IB Biology Unit 10: Chromosomes and Meiosis

Outline conclusions drawn from the images produced using Cairn's autoradiography technique.

-> Cairns was able to produce images of DNA and found out that prokaryotic DNA is circular. -> They were able to produce (later) images of eukaryotic chromosomes (size and measurement). -> Observed DNA replication at the replication fork.

List three events that occur in prophase I of meiosis.

1) DNA super coils from chromatin to chromosome. 2) Nuclear membrane begins to disappear. 3) Crossing over 4) Pairing of homologous chromosomes.

Describe the process of karyogram.

1) cells are frozen in metaphase with the use of chemicals to stop spindle fibers. (Cells are stopped in Metaphase b/c at this point the chromosomes are showing). 2) Hypotonic solution added, therefore cell would swell up and burst. 3) the chromosomes are now spreaded out from each other and are strained to view under microscope. 4) photograph taken and cut out to be re-organized.

State that the number of chromosomes combination possible due to random orientation is 2^n.

2^n is the rule in which will tell us the possible variation of gametes in a species due to independent assortment. The n is for haploid, therefore whatever haploid of the organism is, make 2 to the power of the haploid you get the possible combinations.

State the human cell diploid number.

2n=46 chromosomes.

Define diploid

A cell or organism containing 2 sets of chromosome.

State the result of non-disjunction.

Abnormal number of chromosome in a cell.

Describe two procedure for obtaining fetal cells for production of a karyotype.

Amniocentesis: needle is inserted into uterus though the stomach. Fluid is drawn containing fetus cells. Karyotype is done to make a karyogram. Chronic villus sampling: a thin tube is inserte into the vagina and into the uterus. small sample of DNA of the placenta is removed that contains baby cells, and karyotype.

Outline the advancement of knowledge gained from the development of autoradiography techniques.

Autoradiography is used to produce an image of a radioactive substance. • It is used in cellular/molecular biology to visualize structures. • visualize chromosomes, bands of DNA in gel electrophoresis and tissue examples.

Explain why meiosis must occur as part of a sexual life cycle.

Because ina sexual lifecycle, genetic information of 2 parents are combined. Therefore to produce the correct number of chromosomes in offsprings, the parent needs to undergo meiosis so that gametes only contain half of the genetic information.

Explain why meiosis I is a reductive division.

Because the number of chromosomes went from 2n (diploid) to n (haploid). Thus the number of chromosomes are reduced.

Define bivalent, chiasmata and synapsis.

Bivalent: a pair of homologous chromosome. Chiasmata: the point in which genes the homologous chromosomes are attached for crossing over. Synapsis: The process in which the chromosomes are aligning in their homologous pairs.

Compare meiosis II with mitosis.

Both meiosis II and mitosis separate replicated sister chromatids to different poles. * Just meiosis is haploid and mitosis is diploid.

Outline the process of crossing over.

During *prophase* 1, the chromosomes are paired together and during that pairing, parts of the chromosomes exchange alleles. This occurs at where the chiasmata is. This results new combination of genes and a variation in combination of genes.

State that DNA is replicated in interphase 1 before meiosis.

During interphase, at S stage, the DNA is in is replicated and the 2 identical sister chromatids are held at the centromere.

Define non-disjuction.

Failure of a pair of homologous or sister chromatids to separate normally in meiosis or mitosis.

List example of haploid cells.

Female egg and male sperm.

Outline the role of fertilization as a source of genetic variation.

Fertilization combines 2 sets of chromosomes, 1 from mother and 1 from father. Therefore you can have up to 16 million+ possible combination from both parents. The zygote will always have a very unique genetic identity.

Outline the formation of a diploid cell from two haploid gametes.

Haploid has 1 set of full chromosome. -> Gametes are haploid (egg/sperm) and each has 1 set of chromosome. (When the sperm fertilizes the egg, the 2 sets of chromosomes combine and makes a diploid zygote.)

Define haploid.

Has 1 full set of chromosomes of either mom's or dad's.

Describe the discovery of meiosis.

Hertwig (1876); observed how 1 cell divided into 4 (sea urchins) Van beneden (1883); discovered how chromosomes move during meiosis. Weismann (1890); Theorized how meiotic division could lead to daughter cells with only HALF the genetic information.

Define homologous chromosomes

Homologous chromosomes are pairs of chromosome from a paternal and a maternal origin. They have the same sequences of genes and length, but not necessary the same alleles.

Explain the relationship between the number of human and chimpanzee chromosomes.

Humans have 46 chromosomes and chimpanzees have 48. Research have shown that it was possible that the cimpanzees' chromosome merged (chromosome #2) into 1 human chromosome.

Deduce the sex of an individual given a karyogram.

If the 23rd pair shown in the karyogram is XX, then the baby is a girl, but if it's XY then it's a boy.

Describe the use of a karyogram to diagnose Down Syndrome.

If the karyogram shows a trisomy #21, meaning there's 3 chromosomes in chromosome set 21, then that's diagnosable as Down Syndrome.

Discuss difficulties in microscopic examination of dividing (meiosis) cells.

In the earlier stages of microscopy, microscopes can only be used if the cells were preserved; and that kills them, so we can't see meiosis actions. And many times the microscope doesn't show a clear image or doesn't show any of cells during meiosis. Also, not until DNA staining were we able to see and observe DNA in mitosis and meiosis.

Distinguish between independent assortment of genes and segregation of allele.

Independent assortment is the random arrangement of chromosomes and whatever happens to genes A do not affect genes B Segregation of alleles is the separation of alleles of the same genes to different gametes.

Compare sexual and asexual life cycles.

Life cycle; period of time that organism passes through until producing offspring. Sexual; two parents--> genetic variation. Asexual; one parent --> binary fission, mitosis, budding, less energy and genetically indenticle.

List the characteristics by which chromosomes are arranged on the karyogram.

Matched by size, banding patterns, position of centromere. Orders from largest/longest to smallest/shortest.

Contrast meiosis I with meiosis II.

Meiosis 1: reduction of chromosome number, chromosomes are still replicated; homologous pairs are separated. Meiosis 2: no further reduction in chromosome number, sister chromatids are separated, chromosomes are not in homologous pairs.

Compare divisions of meiosis I and meiosis II.

Meiosis 1: reductive division (dip->hap). chromosomes are still replicated. crossing over. DNA replication. 2 haploid cells results. Meiosis 2: non-reductive division (hap->hap). chromatids of chromosomes separate. non-crossing. proceeded by interkinesis --> no s-phase.

Explain how meiosis leads to genetic variation in gametes.

Meiosis leads to genetic variation due to the process of crossing over and random orientation/independent assortment.

State the outcome of allele segregation during meiosis.

Mendel's law of segregation describes the consequences of the behavior of chromosomes during meiosis. * Pairs of alleles will end up in different gamete cells * Because the homologous chromosomes will move to different cells, separating the alleles.

Explain why the typical number of chromosomes in a species is always an even number.

Most eukaryotic species has an even number in the offspring because of sexual reproduction. • Each parent gives one set of chromosome, resulting in an even number in baby.

State the chromosomes number and type is a distinguishing characteristic of a species.

Organism with a different number of chromosome are *unlikely* to be interbred. • Therefore all interbreeding members of a species needs to have the SAME # of chromosomes.

Outline the events of prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.

Prophase 1: DNA begins to supercoil; nuclear membrane begins to disappear; homologous chromosome begins to pair up; crossing over between non-sister chromatids homologous chromosomes. Metaphase 1: Homologous pairs lines up in the center or "metaphase plate" of the cell which were moved by microtubules. Anaphase 1: Homologous chromosomes are separated by spindle fibers from the centrosomes to opposite poles of the cell. Telophase 1: As the cell elongates, a cleavage furrow forms and eventually connect, creating 2 haploid cells. Cytokinesis happens during the process forming 2 cells. DNA recondenses. Prophase 2: DNA recondenses and remains replicated; nuclear membrane breaks down and spindle fibers reform. Metaphase 2: DNA beings to line up in the metaphase plate, moved by microtubules. Due to cell being haploid, it doesn't need to pair. Spindle fibers attaches. Anaphase 2: Sister chromatids are separated by spindle fibers that attach at the centrosome. Each sister chromatid is pulled to different poles. Telophase 2: nuclear membrane reforms around the chromosomes and it decondenses. Cytokinesis occurs simultaneously. 4 haploid cells as the result.

Describe the random orientation and independent assortment.

Random orientation and independent assortment is the same. This is the idea in which the homologous pairs align at the metaphase plate RANDOMLY. It doesn't matter if the paternal or maternal go on either side as long as they're paired on the metaphase plate. *Orientation of one homologous pair does not affect any other homologous pair*

Describe the mechanism by which the SRY gene regulates embryonic gonad development.

SRY gene is present only in the Y chromosome. SRY is a transcription factor which triggers the development of seminiferous tubules, this is in the gonads. Presence of the SRY genes means that the gonad will turn into testies; which then secretes testerone. Absence of the SRY gene means the gonad becomes the ovaries.

State a similarity and a difference between pairs of homologous chromosomes.

Similarity: same gene loci, majority of DNA sequence is same, same length (bp), same centromere position, same banding patterns. Differences: 2 different origins, different alleles of genes.

Describe the attachment of spindle microtubules to chromosomes during meiosis I

Spindle microtubules; are long protein fibers that attach to the chromosomes and pulls them to different poles of the cells. Meiosis 1: homologous chromosome pairs are pulled into separate poles of the cell. The chromosome alignment is random so it doesn't matter which chromosome pair goes where. It attaches at the chromosome's kinetochore

identify tetrad, bivalent, sister chromatids, and non-sister chromatids.

Tetrad: the lined up pairs of homologous chromosomes. Bivalent: A pair of homologous chromosome. Sister chromatids: are replicated chromatids that have the exact identical sequences. Non-sister chromatids: are the chromatids from different homologous pairs.

Outline the structure of the two human sex chromosomes.

The X chromosome is larger than the Y chromosome. It consists of about 156mbp. and 1,805 genes. The Y chromosome is a lot smaller, consisting of only about 57mbp and 460 genes.

Describe the cause and symptoms of down syndrome.

The cause of down syndrome is due to the extra # of chromosome in chromosome 21, which is also known as "trisomy 21"

State the minimum chromosome in eukaryotes.

The minimum # of chromosomes in a eukaryote is 2n=2. The "Jack jumper ant".

List mechanism by which a species chromosome number can change.

The number of chromosome can change during evolution. Chromosomes *numbers can decreased through fusing* or can *increase by splitting*.

Explain the relationship between parental age and changes of non-disjunction.

The older the mother, the more likely for non-disjunction to happen. In women, eggs are paused in meiosis until it's released each month after puberty. Therefore the older the woman, the more likely it is for them to have a non-disjunction

Outline gender determination by sex chromosomes.

XX = girl. XY = boy. Due to only the male having both chromosomes, they have the chance of passing any of it which determines the offspring's gender.

State an advantage of being diploid.

You'd have a copy of each of your chromosome, therefore 2 copies of your genes. In any case that the chromosome is mutates or broken, the copy of the allele can prevent a negative effect.

Describe Carin's technique for producing images of DNA molecules from E. Coli.

a) cells were grown in a culture with the presence of radioactive thymine. cells would replicate with thymine. b) cells were placed onto a dialysis membrane and cell would be slowly digested leaving only DNA. c) thin film of photographic envision was placed on top of the membrane and left in darkness for 2 months. d) after 2 months, the film is placed under a microscope, where the dark spots are (from radioactivity decay) was where the DNA was. e) image of DNA produced.

Distinguish between a karyogram and a karyotype.

karyograms: photographic results of karyotypes. karyotype are the characteristics of the chromosomes; size, shape, banding patterns.

State the human cell haploid number.

n=23

State the number of nuclear chromosome types in a human type.

• 23 chromosome in a human cell. This consists of 22 autosomes and 2 types of sex chromosomes. Males will have a Y chromosome and female would have an X chromosome.

List three ways in which the types of chromosomes within a single cell are different.

• Chromosomes can differ in length. • The type of gene in different chromosome can differ. • Location of centromere can differ. • Banding patterns can be different for the types. • Base sequence will be different.