The zygote (the cell resulting from the fusion of an ovum and a sperm) and all the somatic cells derived from it are diploid (2n) in chromosome number; thus, their cells have two copies of every chromosome and every gene encoded on this chromosome. These chromosomes are called homologous chromosomes because they are similar but not identical; one set of chromosomes is of maternal origin, the other is from the male parent. The gametes, having only one member of each chromosome pair, are described as haploid (1n). During gametogenesis, reduction in chromosome number to the haploid state (23 chromosomes in humans) occurs through metaphase I continuation of continuation of meiosis as seen meiosis as seen metaphase in spermatogenesis in oogenesis (undergo apoptosis) spermatozoa *note: prophase II, anaphase II, and telophase II are not shown ovum
FIGURE 3.12 • Comparison of mitosis and meiosis in an idealized cell with two pairs of chromosomes (2n). The chromosomes of maternal and paternal origin are depicted in red and blue, respectively. The mitotic division produces daughter cells that are genetically identical to the parental cell (2n). The meiotic division, which has two components, a reductional division and an equatorial division, produces a cell that has only two chromosomes (1n). In addition, during the chromosome pairing in prophase I of meiosis, chromosome segments are exchanged, leading to further genetic diversity. It should be noted that in humans the first polar body does not divide. Division of the first polar body does occur in some species.
FIGURE 3.13 • Atomic force microscopic image of the centromeric region of a human metaphase chromosome.
The facing surfaces of two sister chromatids visible on this image form the centromere, a point of junction of both chromatids. On the opposite side from the centromere, each chromatid possesses a specialized protein complex, the kinetochore, which serves as an attachment point for kinetochore microtubules of the mitotic spindle. Note that the surface of the chromosome has several protruding loop domains formed by chromatin fibrils anchored into the chromosome scaffold. 40,000. (Courtesy of Dr. Tatsuo Ushiki.) meiosis, a process that involves two successive divisions, the second of which is not preceded by an S phase. This reduction is necessary to maintain a constant number of chromosomes in a given species. Reduction in chromosome number to (1n) in the first meiotic division is followed by reduction in DNA content to the haploid (1d) amount in the second meiotic division.
FIGURE 3.15 • Mitotic spindle in anaphase. This immunofluorescent image comes from the same cell type and identical preparation as in Figure 3.13. Connections that hold the sister chromatids together break at this stage. The chromatids are then moved to opposite poles of the cell by microtubule-associated molecular motors (dyneins and kinesins) that slide along the kinetochore microtubules toward the centriole and are also pushed by the polar microtubules (visible between the separated chromosomes) away from each other, thus moving opposite poles of the mitotic spindle into the separate cells. 1,400. (Courtesy of Dr. Thomas U. Mayer.)