Parts 2. Modeling chromosome segregation

Transcription

1 Genome 371, Autumn 2017 Quiz Section 2 Meiosis Goals: To increase your familiarity with the molecular control of meiosis, outcomes of meiosis, and the important role of crossing over in generating genetic diversity. Cell division in meiosis is the moment of chromosome segregation and provides the underpinnings of all of genetic analysis. Outline of exercise Part 1. Pre-worksheet: familiarize yourself with the process of meiosis. Part 2. Model meiosis Part 3. The chromosomal basis of independent assortment. Part 4. How ploidy constrains the success of meiosis. Part 5. Identifying errors in illustrations of meiosis. Part 6. Homework. Parts 2. Modeling chromosome segregation (a) (b) Using the chromosome props you have been given, depict what it is that allows chromosomes to achieve proper attachment to the spindle and arrive at the metaphase plate in Meiosis I. Use the space below to record your particular Metaphase I in (a). spindle pole Examine the Metaphase I at your bench that achieved proper tension what is it that allows tension to be maintained across the homologue pair? Diagram the key elements as best as you can. spindle pole (c) Complete Meiosis I and and agree on the ploidy and replication state of each product of Meiosis I. (d) What would be the ploidy and replication state of each cell produced by Meiosis II? Genome 371 Autumn 2017 QS2 1

2 Part 3. The chromosomal basis of independent assortment. How would you modify your Metaphase I diagram from the previous page to depict the basis of independent assortment? Is there more than one arrangement that would give the same outcome? Mark loci and alleles as you see fit. Part 4. How ploidy constrains the success of meiosis. Examine the cell models at the end tables What is the ploidy of each? From what you know, would either of them be able to achieve a proper Metaphase I arrangement? What problem(s) do you foresee? Genome 371 Autumn 2017 QS2 2

3 Part 5. Identifying errors in illustrations of meiosis. Each card you are given has a depiction of diploid cell at Metaphase of Meiosis I. Are there any depictions that are completely correct? What errors in the depictions can you identify? Conclusions From our exercise and discussion, you should be able to answer the following questions. What is the goal of Meiosis I? What is the goal of Meiosis II? How is tension across a homologue pair achieved? What allows a cell to proceed to Metaphase I? For gamete genotype frequencies, does it matter whether the genes are on the same chromosome or on separate chromosomes? How does the distance between genes influence segregation patterns? What meiosis problems might occur in polyploid cells? Genome 371 Autumn 2017 QS2 3

4 QS2, Part 6. Homework. 1. Errors in depicting chromosome segregation. For us to be able to understand what you know about meiosis, we need to be able to interpret your drawings of meiosis. Each of the following depictions of meiosis has one or several ambiguities that would lose credit if drawn on an exam. For each depiction, study the figure and decide how it could be improved. AMBIGUOUS depictions I II III IV Genome 371 Autumn 2017 QS2 4

5 2. All of the following diagrams are supposed to represent Metaphase I of meiosis in an RrYy diploid. Identify the error(s), if any, in each of them. Genome 371 Autumn 2017 QS2 5

6 3. The arrangement of four loci (A/a, H/h, D/d, and R/r) in a diploid cell is shown: (a) Considering just D/d and R/r, what would it take to get four different gamete genotypes from one cell undergoing meiosis? (b) Now consider D/d and H/h. What would it take for one cell undergoing meiosis to yield DH and dh as the only gamete genotypes? (c) In class we discussed how nondisjunction could lead to abnormal combinations of alleles in the gametes. Suppose one meiosis produced a gamete that had the alleles DRR. Another meiosis produced the alleles DRr. For each of these meioses, is it possible to tell when in meiosis the nondisjunction event occurred? Was it in meiosis I? Meiosis II? Or is it not possible to make a definite conclusion from the information given? Explain using diagrams as necessary to illustrate your answer. Genome 371 Autumn 2017 QS2 6

7 4. The chromosome cartoon below depicts the arrangment of four genes in a male mouse: (a) Using the diagram above as a guide, depict the chromosome arrangement at Metaphase I of meiosis and the resulting Metaphase II of meiosis in this male mouse such that at least one gamete from this meiosis is certain to have the genotype D A B C. Assume that there is no aberrant event or any new mutation. (b) Male mice with the genotype shown above were mated to females of the same genotype. What fraction of the progeny do you expect to be homozygous for D/d, B/b and C/c (i.e., homozygous for all three genes). (On a question like this, you will almost always be asked to show the steps by which you arrived at your answer.) Genome 371 Autumn 2017 QS2 7

8 5. In the minitest, you were asked to determine the ploidy of the cell shown on the right in the metaphase that led to this anaphase. But that wasn t the question I really wanted to ask. Here is what I really wanted to ask: What is the ploidy of this cell as diagrammed? 6. Returning to the A, D, H and R loci marked on the chromosome maps in Question 3 above H is the Huntingtin gene. Certain mutations in this gene cause Huntington's Disease. The dominant H allele is the disease-causing allele; the recessive h allele is healthy. One copy of H is enough to cause the disease. A, D and R are molecular markers: locations at which the genetic material varies from one person to another in length or content. They are used as landmarks on the chromosome. They may or may not be located in genes, and may or may not have any detectable effect on the person carrying them. Like mileposts on a highway, they are important more for where they are than for what they are. (While we have used upper and lower case to distinguish alleles of these loci, for molecular markers it doesn t make sense to talk about dominant or recessive alleles, as these alleles don t necessarily code for anything.) If you could not directly observe inheritance of the Huntingtin gene, which marker would be the best substitute for it? Which would be the worst? 7. Consider four alleles labeled 1, 2, 3, and 4. They belong to two loci, two alleles per locus, but I am not telling you which allele belongs to which locus. However, one cell undergoing meiosis produces gametes of genotypes 1,2 and 3,4 while another cell of the same genotype produces gametes of genotypes 1,3 and 2,4. Which alleles belong to the same locus? What was logic by which you arrived at your conclusion? Support your answer by depicting metaphase I in the two meiotic cells. Genome 371 Autumn 2017 QS2 8

9 A B C D

10 E G H I

11 J K