8 - Meiosis and Mitosis

Example: Brewer's and Baker's yeast

-yeast has an interesting life cycle: it can live as a haploid, or it can live as a diploid. As a haploid it has one copy of genome, reproduces through binary fission as usual by making a copy of genome to become diploid and then splitting into two haploid offspring. OR, two haploid yeasts can fuse together to make a diploid yeast, which can reproduce through binary fission by duplicating its genome to become tetraploid (four copies) and splitting into two diploid daughter cells, which will also live and reproduce as diploids. When times are hard, the diploids can make spores that are haploid, which go dormant until conditions are good again. when they emerge from spores, the population lives as haploid until eventually some fuse to become diploid again.

How was the cell cycle control system identified?

*Geneticists understand systems by breaking part of the system (causing mutations in certain areas) and seeing what changes. So to understand cell cycle, they took yeast cells and gave them mutations in different areas of genome. The mutants that couldn't complete cell cycle (got stuck at a certain point and could go no further) helped identify genes responsible for regulation of cell cycle. Lee Hartwell investigated yeast mutants that become stuck at some point in the cell cycle - he identified many genes involved in the cell cycle and hypothesized where in the cycle their protein products operated. Paul Nurse identified a gene in yeast (cdc2) which encodes a protein kinase (enzyme that puts phosphates on proteins to activate or deactivate) needed for the cell to progress from G2 to M (important transition; entering mitosis). All eukaryotic cells have counterparts of the cdc2 gene as control mechanism (check point in cell cycle).

External Controls

*the controls discussed previously have all been internal controls. The internal controls are modified by external controls (signal molecules originating outside the dividing cell) including peptide hormones and growth factors. Hormones and growth factors are synthesized in one area, is secreted by cells in that area, and travels through body to other cells to encourage them to grow. Hormones and growth factors act on the cell by the reception-transduction-response pattern. Reactions triggered by the activated receptor may speed, slow, or stop the progress of cell division. Cell-surface receptors in animals also recognize contact with other cells or with molecules of the extracellular matrix. Contact inhibition triggers internal reaction pathways that inhibit division by arresting the cell cycle, stabilizing cell growth in fully developed organs and tissues (when there is no room around them, cells will not divide) Cells in contact with one another are shunted into the G0 phase and prevented from dividing. If contacts are broken, the freed cells often enter rounds of division (if more room is created around them, they will go back to replicating). Cells can go to G0 phase by being halted at the G1/S checkpoint. Once they leave cell cycle and enter G0, it is like they are permanently in G1; they are just doing their job and living their life.

Consequences of Meiosis

1) 4 haploid gamete cells (1 round of replication followed by 2 rounds of division). Fusion of two haploid nuclei in sexual reproduction restores the diploid 2n chromosome number--this allows sexual reproduction with maintenance of chromosome number. 2) In metaphase I, each maternal and paternal chromosome can align on either side of the metaphase plate, so has an equal probability of being drawn towards either centriole. This leads to a random combination of paternal and maternal chromosomes. Each has one chromosome from each of the homologous pairs of chromosomes. Because of crossing over, these chromosomes are not exact duplicates of the original chromosomes. So huge amount of possible results.

There are three types of cyclin, one for each checkpoint

1) G1/S cyclin binds to Cdk2 near the end of G1 - required for transition from G1 to S, and to commit to DNA replication. 2) S cyclin binds to Cdk2 in the S phase - required for initiation of DNA replication and progression of the cell through S. 3) M cyclin binds to Cdk1 in G2 - required for the transition from G2 to M, and the progression of the cell through mitosis.

The 3 stages of Interphase

1)G1 2)S 3)G2 *How can scientists tell when DNA is being synthesized? aka discern the different stages of interphase? they can provide cells with deoxyribose nucleotides tagged with radio-isomers and wait to see when those radioactive nucleotides get polymerized during DNA synthesis. They saw that there were two gaps (G1 and G2) in DNA synthesis, and one period of DNA synthesis (S). They did the same experiments with radioactive ribose nucleotides and amino acids, and saw that RNA and protein synthesis happens throughout the entire interphase.

Why must cells divide?

1)to multiply and spread species (in unicellular organisms) 2)to become a multicellular organism-allows cell specialization. 3)to replace damaged or worn out cells (in a multicellular organism). 4)because once they grow past a certain size, they have to. they can get too big to survive (surface to volume ratio is too low). Cells must take in nutrients and get rid of waste products through surface, yet as cell grows volume increases faster than surface area. If we assume a spherical cell, SA = 4(pi)r^2 V = 4/3(pi)r^3 So if you double the radius, the area increases 4 fold but the volume increases 8-fold. This imposes an upper limit on the size of a cell.

Bacteria and Archaea reproduce by binary fission

Binary fission means "dividing in half". It occurs in bacterial and archaeal cells. It is asexual, but has completely different steps than mitosis. Two identical daughter cells arise from one cell. Steps in the process: 1) A single circular chromosome duplicates, and the copies begin to separate from each other 2)The cell elongates, and the chromosomal copies separate further 3)The plasma membrane grows inward at the midpoint to divide the cells *very simple

Cancer

Cancer occurs when cells lose normal controls over division - cancer cells divide continuously and uncontrollably, producing a rapidly growing mass called a tumor. Cancer cells typically lose adhesions to other cells and spread throughout the body (metastasis) producing new tumors in other body regions. Metastasis is promoted by changes that block contact inhibition and alter cell-surface molecules that link cells together or to the extracellular matrix. Cancer cells typically have a number of mutated genes that promote uncontrolled cell division or metastasis. Many of these genes code for components of the cyclin/Cdk system that regulates cell division. Others encode proteins that regulate gene expression, form cell surface receptors, or elements of receptor systems. The mutated form of the genes, called oncogenes, encode altered versions of these products.

Cells arise only from preexisting cells.

Cells arise only from preexisting cells. Cell division, the reproduction of cells, perpetuates life. Virchow's principle states "Every cell from a cell".

Prophase

Chromatin condense into chromosomes. Nucleolus weakens and disappears. The mitotic spindle begins to form between the two centrosomes as they migrate toward the opposite ends of the cell, where they will form spindle poles made of microtubules extending from the two centrosomes to the center dividing line of the cell.

Cytokinesis

Cytokinesis produces two daughter cells, each with one of the two daughter nuclei. In animals, protists, and many fungi, a furrow girdles the cell and deepens until it cuts the cytoplasm into two parts. In plants, a cell plate forms between the daughter nuclei and grows laterally until it divides the cytoplasm in two. The plane of cytoplasmic division is determined by the layer of microtubules that persists at the former spindle midpoint.

The cell cycle has positive and negative controls

Proto-oncogenes are positive control: they encourage the cell cycle to go forward. Tumor repressor genes are negative control: they inhibit the cell cycle. Both must be in accordance for cell cycle to continue. If one or both malfunction badly, you have malignant cancer. If the proto-oncogene mutates to always be activated, always driving cell cycle forward, there is uncontrolled growth. If the tumor repressor gene mutate to always be deactivated, nothing is holding cell cycle back and there is uncontrolled growth.

Cyclins and Cyclin-Dependent Kinases

Direct regulation of the cell cycle itself involves an internal control system consisting of proteins called cyclins and enzymes called cyclin-dependent kinases (Cdks). A Cdk is a protein kinase, which phosphorylates and regulates the activity of target proteins - Cdk enzymes are active only when bound to cyclin. Concentrations of different cyclins change as the cell cycle progresses. If there is not enough of the right cyclin, the Cdk cannot bind to cyclin to activate, and so cannot phosphorylate the proteins that it needs to to pass the checkpoint. Phosphorylation regulates proteins that initiate or regulate key events in the cell cycle (DNA replication, mitosis, and cytokinesis).

Meiosis I

During meiosis I, homologous chromosomes pair and nonsister chromatids undergo a physical exchange of chromosome segments (crossing-over). In meiosis I, the chromosomes align differently at the metaphase plate than in mitosis. In mitosis 46 microtubules from each pole bind to 46 kinetochores, two on each centrosome of each pair of sister chromatids, so that after segregation each pole ends up with 46 unique chromosomes, one from mom and one from dad for each pair of homologous chromosomes. In meiosis, 23 microtubules from each pole bind to 23 centrosomes of the sister chromatids. During metaphase the tetrads assemble at the metaphase plate. After segregation (after meiosis I has been completed), each pole has 46 chromosomes, but those chromosomes are 2 repeats of 23 identical chromosomes (still in sister chromatid form). The only parts of them that are not identical are the parts that crossed over. In a pair of sister chromatids from mom, little sections of one sister will likely be from dad b/c of crossing over, and vice versa at the other pole. You now have two diploid cells. But unlike somatic cells, the two versions of the genome are not slightly different (one from mom one from dad). Instead they are repeat copies of the same genome, with the only differences being the sections that recombined during crossing over. This means that the genetic combinations possible in gametes are limited.

G1 phase:

G1 stands for gap 1 phase. It is the period of regular healthy growth as a newish diploid cell, before DNA is duplicated. Cell is growing and making RNA, proteins, etc. DNA is in loose strands of chromatin inside the nucleolus inside the nucleus.

G2 phase:

G2 stands for gap 2 phase. DNA synthesis is finished and cell is now tetraploid. Cell continues to grow and function as normal. DNA is still in loose strands of chromatin. End of interphase. once this stage ends, cell enters mitosis. It goes first into prophase, and the chromatin starts condensing into chromosomes

Not all centromeres are at the center of the chromosome

If they are in the middle it is called metacentric. If they are close to one end it if called acrocentric. If they are at the end it is called telocentric.

Meiosis II

In meiosis II, the sister chromatids in the two diploid cells separate - daughter chromosomes segregate into four different cells, each with the haploid number of chromosomes. The same thing happens during metaphase as what happens in mitosis, except instead of splitting 46 sister chromatids apart you are splitting 23 sister chromatids apart to make two haploid daughter cells. So of the four gametes produced, two will contain a single copy of grandma's genome and two will contain a single copy of grandpa's genome. The only recombination between maternal and paternal will be what switched during crossing over.

Prophase I of Meiosis I: Crossing over aka Recombination

In prophase I of Meiosis I, strands of chromatin condense into chromosomes just like in prophase of mitosis. Homologous chromosomes (now in the form of two homologous pairs of sister chromatids) are put into alignment with each other by synapsis (tight association between homologs along their entire length). The synaptenemal complex is a zipper like structure that holds them together. Each grouping now has four chromatids (two pairs of identical sister chromatids; the pairs homologs to each other) and is called a tetrad. This alignment of chromosomes in meiosis I puts homologous sister chromatids next to each other (two copies of a chromosome from mom next to two copies of the same chromosome but from dad, aka non-sister chromatids are next to each other). Now, non-sister chromatids can swap a little bit of DNA and make new combinations. This crossing over or recombination happens during prophase I of meiosis I. It happens in every tetrad. Chiasmata are x-shaped structures that appear between non sister chromatids in a tetrad where crossing over has happened: visible evidence of crossing over.

Aneuploids

Individuals with extra or missing chromosomes are called aneuploids - individuals with a normal set of chromosomes are called uploads. This is a huge problem. 1 copy of a chromosome = monosomy. 3 copies of a chromosome = trisomy. Aneuploid embryos are often completely inviable and are naturally aborted early in pregnancy. 15-20% of human conceptions result in spontaneous abortions because of chromosome problems that make it so the embryo could not have survived. However, there are some exceptions: Down's syndrome is caused by an extra copy of chromosome 21. 1/1,000 born in US has down syndrome. Also there are people who have an extra sex chromosome: xxx or xxy.

Meiosis

Life cycle begins with starts with fusion of gametes. If you were to fuse two diploid gametes you'd have a tetraploid daughter cell. So need a mechanism to reduce from diploid to haploid, to make haploid gametes. This is meiosis. Meiosis separates homologous pairs, reducing the diploid (2n) number of chromosomes to the haploid (n) number. Somatic cells have pairs of homologous chromosomes, receiving one member of each pair from each parent. Each gamete produced by meiosis receives one member of each homologous pair. Humans have 46 chromosomes in their diploid cells, which make up 23 homologous pairs (2n). A human egg or sperm cell contains 23 chromosomes, one of each pair (n).

Living organisms reproduce by two methods

Living organisms reproduce by two methods: 1) Asexual reproduction: Offspring are identical to the original cell or organism. Offspring inherits all genes from one parent. (binary fission in single celled organisms, mitosis in multicellular organisms, produces two identical daughter cells) 2) Sexual reproduction: Offspring are similar to parents, but show variations in traits. Offspring inherits a unique sets of genes from two parents. (meiosis, produced gametes)

Prometaphase

Looks a lot like prophase, but begins when the nuclear envelope breaks down and nucleus disappears. Spindle microtubules grow from centrosomes at opposite spindle poles toward the center of the cell. A kinetochore forms on each sister chromatid at the centromere (the point where chromatids are joined). Kinetochore microtubules bind to the kinetochores. Nonkinetochore microtubules overlap those from the opposite spindle pole. Each pair of sister chromatids has one centromere and two kinetochore. one microtubule from each side binds to one kinetochore on each centromere, so the two sister chromatids in a pair are tied together at the centromere, but they are also tied to different poles of the cell by different microtubules.

The Meitotic Cell Cycle has two large parts.

Meiosis one and Meiosis two each have very similar internal steps to mitosis. The chromosome is replicated into tetraploid state during S interphase, just like with mitosis. But unlike mitosis which has one cell division, meiosis has two cell divisions so cell goes from one tetraploid (4n) to two diploid (2n) to four haploid (n) gametes.

The five stages of mitosis

Mitosis Proceeds in Five Stages. Following interphase, mitosis can be divided into five sequential stages: 1)prophase 2)prometaphase 3)metaphase 4)anaphase 5)telophase (Cytoplasmic division (cytokinesis) coincides with telophase, and is the actual division between the two new cells. So after telophase there are two cells.)

Review of comparison of results Mitosis and Meiosis.

Mitosis—two genetically identical daughters that are identical to their parent. Meiosis—four genetically distinct gametes that are haploid and whose chromosomes have been altered by recombination from the parental chromosomes.

The Eukaryotic Cell cycle

Overall split into two: interphase (just living and getting ready for division and duplicating genome; much longer than mitosis; genome is not very compact) and mitosis (dividing). Interphase is split into 3 stages: G1, S, and G2. Mitosis is split into 5 stages.

G2/M checkpoint

Passing the G2/M checkpoint commits a cell to mitosis. Cells arrest at the G2/M checkpoint if DNA was not replicated accurately in S, or if the DNA has been damaged by radiation or chemicals. Accurate DNA replication is essential for producing genetically identical daughter cells. To pass through, proto-oncogenes are activated and tumor repressor genes are deactivated. During S phase, the cell produces S cyclin. If enough S cyclin is present to bind to Cdk2, then enough Cdk2 will be activated to get the cell through the G2/M checkpoint. The S cyclin will degrade right after this checkpoint.

Cell Theory: Cells arise only from preexisting cells

Roles of cell division: 1)Asexual reproduction: Reproduction of an entire single-celled organism, Growth of a multicellular organism, Growth from a fertilized egg into an adult, Repair and replacement of cells in an adult 2) Sexual reproduction: Sperm and egg production Mitosis (an asexual growth procession multicellular organisms) divides the replicated DNA equally and precisely, generating daughter cells which are exact genetic copies of the parent cell. Meiosis (a process of sexual reproduction in multicellular organisms) produces daughter nuclei with half the number of chromosomes of the parental nucleus - the arrangements of genes on chromosomes are different from those in the parent cell

S phase: synthesis

S stands for synthesis phase: all DNA AND nuclear proteins are duplicated, and cell goes from diploid to tetraploid. regular cellular functions and synthesis of other cellular proteins go on as usual. DNA is still in loose strands of chromatin.

Metaphase

Spindle microtubules move chromosomes into alignment at the spindle midpoint (metaphase plate). *All the chromosomes line up along center of cell, with sister chromatid tied to one side and the other sister chromatid tied to the other side and the centromere in the middle. Condensation gives each chromosome a characteristic shape, determined by length and centromere location. This is how it becomes clear which chromosomes are homologous pairs: they are parallels of each other in size, shape, and banding pattern. An image of a complete set of metaphase chromosomes, arranged according to size and shape, forms a karyotype. A human cell at this point (metaphase) would be 4n (tetraploid), with each of the 46 chromosomes having been duplicated into a pair of sister chromatids. ***mammals' Red Blood Cells (oxygen transport cells) do not have nuclei! they don't have DNA other than in mitochondria. White Blood Cells (immune system cells) do still have their nuclei.

G1/S checkpoint

The cell cycle arrests at the G1/S checkpoint if DNA is damaged by radiation or chemicals - if the DNA damage is repaired, the cycle starts again. Cell cycle arrest also occurs at this checkpoint if the cell is nutritionally deficient, or growth factors are absent. This is the cell deciding whether or not to replicate genome (go from being diploid to tetraploid). To pass through, the proto-oncgenes are activated and the tumor repressor genes are deactivated. During G1, the cell makes G1/S cyclin. The amount of G1/S cyclin present reflects how successful the cell was during G1. If it was not successful enough to perform S phase, there will not be enough G1/S cyclin to bind to enough Cdk2 to get the cell through the G1/S checkpoint. If it does pass, the G1/S cyclin degrades right after the checkpoint.

The three checkpoints

The cell cycle has three key checkpoints to prevent critical phases from beginning until the previous phases are completed correctly. 1)The G1/S checkpoint (just before entering S phase) 2)The G2/M checkpoint (just before entering mitosis) 3)The mitotic spindle checkpoint (during mitosis)

Cell Cycle Control System

The cell cycle is tightly controlled/regulated by the cell cycle control system. The cell cycle control system: A set of molecules, including growth factors, that triggers and coordinates events of the cell cycle. It is set up as three checkpoints. Checkpoints are signals to stop - certain things must be present in order to deactivate the check point. Deactivation of a checkpoint allows the cell cycle to proceed to the next stage of the cycle.

Centrosome

The centrosome is an organelle that contains two centrioles and a bunch of microtubules. Animal cells need these to perform mitosis. Other types of life forms don't need these. It is simply a way to organize microtubules, which are essential for cell division to work properly (they cause the chromosomes to segregate evenly). During S phase, the centrosome duplicates and one goes to each end of the mother cell and sends out its own microtubules to help the cell divide.

How much diversity of combination is there in sexual reproduction?

The fact that maternal and paternal chromosomes don't have to segregate together allows for large numbers of combinations: 2^n, where n = # chromosome pairs. Drosophila has 3 chromosomes, so 2^3 = 8 combinations. Humans have 23 chromosomes, so 2^23 = more than 8 million combinations. This diversity of combination is enhanced by recombination, which mixes paternal and maternal combinations. So what is the probability that two siblings will be genetically identical? Mom can make 8.4 distinct eggs, dad can make 8.4 million sperm, so 1 / (8.4 x 8.4) = 1/70.7 trillion. and this is ignoring crossing over, which adds more random diversity to every gamete!

Mitosis in the Eukaryotic Cell

The large, complex chromosomes of eukaryotes duplicate with each cell division (each cycle of mitosis). Eukaryotic chromosomes are composed of chromatin Chromatin = DNA + proteins (histones and others). The DNA is wrapped and folded around structural proteins in a very complex way, in order to make it compact enough to fit neatly in nucleus, and to activate or deactivate large areas of the genome. This structure is chromatin, which forms the chromosomes. *structure of DNA has nucleotides on inside of helix and phosphate backbone on outside. the phosphate backbone is negatively charged (acidic, excess of H+). SO, histones and other proteins meant to interact with DNA are positively charged (basic, excess of OH-). To prepare for division, the chromatin becomes highly compact (~100,000-fold—1 m of DNA in a 10 µm nucleus), and the chromosomes are visible with a microscope. Early in the division process, chromosomes duplicate (now there are 46). Each chromosome now appears as two sister chromatids, containing identical DNA molecules. Sister chromatids are joined at the centromere, a narrow region, by cohesion. One sister chromatid goes to each daughter cell, so they all get 46 chromosomes *Humans have 23 pairs of homologous chromosomes, so 46 total.

Mitotic Spindle Checkpoint.

The mitotic spindle checkpoint during mitosis, just before metaphase. This checkpoint assesses whether chromosomes are attached properly to the mitotic spindle so that they align correctly at the metaphase plate. Once the cell begins anaphase, it is irreversibly committed to completing mitosis, so if the spindle isn't set up or attached to the kinetochores properly, the two daughter cells might have uneven numbers of or the wrong chromosomes. During G2 phase, the cell makes M cyclin. If enough M cyclin is present, it will activate enough Cdk1 in order to get the cell through the mitotic spindle checkpoint. The M cyclin will then degrade and the cell will begin mitosis.

Ploidy

The number of copies of your genome (the number of sets of chromosomes) = your ploidy. If 1 copy of your genome = haploid = n. If 2 copies of your genome = diploid = 2n. If 4 copies of your genome = tetraploid = 4n. Generic term for all conditions with >2 copies of your genome = polyploid. -humans are diploid: one complete copy of our genome from our mom and one from our dad. -most animals do not tolerate polypoidy very well (diploid is usually the max), but plants do it quite well, and can be triploid or tetraploid, etc.

Telophase

The spindle disassembles and chromosomes at each spindle pole decondense, returning to the extended state typical of interphase (looser strands of chromatin as opposed to dense, highly structured chromosomes). The nucleolus reappears, RNA transcription resumes. A new nuclear envelope forms around the DNA at each pole producing the two daughter nuclei. At this point, nuclear division is complete - the cell has two nuclei.

Anaphase

The spindle separates sister chromatids and pulls them toward opposite spindle poles. Movement continues until the separated chromatids (daughter chromosomes) have reached the two poles. At this point, chromosome segregation is complete. -The centromere is pulled apart; sister chromatids are pulled apart from each other, each to different poles. So at end of anaphase, there are 43 chromosomes at each pole.

Why do cells need check points?

This is like the cell reviewing its work: before it goes on to the next stage it needs to pass a checkpoint to make sure everything has been done properly and is ready to embark on the next step. You don't want to: a)replicate and spread mutations b)exhaust yourself if you don't have enough food to replicate or if the conditions are bad 4)replicate if the spindle is not properly set up and the chromosomes won't distribute evenly between the two daughter cells. The check points are really important because cells that replicate heedlessly without review and consideration, spreading mutations and/or replicating in situations that they shouldn't = cancer! Proto-oncogenes are positive regulatory proteins that encourage cell division. When these don't function properly, they are called oncogenes. Oncogenes are the cause of cancer; they cause limitless cell division.

Review of Mitosis

When born, a human cell has 23 pairs of homologous chromosomes (homologs), one from mom and one from dad, making it diploid (two copies of genome). Each chromosome is one strand of DNA, folded very compactly as chromatin (DNA and proteins). During G1 interphase, the cell grows. During S interphase, the cell duplicates its chromatin. Now the cell is tetraploid (2 copies of each of 46 chromosomes, so 4 copies of genome total. pairs of sister chromatids (identical chromosomes) are attached at centromere). During G2 interphase, the tetraploid cell waits and grows. Then it leaves interphase and enters mitosis. During Prophase, the DNA which was in loose chromatin strands condenses into chromosomes and the centrosomes go to opposite ends of the cell and start making the mitotic spindle. During prometaphase, the nuclear envelope breaks down releasing chromosomes into cytoplasm, the microtubules grow out from the spindles and attach to the chromosomes at the kinetochores (two on each centromere, to pull the sister chromatids to opposite poles of the cell). During metaphase, the microtubules align the sister chromatids at the metaphase plate (center line of cell), so that one can be pulled to each pole. During anaphase, the spindles pull apart the sites chromatids by their kinetochores, breaking the centromere and pulling one identical set of 46 chromosomes to each pole. During telophase the spindle disasembles, two nucleuses form around the two sets of chromosomes, which dissolve back into loose strands of chromatin, and cytokinesis happens (the cell membrane grows between the two cells, separating them and making two genetically identical diploid daughter cells).

A possible mistake that can happen in cell division is change in chromosome number, aka nondisjunction.

Whole, single chromosomes may go to the wrong daughter cell through nondisjunction - the failure of homologous pairs to separate during the first meiotic division or of chromatids to separate during the second meiotic division. This leads to two out of four gametes having fewer than 46 chromosomes and the other two having more than 46 chromosomes. If these are combined with another haploid during sexual reproduction, the resulting embryo