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Genetics Post Lab

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  Lab I Assignment Due next week IN DISCUSSION   Prepare this assignment as a problem set, not as a formal lab report. We will provide an e-copy of this document, posting it on Glow.   Please type your answers into this document, print it out, and don’t forget to sketch your answer to 1.c. on the hard copy and to fill out the table in Question 4. You will need internet access to view some images necessary to complete the assignment. Remember to staple or paper clip this and all other multiple page lab assignments.   1.   This picture shows two stained diploid cells undergoing mitosis (blue=DNA, green=tubulin, a microtubule component). Use this picture to answer a. and b.   a. What cell cycle stage is each of these two cells in? Cell 1 is in metaphase and Cell 2 is in anaphase.    b. Are there single chromatid or double chromatid chromosomes in each of these two cells?   Double chromatid chromosomes   c. Sketch two homologous pairs of chromosomes and their attached spindle fibers at metaphase I of meiosis, then sketch the chromosomes and attached spindle fibers that you would expect to see later in each of the daughter cells at anaphase II of meiosis. Shade the paternal copies of each chromosome in your picture, and show one possible outcome of recombination and chromosome segregation in each case.   (continued on next page)    1   2    d.   Why is the outcome of meiosis consistent with Mendel’s Law of Independent Assortment of different loci? Use your answer in 1.c. to explain your reasoning. Mendel’s second law states that each pair of alleles segregates independently during gamete formation. Recombination occurs in metaphase I only between homologous pairs and  before any segregation occurs. During Anaphase II each sister chromatid is in its own gamete where it is independent of other chromatids. Importantly, neither paternal nor maternal chromosomes have separated together.   2.  Open your polytene chromosome photo file (or one you had to borrow if your image is not sufficient). PRINT A COPY OF THIS PHOTO AND ATTACH IT TO YOUR ASSIGNMENT.  Select a chromosome end with prominent banding. Now open the Helsinki website you we used at the start of Part 4 and address the following issues:   a. Which end is shown in your picture? Make sure you indicate which end you are focusing on if there is more than one shown in the picture.   This is most likely Region 1 on chromosome X.  b. Using the Part 4 instructions in the lab procedure, identify:   -one gene within a specific band ( indicate which band on your photo )   -the hypothesized function and expression pattern of that gene   -a transposable element insertion within the gene   -one example of a deletion and one of a duplication that span the gene (if available)   I identified tyn (Trynity) gene as the first band of the chromosome. It is a protein coding gene. It is involved in actin filament organization, regulation of embryonic cell shape, apical construction and cell-matrix adhesion. It expresses at high levels in larval hindgut. P{PZ} JC01JC01 is one of the transposable element insertion within the gene. There is no deletion of a gene and DC 426 is a duplication that span the gene.    3. Examine the pair of human chromosome photos below. One is from one normal human cell and the other from one HeLa cell. Which is which and why? Given what you have learned so far in lecture as well as lab, by what process do you think this abnormality in the HeLa cell developed? The picture on the right is the HeLa cell because there are duplicated chromosomes and some of the chromosomes are broken and not the correct length and size. The transcription regulators in HeLa cells that prevent over transcription and also tumor cells like HeLa cells are losing checks that prevent them from dividing. If normal cells have an abnormal karyotype they would not undergo mitosis, but since HeLa cells are missing this checkpoint, they will continue dividing even if their karyotype is not correct.   4. On Glow, access the pictures of the salivary gland chromosomes of various Drosophila  species. Images were obtained from females for simplicity (see a. below). These images were taken by the instructors to ensure that all chromosome arms were distinguishable.   a. Examine the photos and count the number of chromosome ends radiating from the chromocenter of each Drosophila  species. Fill these values into the table below. Using your counts and the information given in the table, calculate the number of telocentric chromosome pairs and the total number of chromosome pairs for each species. Enter these values into the table as well. (Note: all counts refer to the haploid karyotype, or considered another way, the number of chromosome pairs in a diploid cell. Glands were obtained from females for these pictures to avoid confusion. So, for example, the somatic cells of a female Drosophila melanogaster have 4 pairs of homologous chromosomes: two X’s, two chromosome 2’s, two chromosomes 3’s and 1 pair of dot chromosomes (#4), but as shown in  Fig. 1-4c  , the two copies of a specific homologous pair are aligned and therefore indistinguishable from each other in the salivary glands.)       b. The phylogeny below represents our current understanding of how the five Drosophila  species under consideration are related. Knowing that: (1) D. virilis  exhibits the ancestral karyotype, (2) the chromosome arms are in synteny across all Drosophila  species (that is, the homologous genes are in more or less the same order between species – but gross segments of the genome have sometimes been inverted or translocated), (3) all species have about the same number of genes, and (4) all species are completely diploid, what processes could have caused the evolutionary changes in chromosome number and type distribution for the four more derived species? Describe two specific examples in your answer. ( Hint and take-home message: consider how this process contrasts with the one at work in Question 3. )   Four of the telocentric pairs become 2 metacentric pairs by combining. This explains virilis and willistoni. And then they break apart again to form 2 telocentric pairs from willistoni to pseudoobscura.  

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Jul 23, 2017

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Jul 23, 2017
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