The Process of Meiosis

In contrast to mitosis, meiosis results in the division of a diploid parental cell into haploid progeny, each containing only one member of the pair of homologous chromosomes that were present in the diploid parent (Figure 14.32). This reduction in chromosome number is accomplished by two sequential rounds of nuclear and cell division (called meiosis I and meiosis II), which follow a single round of DNA replication. Like mitosis, meiosis I initiates after S phase has been completed and the parental chromosomes have replicated to produce identical sister chromatids. The pattern of chromosome segregation in meiosis I, however, is dramatically different from that of mitosis. During meiosis I, homologous chromosomes first pair with one another and then segregate to different daughter cells. Sister chromatids remain together, so completion of meiosis I results in the formation of daughter cells containing a single member of each chromosome pair (consisting of two sister chromatids). Meiosis I is followed by meiosis II, which resembles mitosis in that the sister chromatids separate and segregate to different daughter cells. Completion of meiosis II thus results in the production of four haploid daughter cells, each of which contains only one copy of each chromosome.

Figure 14.32

Comparison of meiosis and mitosis. Both meiosis and mitosis initiate after DNA replication, so each chromosome consists of two sister chromatids. In meiosis I, homologous chromosomes pair and then segregate to different cells. Sister chromatids then separate (more...)

The pairing of homologous chromosomes after DNA replication is not only a key event underlying meiotic chromosome segregation, but also allows recombination between chromosomes of paternal and maternal origin. This critical pairing of homologous chromosomes takes place during an extended prophase of meiosis I, which is divided into five stages (leptotene, zygotene, pachytene, diplotene, and diakinesis) on the basis of chromosome morphology (Figure 14.33). The initial association of homologous chromosomes is thought to be mediated by base pairing between complementary DNA strands during the leptotene stage, before the chromatin becomes highly condensed. The close association of homologous chromosomes (synapsis) begins during the zygotene stage. During this stage, a zipperlike protein structure, called the synaptonemal complex, forms along the length of the paired chromosomes (Figure 14.34). This complex keeps the homologous chromosomes closely associated and aligned with one another through the pachytene stage, which can persist for several days. Recombination between homologous chromosomes is completed during their association at the pachytene stage, leaving the chromosomes linked at the sites of crossing over (chiasmata; singular, chiasma). The synaptonemal complex disappears at the diplotene stage and the homologous chromosomes separate along their length. Importantly, however, they remain associated at the chiasmata, which is critical for their correct alignment at metaphase. At this stage, each chromosome pair (called a bivalent) consists of four chromatids with clearly evident chiasmata (Figure 14.35). Diakinesis, the final stage of prophase I, represents the transition to metaphase, during which the chromosomes become fully condensed.

Figure 14.33

Stages of the prophase of meiosis I. Micrographs illustrating the morphology of chromosomes of the lily. (C. Hasenkampf/Biological Photo Service.)

Figure 14.34

The synaptonemal complex. Chromatin loops are attached to the lateral elements, which are joined to each other by a zipperlike central element.

Figure 14.35

A bivalent chromosome at the diplotene stage. The bivalent chromosome consists of paired homologous chromosomes. Sister chromatids of each chromosome are joined at the centromere. Chromatids of homologous chromosomes are joined at chiasmata, which are (more...)

At metaphase I, the bivalent chromosomes align on the spindle. In contrast to mitosis (see Figure 14.27), the kinetochores of sister chromatids are adjacent to each other and oriented in the same direction, while the kinetochores of homologous chromosomes are pointed toward opposite spindle poles (Figure 14.36). Consequently, microtubules from the same pole of the spindle attach to sister chromatids, while microtubules from opposite poles attach to homologous chromosomes. Anaphase I is initiated by disruption of the chiasmata at which homologous chromosomes are joined. The homologous chromosomes then separate, while sister chromatids remain associated at their centromeres. At completion of meiosis I, each daughter cell has therefore acquired one member of each homologous pair, consisting of two sister chromatids.

Figure 14.36

Chromosome segregation in meiosis I. At metaphase I, the kinetochores of sister chromatids are either fused or adjacent to one another. Microtubules from the same pole of the spindle therefore attach to the kinetochores of sister chromatids, while microtubules (more...)

Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. At metaphase II, the chromosomes align on the spindle with microtubules from opposite poles of the spindle attached to the kinetochores of sister chromatids. The link between the centromeres of sister chromatids is broken at anaphase II, and sister chromatids segregate to opposite poles. Cytokinesis then follows, giving rise to haploid daughter cells.

Regulation of Oocyte Meiosis

Vertebrate oocytes (developing eggs) have been particularly useful models for research on the cell cycle, in part because of their large size and ease of manipulation in the laboratory. A notable example, discussed earlier in this chapter, is provided by the discovery and subsequent purification of MPF from frog oocytes. Meiosis of these oocytes, like those of other species, is regulated at two unique points in the cell cycle, and studies of oocyte meiosis have illuminated novel mechanisms of cell cycle control.

The first regulatory point in oocyte meiosis is in the diplotene stage of the first meiotic division (Figure 14.37). Oocytes can remain arrested at this stage for long periods of time—up to 40 to 50 years in humans. During this diplotene arrest, the oocyte chromosomes decondense and are actively transcribed. This transcriptional activity is reflected in the tremendous growth of oocytes during this period. Human oocytes, for example, are about 100 μm in diameter (more than a hundred times the volume of a typical somatic cell). Frog oocytes are even larger, with diameters of approximately 1 mm. During this period of cell growth, the oocytes accumulate stockpiles of materials, including RNAs and proteins, that are needed to support early development of the embryo. As noted earlier in this chapter, early embryonic cell cycles then occur in the absence of cell growth, rapidly dividing the fertilized egg into smaller cells (see Figure 14.2).

Figure 14.37

Meiosis of vertebrate oocytes. Meiosis is arrested at the diplotene stage, during which oocytes grow to a large size. Oocytes then resume meiosis in response to hormonal stimulation and complete the first meiotic division, with asymmetric cytokinesis (more...)

Oocytes of different species vary as to when meiosis resumes and fertilization takes place. In some animals, oocytes remain arrested at the diplotene stage until they are fertilized, only then proceeding to complete meiosis. However, the oocytes of most vertebrates (including frogs, mice, and humans) resume meiosis in response to hormonal stimulation and proceed through meiosis I prior to fertilization. Cell division following meiosis I is asymmetric, resulting in the production of a small polar body and an oocyte that retains its large size. The oocyte then proceeds to enter meiosis II without having re-formed a nucleus or decondensed its chromosomes. Most vertebrate oocytes are then arrested again at metaphase II, where they remain until fertilization.

Like the M phase of somatic cells, the meiosis of oocytes is controlled by MPF. The regulation of MPF during oocyte meiosis, however, displays unique features that are responsible for metaphase II arrest (Figure 14.38). Hormonal stimulation of diplotene-arrested oocytes initially triggers the resumption of meiosis by activating MPF, as at the G₂ to M transition of somatic cells. As in mitosis, MPF then induces chromosome condensation, nuclear envelope breakdown, and formation of the spindle. Activation of the anaphase-promoting complex B then leads to the metaphase to anaphase transition of meiosis I, accompanied by a decrease in the activity of MPF. Following cytokinesis, however, MPF activity again rises and remains high while the egg is arrested at metaphase II. A regulatory mechanism unique to oocytes thus acts to maintain MPF activity during metaphase II arrest, preventing the metaphase to anaphase transition of meiosis II and the inactivation of MPF that would result from cyclin B proteolysis during a normal M phase.

Figure 14.38

Activity of MPF during oocyte meiosis. Hormonal stimulation of diplotene oocytes activates MPF, resulting in progression to metaphase I. MPF activity then falls at the transition from metaphase I to anaphase I. Following completion of meiosis I, MPF activity (more...)

The factor responsible for metaphase II arrest was first identified by Yoshio Masui and Clement Markert in 1971, in the same series of experiments that led to the discovery of MPF. In this case, however, cytoplasm from an egg arrested at metaphase II was injected into an early embryo cell that was undergoing mitotic cell cycles (Figure 14.39). This injection of egg cytoplasm caused the embryonic cell to arrest at metaphase, indicating that metaphase arrest was induced by a cytoplasmic factor present in the egg. Because this factor acted to arrest mitosis, it was called cytostatic factor (CSF).

Figure 14.39

Identification of cytostatic factor. Cytoplasm from a metaphase II egg is microinjected into one cell of a two-cell embryo. The injected embryo cell arrests at metaphase, while the uninjected cell continues to divide. A factor in metaphase II egg cytoplasm (more...)

More recent experiments have identified a protein-serine/threonine kinase known as Mos as an essential component of CSF. Mos is specifically synthesized in oocytes around the time of completion of meiosis I and is then required both for the increase in MPF activity during meiosis II and for the maintenance of MPF activity during metaphase II arrest. The action of Mos results from activation of the ERK MAP kinase, which plays a central role in the cell signaling pathways discussed in the previous chapter. In oocytes, however, ERK plays a different role; it activates another protein kinase called Rsk, which inhibits action of the anaphase-promoting complex and arrests meiosis at metaphase II (Figure 14.40). Oocytes can remain arrested at this point in the meiotic cell cycle for several days, awaiting fertilization.

Figure 14.40

Maintenance of metaphase II arrest by the Mos protein kinase. The Mos protein kinase maintains metaphase II arrest by inhibiting the anaphase-promoting complex. The action of Mos is mediated by MEK, ERK, and Rsk protein kinases.

Fertilization

At fertilization, the sperm binds to a receptor on the surface of the egg and fuses with the egg plasma membrane, initiating the development of a new diploid organism containing genetic information derived from both parents (Figure 14.41). Not only does fertilization lead to the mixing of paternal and maternal chromosomes, but it also induces a number of changes in the egg cytoplasm that are critical for further development. These alterations activate the egg, leading to the completion of oocyte meiosis and initiation of the mitotic cell cycles of the early embryo.

Figure 14.41

Fertilization. Scanning electron micrograph of a human sperm fertilizing an egg. (David M. Philips/Visuals Unlimited.)

A key signal resulting from the binding of a sperm to its receptor on the plasma membrane of the egg is an increase in the level of Ca²⁺ in the egg cytoplasm, probably as a consequence of stimulation of the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) (see Figure 13.27). One effect of this elevation in intracellular Ca²⁺ is the induction of surface alterations that prevent additional sperm from entering the egg. Because eggs are usually exposed to large numbers of sperm at one time, this is a critical event in ensuring the formation of a normal diploid embryo. These surface alterations are thought to result, at least in part, from the Ca²⁺-induced exocytosis of secretory vesicles that are present in large numbers beneath the egg plasma membrane. Release of the contents of these vesicles alters the extracellular coat of the egg so as to block the entry of additional sperm.

The increase in cytosolic Ca²⁺ following fertilization also signals the completion of meiosis (Figure 14.42). In eggs arrested at metaphase II, the metaphase to anaphase transition is triggered by a Ca²⁺-dependent activation of the anaphase-promoting complex. The resultant inactivation of MPF leads to completion of the second meiotic division, with asymmetric cytokinesis (as in meiosis I) giving rise to a second small polar body.

Figure 14.42

Fertilization and completion of meiosis. (A) Fertilization induces the transition from metaphase II to anaphase II, leading to completion of oocyte meiosis and emission of a second polar body (which usually degenerates). The sperm nucleus decondenses, (more...)

Following completion of oocyte meiosis, the fertilized egg (now called a zygote) contains two haploid nuclei (called pronuclei), one derived from each parent. In mammals, the two pronuclei then enter S phase and replicate their DNA as they migrate toward each other. As they meet, the zygote enters M phase of its first mitotic division. The two nuclear envelopes break down, and the condensed chromosomes of both paternal and maternal origin align on a common spindle. Completion of mitosis then gives rise to two embryonic cells, each containing a new diploid genome. These cells then commence the series of embryonic cell divisions that eventually lead to the development of a new organism.