Lecture 2 BIO 311D 2nd EditionOutline of Last Lecture I. Review of levels of organization in biologyII. ChromosomesIII. Cell division and life cyclesIV. Significance of meiosisOutline of Current Lecture I. Meiosis, “ploidy”, and life cyclesII. Sources of genetic variability in meiosisIII. Applying meiosis to Mendelian inheritance Current LectureI. MeiosisA. (2n) Mitosis (2n) Meiosis (n) Mitosis (n)B. If human life cycles were like plant life cycles, a multicellular sperm(made by mitosis) would be equivalent to the independentmulticellular haploid structures seen in plants. C. Meiosis I: Chromosomes condense, tetrads form, homologsseparate from each otherMeiosis II: Twin chromatids separate from each other II. Sources of genetic variability in meiosis A. Non-disjunction is failure of chromosomes to separate duringmeiosis- If a cell with 2n=40 undergoes meiosis, but one non-disjunction event occurs in meiosis II, how many chromosomes will be in each of the four daughter cells? What if the event occurred in meiosis I? Meiosis II: 21, 19, 20, 20 (n+1), (n-1), n, nMeiosis I: 21, 21, 19, 19 (n+1), (n+1), (n-1), (n-1)B. Segregation: When a gamete is formed, the members of a homologous pair are separated from one another and segregate (separate) into different gametes (this is called the law of segregation) this results in gametes with only 23 chromosomes (Each has 1 member of each homologous pair)C. Independent assortment during meiosis produces 2n combinations of chromosomes- The way the members of one homologous pair separate into gametes is totally independent from the way other homologous pairs segregate- Since each gamete is produced by independent assortment, the combination of genes is uniqueD. Another source of genetic variation is recombination through crossing over: maternal and paternal homologs break and exchange segments at random sites along the chromosome.III. Meiosis to Mendelian Inheritance A. We care for what happens in meiosis because that’s how we get our genes.- Meiosis is the basis of transmission genetics and of Mendelian Inheritance
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