Topic 4 Mitosis and Meiosis Homologous chromosomes will segregate equally from

Topic 4 mitosis and meiosis homologous chromosomes

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Topic 4: Mitosis and Meiosis Homologous chromosomes will segregate equally from one another during meiosis - Each allele of a gene pair carried on the chromosomes will segregate equally - Gg organism will produce 50% G gametes and 50% g gametes Figure 3.16: Different chromosomes will assort independently of one another during meiosis - Different gene pairs carried on different chromosomes will assort independently of each other - Not necessarily true for gene pairs on the same chromosome 14
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Topic 4: Mitosis and Meiosis Figure 3.17: Diagram the possible meiotic products from this diploid cell (2n = 4): 6. Mechanisms of Recombination First model proposed: Holliday Model Not entirely accurate, but basis for future models We talked about results, what about the mechanism? Holliday Model (first model proposed) 15
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Topic 4: Mitosis and Meiosis start with synapsed chromatids nick/break in identical spot in both chromatids o broken strands invade opposite chromatid, base pair covalent linkage to form Holliday Junction Modifications to the Holliday Model Holliday model: single strand nick in identical location on non- sister chromatids Modification: single nick (one chromatid) or double stranded break (sister chromatids) o Both nicks on the same homologue o double-strand break model o this is what happens during meiosis Later steps are the same as the holiday models – just starts differently Double Strand Break Model Fig. 17-5 Homologous Recombination between 2 non-sister chromatids during prophase 1 of meiosis involves the Rec proteins (for recombination) o don’t need to know the names or functions initiated by a double-strand break, followed by invasion of a non-sister chromatid by single-stranded DNA from the broken chromatid Site-specific Recombination does not require meiosis does not require synapsis during meiosis 1 (or even related sources of DNA) o identical in both site involved in recombination specific enzymes catalyze breakage and rejoining of DNA fragments 16
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Topic 4: Mitosis and Meiosis used in our bodies for antibody variation o allows a recognition of a huge number of antigens used by lysogenic viruses to integrate into host genome Topic 4 Summary: 1. Mitosis maintains the correct chromosome complement during somatic cell divisions 2. The process of meiosis occurs in diploid cells (meiocytes), and produces four haploid nuclei (meiotic products; gametes) 3. The synapsis of structurally and genetically similar chromosomes (homologues) leads to the reduction in chromosome numbers in the first division of meiosis 4. Random segregation of chromosomes and chromosome crossing-over are two features that result in genetic diversity 5. Crossing over occurs through the double-strand break mechanism, which can result in recombinant chromosomes 17
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