FIGURE 3.14 • Mitotic spindle in metaphase. Using indirect immunofluorescence techniques, the mitotic spindle in a Xenopus XL-177 cell was labeled with an antibody against -tubulin conjugated with fluorescein (green). DNA was stained blue with fluorescent DAPI stain. In metaphase, the nuclear membrane disassembles, DNA is condensed into chromosomes, and microtubules form a mitotic spindle. The action of microtubuleassociated motor proteins on the microtubules of the mitotic spindle creates the metaphase plate along which the chromosomes align in the center of the cell. 1,400. (Courtesy of Dr. Thomas U. Mayer.)
During meiosis, the chromosome pair may exchange chromosome segments, thus altering the genetic composition of the chromosomes. This genetic exchange, called crossing-over, and the random assortment of each member of the chromosome pairs into haploid gametes give rise to infinite genetic diversity.
FIGURE 3.16 • Mitotic spindle in telophase. In this phase, DNA is segregated and a nuclear envelope is reconstituted around the chromosomes at each pole of the mitotic spindle. The cell divides into two during cytokinesis. In the middle of the cell, actin, septins, myosins, microtubules, and other proteins gather as the cell establishes a ring of proteins that will constrict, forming a bridge between the two sides of what was once one cell. The chromosomes uncoil and become indistinct except at regions that remain condensed in interphase. The cell types and preparation are the same as those in Figures 3.13 and 3.14. 1,400. (Courtesy of Dr. Thomas U. Mayer.)
The cytoplasmic events associated with meiosis differ in the male and female.
The nuclear events of meiosis are the same in males and females, but the cytoplasmic events are markedly different. Figure 3.12 illustrates the key nuclear and cytoplasmic events of meiosis as they occur in spermatogenesis and oogenesis. The events of meiosis through metaphase I are the same in both sexes. Therefore, the figure illustrates the differences in the process as they diverge after metaphase I.
In males, the two meiotic divisions of a primary spermatocyte yield four structurally identical, although genetically unique, haploid spermatids. Each spermatid has the capacity to differentiate into a spermatozoon. In contrast, in females, the two meiotic divisions of a primary oocyte yield one haploid ovum and three haploid polar bodies. The ovum receives most of the cytoplasm and becomes the functional gamete. The polar bodies receive very little cytoplasm and degenerate.
The nuclear events of meiosis are similar in males and females.
Meiosis consists of two successive mitotic divisions without the additional S phase between the two divisions. During the S phase that precedes meiosis, DNA is replicated forming sister chromatids (two parallel strands of DNA) joined together by the centromere. The DNA content becomes (4d), but the chromosome number remains the same (2n). The cells then undergo a reductional division (meiosis I) and an equatorial division (meiosis II).