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BIOL 1020 13

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BIOL 1020
Anne- Marie Singh

BIOL 1020 – CHAPTER 13 LECTURE NOTES Chapter 13: Meiosis and Sexual Life Cycles I. Different modes of reproduction require different types of cell division a. asexual reproduction creates offspring that are genetically identical to each other and to the parent cell (clones) i. only mitotic cell division, or something very similar, is required ii. the parent may split, bud, or fragment; sometimes, this involves mitotic cell division with unequal partitioning during cytokinesis (cellular budding) iii.asexual reproduction is typically rapid and efficient compared to sexual reproduction b. sexual reproduction occurs when specialized sex cells called gametes fuse to form a single cell called a zygote i. usually the gametes that fuse are produced by different individuals, but they may be produced by the same individual ii. in plants and animals the gametes are called the egg and the sperm iii.the offspring are not genetically identical to their parents iv. this genetic recombination may render the offspring better adapted to the environment than either parent, or it may be more poorly adapted than either parent v. sexual reproduction must contain a mechanism to half the number of chromosomes at some point 1. without such a mechanism, the number of chromosomes would double with each generation 2. halving the chromosome number is accomplished through meiosis II. Diploid cells give rise to haploid cells during meiosis a. the somatic (body) cells of animals and higher plants are diploid cells i. each chromosome in a diploid cell has a partner chromosome ii. the partners are called homologous chromosomes member of each pair came from the father (paternal homolog), and one from the mother (maternal homolog) iv. thus, for humans, the 46 chromosomes are in 23 pairs v. most pairs of homologous chromosomes contain very similar, but not identical, genetic information in each member of the pair (more on this in the next unit, genetics) vi. sex chromosomes aren’t strictly homologous (an X chromosome has different genes than a Y chromosome), but they act as if they are homologous during meiosis vii. a set of chromosomes (n) has one member for each homologous pair; a diploid cell has two complete sets (2n), while a haploid cell has one set (n) viii. sometimes, cells have extra sets (3n or more; polyploid cells – common in plants, rare and usually fatal in animals) b. meiosis reduces chromosome number, producing up to 4 haploid cells from one diploid cell i. meiosis has two successive cell divisions after only one DNAreplication ii. the two cell divisions are called meiosis I and meiosis II 1. homologous chromosomes separate during meiosis I 2. sister chromatids separate during meiosis II 3. meiosis is best understood by following the chromosomes (and their kinetochores) – don’t confuse homologous chromosomes with sister chromatids III. Meiosis I and meiosis II are each divided into prophase, metaphase, anaphase, and telophase (PMAT) with accompanying cytokinesis a. prophase I – chromatin condenses to form chromosomes, and homologous chromosomes pair i. the process of homologous chromosomes pairing lengthwise is called synapsis 1. the resulting structure, with 4 total chromatids (two sisters from each homologous chromosome), is called a bivalent or a tetrad 2. the tetrad is held together by a synaptonemal complex during early prophase I 3. typically, enzymes cause breaks in the chromatids and genetic material may be exchanged between chromatids (crossing-over or genetic recombination) 4. genetic recombination greatly increases the
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