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...
Nội dung trích xuất từ tài liệu:
Chapter 063. Chromosome Disorders (Part 5) Chapter 063. Chromosome Disorders (Part 5) Chromosome segregation in meiosis. A. In meiosis I, each of the 23 pairsof chromosomes finds its partner, or homologue, and exchanges genetic material(recombines) with it. At metaphase, each homologous pair aligns on the equatorialplate; at anaphase, each member of the homologous pair segregates from itspartner. Thus, at the end of meiosis I, each daughter cell contains 23chromosomes, with each chromosome consisting of two sister chromatids. B. Inmeiosis II, each chromosome aligns on the metaphase plate, and at anaphase, eachof the two sister chromatids divides from the other. Thus, at the end of meiosis II,each daughter cell (e.g., the oocyte or spermatocyte) contains 23 chromosomes,with each chromosome consisting of one sister chromatid. In mitosis, thechromosomes behave exactly as they do in meiosis II, except that somaticallydividing cells contain 46 chromosomes, not the 23 that are present in the meiosisII cell. Chromosome segregation is more complicated in germ cell division, sincethe number of chromosomes must be reduced from 46 to 23 in the mature spermand eggs. This is accomplished by two rounds of division—meiosis I and meiosisII (Fig. 63-3). In meiosis I, homologous chromosomes pair and exchange geneticmaterial, 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 arerepresented in each of the two daughter cells. Meiosis II quickly follows meiosis Iand is essentially a haploid mitosis, involving separation of the sister chromatidsin 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. Inmales, meiosis begins with puberty and continues throughout the individualslifetime. In females, meiosis begins prenatally, with oocytes proceeding throughthe first stages of meiosis I but arresting at mid-prophase. At the time of birth, thefirst meiotic division is suspended in oocytes. Only after ovulation many yearslater do oocytes complete meiosis I and proceed to the metaphase stage of meiosisII; if fertilized, the oocyte then completes the second meiotic division. Thus, infemales, the first meiotic division takes at least 10–15 years and as many as 40–45years to complete. Maternal age–related increases in the incidence of trisomy arelikely the consequence of this protracted process of cell division. Incidence and Types of Chromosome Abnormalities Errors in meiosis, or in early cleavage divisions, occur with extraordinaryfrequency. At least 10–25% of all pregnancies, for example, involvechromosomally abnormal conceptions. A large proportion of these terminate in theearliest stages of pregnancy, many of which go unrecognized. Nevertheless, evenamong clinically recognized pregnancies, nearly 10% of fetuses arechromosomally unbalanced. For the three types of clinically recognizedpregnancies—spontaneous abortions, stillbirths, and livebirths—the frequencies ofdifferent chromosomal abnormalities are summarized in Table 63-2. The mostcommon abnormalities are numerical, involving fetuses with additional (trisomy)or missing (monosomy) chromosomes, or those with one (triploidy) or two(tetraploidy) additional sets of chromosomes. Structural chromosomeabnormalities are much less common, although several of the most importantclinical chromosomal disorders involve structural rearrangements (see below). Table 63-2 Frequency and Distribution of Chromosome Abnormalitiesin Different Types of Clinically Recognizable Pregnancies Frequency of Abnormality Chromoso Spontane Stillbir Livebi Probabilme Abnormality ous Abortion th rth ity of Surviving to Term, % Trisomy, 25.1 4.0 0.3 5all +13, 18, 4.5 2.7 0.14 1521 +16 7.5 — — 0 Sex 8.7 0.1 0.01 1chromosomemonosomy (45,X) Triploidy 6.4 0.2 — 0 Tetraploid 2.4 — — 0y Structural 2.0 0.8 0.3 45abnormality Total 50.0 5.1 0.6 5abnormalities By far the most common abnormality is trisomy, which is identified in~25% of spontaneous abortions and 0.3% of newborns. Trisomies for allch ...