18%) had white eyes, which, although not a 3:1 ratio, did prove Mendel's Law of Segregation. 3) However, while some flies had red eyes, and others had white, all of the flied that *did* have white eyes, were males. Morgan wanted to figure out why this was. He tested one hypothesis (white-eyed female flies simply did not exist) by crossing his F1 females with his original white-eyed male. He obtained white and red-eyed flies of both sexes. Having sorted out this hypothesis, Morgan began studying chromosomes from male and female flies to attempt to find an explanation.

stages of interphase

1. G1 2. S 3. G2 during this portion of the cell cycle, the cell has not actually begun to divide yet

binary fission in detail

1. bacterial DNA is replicated at the origin of replication. The DNA replicates by starting at that point, and then branching out in both directions, down the circle, until both 'branches' meet in the middle (site of termination- across from the origin; lets the cell know when to stop) and the whole genome has been copied. 2. we now have 2 copies of bacterial DNA. 3. the cell "grows" by elongating itself; the DNA split off from one another 4. the cell divides when a cell is not dividing, the origin of replication is either at the 'top' or 'bottom' of the ring (closer to midcell). Once the cell has divided, however, both origins of replication move as far away from each other as they can (left copy, o.r. is on the left side of the ring; right copy, o.r. is on the right). By moving into position like this, they are positioned in such a way that their origins of replication WILL be in the middle once the 2 cells divide.

chromosome segregation in binary fission

1. chromosomes are moved, origin-first, to opposite ends of the cell 2. nucleoids begin to assemble 3. decatenation (untangling; organization) of the chromosomes 4. division occurs; other components in the cell are partitioned onto both sides by the growing membrane + septum (septation) 5. septation begins by forming a ring of FtsZ proteins. Then, other proteins accumulate around that ring. After this, the FtsZ structure contracts, which pinches the cell off into two new cells. - FtsZ is grown/moved to the midcell by switching the two inhibitor poles on both sides of the ring on and off (as it grows, it is 'pulled' back and forth to be centered as needed, while still being shaped evenly). - FtsZ is found in most prokaryotes

universal basics of cell division

1. genetic duplication (multiple copies) 2. segregating genetic information into daughter cells (organization) 3. division

structure of eukaryotic chromosomes

COMPOSITION OF CHROMATIN: - chromosomes are made up of chromatin, a DNA/protein mixture (40/60). Because chromosomes are the site of RNA synthesis, there's usually a good amount of RNA floating around near them, too. - each chromosome contains a very, very long molecule of DNA that is 'woven' back and forth to create a compact shape - we're not sure what chromatin is really like when it's in a non-dividing state. The non-expressed DNA within the chromatin is called heterochromatin, and the expressed is called euchromatin. CHROMOSOME STRUCTURE: - the DNA within a eukaryotic nucleus is like a string of beads. Every 200 nucleotides, the DNA complex (double helix) is coiled around 8 histone proteins. - Because histones have many amino acids (specifically, arginine and lysine), they are positively charged, which makes them strongly attracted + well bound to the negatively-charged phosphate groups in DNA. This allows the histone cores to act as "magnets" that guide DNA during coiling. - DNA + histone protein = nucleosome - nucleosomes are then further coiled into solenoids, which are coiled into 30-nm fibers. When chromatin is not dividing, this is the "final step" of storage. - during mitosis (non-reproductive cell division), proteins are assembled into a scaffold that provides a framework for DNA 'compacting'. It also aids in separation. - some people thing that DNA is compacted by using 'loops' of chromatin fibers found on scaffolding proteins - condensin proteins (evolutionarily related to SMC proteins) aid in eukaryotic DNA compaction

multicellular eukaryotes + external influences on the cell cycle

DIFFERENCES BETWEEN COMPLEX AND SIMPLE EUKARYOTES (single-cell eukaryotes, fungi, protists): 1) multiple Cdks control the cell cycle within complex eukaryotic organisms; simple eukaryotic organisms seem to have only one 2) complex eukaryotic organisms respond to a greater variety of external signals than simple eukaryotes do - higher eukaryotes have more Cdk enzymes/cyclins (proteins that aid in cell division) than they do Cdks themselves - because eukaryotes have more complex cell cycles, they also have more more 'input' on them b/c the organisms can mess with it at more point. - Complex forms of cell cycle control evolved as a response to more complex forms of organic organization (tissues, organs, organ systems) - in order to run a body, a multicellular organism must be very selective about which cells it allows to multiply, and which it does not. Cells inhibit the growth/division of other cells by recognizing cell 'borders'. Cells are divided into regions, and once one region hits another, both stop growing. Conversely, if part of one region is destroyed, the other will fill the gap by growing bigger until it hits the remaining border. - cells can 'sense' whether or not they are coming into contact with one another through receptor proteins within their plasma membranes. A specific type of receptor protein inhibits Cdk action when sensing contact w/ another cell, preventing both cells from any further division.

research + cell cycle control factors

DISCOVERY OF MPF: - researchers were looking at frog oocytes (ovary cells w/ ability to divide in order to form an ovum) when they discovered a substance they dubbed the maturation-promoting factor, or MPF. - frog oocytes 'pause' just before they're ready to go into meiosis (sex cell cell division). Before they can divide, they must get the 'okay' from hormonal signals. - when cytoplasm from all kinds of dividing cells was injected into these ooctyes, it trigger cell division. - so, MPF is a substance found within the cytoplasm of the cell that triggers cell division. - additionally, a version of this meiotic 'trigger' was found in the cytoplasm of cells undergoing mitosis (people were trying to fuse mitotic + interphase cells together) KEY ASPECTS OF MPF: 1) MPF activity varies throughout different stages of the cell cycle - increases HELLA during G2 (during which the cell endures a period of rapid growth) - peaks during mitosis (growth before split) 2) MPF is an enzyme that phosphorylates proteins (phosphorylation "switches" protein activity on and off). - MPF is not always active, because it can be turned off - MPF's activity is regulated by the cell cycle DISCOVERY OF CYCLINS: - researchers examined the proteins that were being produced during early division of sea urchin embryos. The found a group of proteins that was produced at the same time the cell cycle was being performed, called *cyclins*. - this type of protein also seemed to extend to other forms of like (surf clams). However, these two forms of cyclin both cycled with different timing, and were not associated with any form of enzymatic activity within the cell. CELL CYCLE RESEARCH + YEAST: - genetic studies were performed on both budding yeast and fission yeast in order to try and find the gene/genes that controlled the cell cycle - by studying mutants that had all stopped somewhere within the division process, they identified two genes that were necessary for furthering the cell cycle. 1) the START gene commits the cell to DNA synthesis (and DNA synthesis commits a cell to performing mitosis from start to finish) 2) cdc2 (fission yeast) seems to "trigger" both the beginning of mitosis and cell division

F1, F2, and F3 generations

F1: - crossed plants with both white and purple flowers - hybrid offspring did not have lilac flowers (purple + white) like the blended inheritance model would suggest (blending of both traits). Instead, offspring all resembled their purple-flowered parent. - because of this, Mendel concluded that purple was the dominant flower color, and white recessive. This dominant/recessive model turned out to be the case for all the traits he studied. - this first 'set' of offspring is referred to as the *first filial generation, or F1*. F2: - allowed F1 generation to self-fertilize - Mendel then collected these self-fertilized seeds and planted them to see what they would look like - these self-fertilized seeds are referred to as the *second filial, or F2, generation*. - Mendel found that, while most F2 plants had purple flowers, some had white flowers - Mendel then counted how many plants (in the F2 generation) had purple flowers, and how many had white. Out of 929 individuals, 705 (75.9%) had purple flowers, and 224 (24.1%) had white flowers. Therefore, this recessive trait seemed to occur in about 25%, or 1/4, of F2 individuals. - Mendel observed this same dominant-to-recessive ratio in the remaining 6 traits he examined. No matter what the 'trait pair' was, 3/4 of F2 individuals exhibited one trait (dominant), and 1/4 exhibited another (recessive). F2 plants always had a 3:1 dominant:recessive ratio. F3: - Mendel then went on to see how the F2 generation passed its own traits down the line - Mendel found that pea plants that exhibited recessive traits were always true-breeding plants (offspring has the same phenotype as their parent. For example, white flowers only produced other white-flowered offspring when self-fertilizing). - out of all the F2 individuals exhibiting dominant traits, 1/3 were true-breeding (always produced purple flowers w/ self-fertilization), and 2/3 were not. - these results led to a dominant:recessive phenotype ratio of 3:1 in the *third filial generation, or F3*. However, the 'true-breeding' ratio of the same generation was actually 1:2:1- 1 true-breeding recessive (rr), 2 non-true-breeding dominants (Rr), and 1 true-breeding dominant (RR)

phases of the eukaryotic cell cycle

GAP PHASE 1 (G1): - the cell begins to grow. Most of the cell's growth occurs during this time. - refers to the 'gap' between the last and first stages of cell division (the cell is "resting"- it's not ready to start dividing again yet) - the longest phase of the cell cycle S PHASE (SYNTHESIS): - the cell replicates its genome GAP PHASE 2 (G2): - another growth phase; this time, it's because the cell is preparing to split - the "link" between DNA synthesis and mitosis - microtubules form from the cytoplasm of the cell; they begin to organize in order to form spindles, which will drag identical chromosomes to opposite sides of the cell. - chromosomes coil as much as they can, preparing to be pulled apart quickly and efficiently MITOSIS: - spindles fully assemble + bind to chromosomes - sister chromatids are then moved apart CYTOKINESIS: - the cell divides - in animal cells, microtubules help to position a contracting ring of actin protein that splits the cell in two - in cells w/ cell walls (eg. plant cells), plates form to divide the two cells

Carl Correns

German geneticist; first to link chromosomes to heredity after rediscovering Mendel's lost work

mitosis v. meiosis

MITOSIS: - one division, one replication - starts with chromosome pairs (X X ---> X ---> /) MEIOSIS: - two divisions, no replication - deals w/ individual chromosomes (X ---> XX ---> X)

mitosis v meiosis cell types

MITOSIS: cell division occurring only in somatic cells (non-sex cell) MEIOSIS: cell division occurring only in sex cells (gametes)

meiosis II

PROPHASE II - chromosomes are clustered evenly at both poles of the cell - nuclear envelope breaks down; new spindle fibers form METAPHASE II - spindle fibers from opposite poles each bind to kinetochores of one sister chromatid - chromosomes are moved to the metaphase plate ("pulled" by tension created from microtubules) ANAPHASE II - spindle fibers contract; cohesin complex in centromeres is destroyed - sister chromatids are pulled apart to opposite poles of the cell TELOPHASE II - nuclear envelope reforms - cytokinesis occurs - results in four daughter cells with 1/2 genetic material - chromatids ---> chromosomes - haploid (singular)

synapsis and crossing over

SYNAPSIS: - occurs during interphase - ends of chromatids are all attached to nuclear envelope at specific sites. Sister chromatids have 'assigned' spots right next to one another in order to make pairing easier. (Ends up producing a homologous pair of chromosomes.) - (interphase 'prepares' for pairing; cell actually goes through pairing in prophase) - homologous pairing (chromosome + chromosome) guided by heterochromatin sequences - both homologues are joined together; sister chromatids of each homologue are joined together (via the cohesin complex in *sister chromatid cohesion*) - sister chromatid cohesion occurs in both mitosis and meiosis CROSSING OVER: - recombination occurs at the same time as the formation of the synaptonemal complex (prophase I) - structure aiding in recombination are called *recombination nodules* - recombination nodules work by using enzymes with the ability to break/rejoin chromatids - DNA segments are only exchanged between non-sister chromatids - doesn't "go crazy" with switching traits- while not limited by the size of the chromosome, most chromosome arms only undergo 1-4 crossovers (humans, for example, undergo 2-3) - synaptonemal complex breaks down once crossing over has been completed. Chromosomes are still attached by the chiasmata, but they're not as psychically attached/'blended' into each other as they were before - chromatids held together in two ways: 1) sister chromatids paired together by cohesin proteins (sister chromatid cohesion) 2) crossing over links non-sister chromatids/homologous pairs together

cancer + cell cycle control

cancer is caused by completely uninhibited cell growth (cells *always* growing and dividing) THE P53 GENE: - inhibitor within the G1 checkpoint - produces P53 proteins which 'make sure' that DNA replicated during the cell cycle is not damaged. if the P53 protein detects damaged DNA, it halts cell division and brings in enzymes that will repair the damage. Once the DNA has been repaired, P53 restarts the division. In cases where DNA *cannot* be prepared, P53 basically makes the cell kill itself. - preventing development of damaged cells prevents development of cells with DNA mutations. - when P53 can't do it's job (either absent or broken), damaged cancer cells are able to divide unchecked. The majority of cancerous cells that scientists have examined have flawed P53 proteins. PROTO-ONCOGENES: - oncogenes cause regular cells to become cancer cells - proto-oncogenes are normal cell genes that turn into ocogenes when mutated, causing cancer - proto-oncogenes signal growth factors all willy-nilly. Some proto-oncogenes create more receptors for growth factors, and others create proteins that are then used in the process of signal transduction. - proto-oncogenes mutate growth factor receptors so that they are always "on"- cell division is always being triggered, and the cell doesn't need any actual receptors in order to begin dividing. - PDGF receptors are a type of proto-oncogene; once even one receptor is mutated, uncontrolled division will begin TUMOR-SUPPRESSOR GENES: - a category of cancer-suppressing (specifically, tumor suppressing) genes. Includes P53. - each tumor-suppressor genes produces two copies that must *both* lose function in order for completely uninhibited cell division to begin - includes the retinoblastoma susceptibility gene (Rb). Rb makes individuals more susceptible to retinal cancer. - this gene is most often heterozygous (Aa Aa, where a = non-mutant and A = mutant). There is a 1/8 chance of inheriting two mutant copies of Rb (which will immediately result in a cancerous cell), and a 1/4 of inheriting only one. If you inherit only one, the mutant is suppressed by the "good" copy, but if the good copy is damaged, both copies can then start to produce cancerous cells, which can lead to the formation of retinal tumors. - within the cell cycle, the Rb protein's role is to combine all the different signals from various growth factors in order to initiate cell division. The Rb protein has binding pockets for regulatory proteins that prevents those proteins from floating off and helping to produce other proteins used w/in the cell cycle (cyclins, Cdks). - Rb binds to other proteins through dephosphorylation, and loses this ability when phosphorylated. Growth factors 'tell' Cdks to phosphorylate Rb proteins, which releases regulatory proteins. This leads to the production of cyclins, including S phase cyclins, which allow cells to pass the G1/S boundary and begin replicating their DNA

prokaryotic v. eukaryotic cell cycle

eukaryotes have bigger and more complexly organized genomes; as a result, as organisms evolved, cell division did, too

eukaryotes + binary fission

eukaryotes have much more complex genomes (multiple chromosomes rather than 1 nucleoid) than bacteria. This may be became organisms evolved the ability to delay chromosome separation until replication

the chromosomal theory of inheritance

first proposed by Walter Sutton in 1902

the role of meiosis is to create...

genetic diversity

the "purpose" of cell division is to...

produce new cells containing genetic information from the parent cell

sex chromosomes

structure/number of chromosomes varies between species - fruit flies, mammals, and humans have XX (female) and XY (male) chromosomes - birds have ZZ (male) AND ZW (female) chromosomes - insects (eg grasshoppers) have XX (female) and XO (no y chromosome, just one x/male) chromosomes