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Lecture 6

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Biology 1001A
Tom Haffie

September 29, 2011 10.3-10.4- Genetic Recombination in Eukaryotes: Meiosis (Lecture 6) Genetic Recombination in Eukaryotes: Meiosis • Sexual reproduction depends on meiosis, a specialized process of cell division that recombines DNA sequences and produces cells with half the number of chromosomes present in the somatic cells of a species. • Fertilization (egg and sperm fuse, producing a zygote) restores the diploid number of chromosomes in the cell • Meiosis and fertilization mix genetic information into new combinations Meiosis Occurs in Different Places in Different Organismal Life cycles • The assumption that meiosis makes gametes is true only for animals • Animals o follow the pattern in which the diploid phase dominates the life cycle o Only zygotes divide by mitosis (somatic cells) • Most Plants and Fungi o Alternate between haploid and diploid generations in which, depending on the organism, either generation may dominate the life cycle o Mitotic divisions occur in both phases (spores and zygotes) o Fertilization produces diploid sporophytes, in which some undergo meiosis producing haploid spores o Spores grow into haploid gametophytes o Sperm/egg nuclei produced by gametophyte’s arise through mitosis • Some Fungi o Diploid stage is limited to just the zygote, produced by fertilization (unicellular) o After fertilization, zygote undergoes meiosis Meiosis Changes both chromosome Number and DNA Sequence • The essence of mitosis is SAMENESS • The essence of meiosis is DIFFERENCE o Halved chromosome number o Recombined chromosomal DNA sequence • Homologous Chromosomes carry the same genes, but different alleles (In picture, one pair of chromosomes paternal, other maternal) • Each cell produced by meiosis carries only one member of each homologous pair • Females begin and finish meiosis as a fetus • Males begin meiosis after they reach maturity and never stop Meiosis Produces Four Genetically Different Daughter Cells • MEIOSIS I- homologous chromosomes find their partners and pair lengthwise, gene for gene, in a process called synapsis Recombination occurs • MEIOSIS II- Sister chromatids are separated into different cells -Chromosome number reduced from diploid to haploid Haploid (n) Prophase I 1. Chromosomes begin to condense, each consisting of two sister chromatids 2. Homologous chromosomes come together and pair (tetrad: a homologous pair consisting of four chromatids) 3. Chromatids of homologous chromosomes undergo recombination by exchanging segments Prometaphase I 4. Nuclear envelop breaks down, and the spindle moves into the former nuclear area. o Kinetochore microtubules connect to the chromosomes Metaphase I 5. Spindle Microtubules align the tetrads in the equatorial plane (metaphase plate) between the two spindle poles (Random Assortment) Anaphase I 6. Spindle microtubules separate the two chromosomes of each homologous pair and move them to opposite spindle poles o The poles now contain the HAPLOID NUMBER OF CHROMOSOMES, but each chromosome at the pole still contains two CHROMATIDS Telophase I 7. Spindle of the first meiotic division disassembles, and two new spindles form for the second division INTERKINESIS- NO DNA REPLICATION BETWEEN FIRST AND SECOND MEIOTIC DIVISION Prophase II 8. The chromosomes condense and a spindle forms Prometaphase II 9. Nuclear envelope breaks down, spindle enters the former nuclear area o Kinetochore microtubules from the opposite spindle poles a
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