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Sexual Reproduction and Meiosis

12.1 Meiosis produces haploid cells from diploid cells.
Discovery of Reduction Division
•The fusion of gametes to form a new cell is referred to as fertilization, or syngamy. (p. 228)
•Gamete formation must involve some mechanism to reduce the number of chromosomes to half that found in other cells. (p. 228)
•Sexual reproduction involves an alternation of meiosis and fertilization, and follows a pattern of alternation between diploid and haploid chromosome numbers. (p. 228)

12.2 Meiosis has three unique features.
Meiosis
•Meiosis in diploid organisms consists of two rounds of division, meiosis I and meiosis II, but replication only occurs at the beginning of meiosis I. (p. 230)
•In prophase I of meiosis, homologues pair in synapsis, and crossing over occurs, allowing nonsister chromatids to exchange chromosomal material. (p. 230)

12.3 The sequence of events during meiosis involves two nuclear divisions.
Prophase I
•During prophase, the ends of the chromatids attach to the nuclear envelope at specific sites, and synapsis occurs. (p. 232)
•Crossing over occurs between nonsister chromatids, forming a chiasma. (p. 232)
Metaphase I
•By metaphase I, the nuclear envelope has dispersed, and microtubules form a spindle. (p. 233)
•Terminal chiasmata hold the homologous chromosomes together, and each joined pair of homologues lines up on the metaphase plate. (p. 233)
Completing Meiosis
•In anaphase I, the microtubules of the spindle fibers begin to shorten, pulling the centromeres toward the poles, and thus dragging the chromosomes toward the poles as well. (p. 236)
•Because of the random orientation of homologous chromosomes on the metaphase plate, meiosis I results in independent assortment of maternal and paternal chromosomes into the gametes. (p. 236)
•In telophase I, the chromosomes have segregated into clusters at each pole, and the nuclear membrane re-forms around each new daughter nucleus. (p. 236)
•Meiosis II resembles a normal mitotic division, except that at the end, each of the four haploid cells contains only one set of every chromosome instead of two sets. (p. 236)

12.4 The evolutionary origin of sex is a puzzle.
Why Sex?
•Several hypotheses exist as to the origin and maintenance of sex, including the DNA repair, contagion, Red Queen, and Muller's Ratchet hypotheses. (pp. 237-238)
•Paradoxically, the evolutionary process is both revolutionary and conservative. (p. 238)










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