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Mitosis, Meiosis (nondisjunction).pdf

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Steven M Short

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Lecture 5 Lecture 5▯ Until now our analysis of genes has focused on gene function as determined by phenotype differences brought about by different alleles or by a direct test of fu▯nction – the complementation test. For the next six lectures our analysis will be concerned with the tests ▯of geneposition starting with the position of genes on chromosomes and finally mapping p▯oint mutations at the resolution of single nucleotide pairs. We’ve taken it for granted that genes reside on chromosomes, but how ▯do we know this?▯ Let’s review the properties of gene segregation.▯ Consider two different traits.▯ A/ , / x a/ , / A B a b The gametes from one parent willA B and from the other pareaa b These gametes will then give 1n F generation ofaa, /b Crosses between 1 individuals will giv2 an F generation with a 9 : 3 : 3 : 1 phenotypic ratio as shown before. A better way to look at segregation is by a test cross1of the F heteroxygote to a homozygous recessive individual. A/a, /b x aaaa, /b a b aa b will be: The possible gamete genotypes from th1 F A bb a bb A BB a BB (recomb.) (parental) (parental) (recomb.) The corresponding genotypes of the offspring in the testcross will be: A/a, /b a/a, /b A/a, /b a/a, /b Each offspring receives either one or the other parentagene segregaion. For most gene pairs, the frequency of each of the four classes of gamete▯s is the same indicating that the two genes segregate independndependeentassortmentt. At the turn of the century microscopes allowed people to watch chromosom▯es in the nuclei of dividing cells. (human cells, for example, contain 46 chromos▯omes). The chromosomes in dividing somatic cells go through Miitosis:▯ The net result of mitosis is to distribute a replica of each chromosome ▯into the two daughter cells. The stages of mitosis are as follows: i) Innterphasse.NA replication i) Prophaase.Chromosomes condense and centromeres attach to microtubule spindle ii) MMettaphasee.hromosomes align iv) Annaphaase.ister chromatids move apart v) TTelophaase.uclei reform The cell has evolved a simple mechanical mechanism to insure that after ▯mitosis each daughter cell has received exactly one copy of each chromosome. (Failure of proper chromosome segregation is known as nonddsjuncctono). The steps in the mechanism are as follows: 1) After DNA replication two daughter chromosomes known as sis▯ter chromatids are held together by special proteins known as cohesins. 2) As chromosomes align in metaphase microtubule spindles attach to centromeres on each chromatid. ▯ 3) Once all of the chromatids are attached to spindles a protease known as separase bec▯omes active (Actually unattached chromatids produce a signal to keep separase inact▯ive and
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