rav65819_ch11_205-218

rav65819_ch11_205-218 - part III 11 16.6 m Genetic and...

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;;;;;;;;;; part III 11 16.6 m μ Genetic and Molecular Biology Sexual Reproduction and Meiosis
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chapter introduction MOST ANIMALS AND PLANTS reproduce sexually. Gametes of opposite sex unite to form a cell that, dividing repeatedly by mitosis, eventually gives rise to an adult body with some 100 trillion cells. The gametes that form the initial cell are the products of a special form of cell division called meiosis, visible in the photo to the left, and the subject of this chapter. Meiosis is far more intricate than mitosis, and the details behind it are not as well understood. The basic process, however, is clear. Also clear are the profound consequences of sexual reproduction: It plays a key role in generating the tremendous genetic diversity that is the raw material of evolution. 11.4 Summing Up: Meiosis Versus Mitosis Anaphase I results from the differential loss of sister chromatid cohesion along the arms Telophase I completes meiosis I Achiasmate segregation of homologues is possible Meiosis II is like a mitotic division without DNA replication Errors in meiosis produce aneuploid gametes concept outline 11.1 Sexual Reproduction Requires Meiosis Meiosis reduces the number of chromosomes Sexual life cycles have both haploid and diploid stages Germ-line cells are set aside early in animal development 11.2 Features of Meiosis
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Homologous chromosomes pair during meiosis Meiosis features two divisions with one round of DNA replication 11.3 The Process of Meiosis Prophase I sets the stage for the reductive division During metaphase I, paired homologues align Homologous pairing and crossing over may involve meiosis-speciFc cohesins Sister chromatid cohesion is maintained through meiosis I but released in meiosis II Sister kinetochores are attached to the same pole during meiosis I Replication is suppressed between meiotic divisions Meiosis produces cells that are not identical 205 rav65819_ch11_205-218.indd 205 rav65819_ch11_205-218.indd 205 1/2/07 5:59:56 PM 1/2/07 5:59:56 PM 11.1 Sexual Reproduction Requires Meiosis The essence of sexual reproduction is the genetic contribution of two cells. This mode of reproduction imposes difficulties for sexually reproducing organisms that biologists recognized early on. We are only recently making progress on the underlying mechanism for the elaborate behavior of chromosomes during meiosis. To begin, we briefly consider the history of meiosis and its relationship to sexual reproduction. Meiosis reduces the number of chromosomes Only a few years after Walther Flemming’s discovery of chromosomes in 1879, Belgian cytologist Edouard van Beneden was surprised to find different numbers of chromosomes in different types of cells in the roundworm Ascaris. Specifically, he observed that the gametes (eggs and sperm) each contained two chromosomes, while all of the nonreproductive cells, or somatic cells, of embryos and mature individuals each contained four. From his observations, van Beneden proposed in 1883 that an egg and a sperm, each containing half
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rav65819_ch11_205-218 - part III 11 16.6 m Genetic and...

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