TAILIEUCHUNG - Chapter 063. Chromosome Disorders (Part 5)

Chromosome segregation in meiosis. A. In meiosis I, each of the 23 pairs of chromosomes finds its "partner," or homologue, and exchanges genetic material (recombines) with it. At metaphase, each homologous pair aligns on the equatorial plate; at anaphase, each member of the homologous pair segregates from its partner. Thus, at the end of meiosis I, each daughter cell contains 23 chromosomes, with each chromosome consisting of two sister chromatids. B. In meiosis II, each chromosome aligns on the metaphase plate, and at anaphase, each of the two sister chromatids divides from the other. Thus, at the end of. | Chapter 063. Chromosome Disorders Part 5 Chromosome segregation in meiosis. A. In meiosis I each of the 23 pairs of chromosomes finds its partner or homologue and exchanges genetic material recombines with it. At metaphase each homologous pair aligns on the equatorial plate at anaphase each member of the homologous pair segregates from its partner. Thus at the end of meiosis I each daughter cell contains 23 chromosomes with each chromosome consisting of two sister chromatids. B. In meiosis II each chromosome aligns on the metaphase plate and at anaphase each of the two sister chromatids divides from the other. Thus at the end of meiosis II each daughter cell . the oocyte or spermatocyte contains 23 chromosomes with each chromosome consisting of one sister chromatid. In mitosis the chromosomes behave exactly as they do in meiosis II except that somatically dividing cells contain 46 chromosomes not the 23 that are present in the meiosis II cell. Chromosome segregation is more complicated in germ cell division since the number of chromosomes must be reduced from 46 to 23 in the mature sperm and eggs. This is accomplished by two rounds of division meiosis I and meiosis II Fig. 63-3 . In meiosis I homologous chromosomes pair and exchange genetic material then align on the metaphase plate and finally separate from one another. Thus by the end of meiosis I only 23 of the original 46 chromosomes are represented in each of the two daughter cells. Meiosis II quickly follows meiosis I and is essentially a haploid mitosis involving separation of the sister chromatids in each of the 23 chromosomes. Although the fundamentals of meiosis are the same in males and females there are important distinctions particularly in the timing of meiotic divisions. In males meiosis begins with puberty and continues throughout the individual s lifetime. In females meiosis begins prenatally with oocytes proceeding through the first stages of meiosis I but arresting at mid-prophase. At the time

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