why do sperm and egg go through meiosis


is a type of cell division that reduces the number of chromosomes in the parent
cell by half and produces four gamete cells. This process is required to
produce egg and sperm cells for sexual reproduction. During reproduction, when
the sperm and egg unite to form a single cell, the number of chromosomes is
restored in the offspring. Meiosis
begins with a parent cell that is diploid, meaning it has two copies of each
chromosome. The parent cell undergoes one round of DNA replication followed by
two separate cycles of nuclear division. The process results in four daughter
cells that are haploid, which means they contain half the number of chromosomes
of the diploid parent cell. Meiosis
has both similarities to and differences from mitosis, which is a cell division
process in which a parent cell produces two identical daughter cells. Meiosis
begins following one round of DNA replication in cells in the male or female
sex organs. The process is split into meiosis I and meiosis II, and both
meiotic divisions have multiple phases. Meiosis I is a type of cell division
unique to germ cells, while meiosis II is similar to mitosis. Meiosis
I, the first meiotic division, begins with prophase I. During prophase I, the
complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they
remain joined at a central point called the centromere. A large structure
called the meiotic spindle also forms from long proteins called microtubules on
each side, or pole, of the cell. Between prophase I and metaphase I, the pairs
of homologous chromosome form tetrads. Within the tetrad, any pair of chromatid
arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA
to produce new combinations of genes.


In metaphase I, the homologous pairs of
chromosomes align on either side of the equatorial plate. Then, in anaphase I,
the spindle fibers contract and pull the homologous pairs, each with two
chromatids, away from each other and toward each pole of the cell. During
telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a
process called cytokinesis that divides the cytoplasm of the original cell into
two daughter cells. Each daughter cell is haploid and has only one set of
chromosomes, or half the total number of chromosomes of the original cell. Meiosis
II is a mitotic division of each of the haploid cells produced in meiosis I. During prophase II, the chromosomes condense, and a new set of spindle fibers
forms. The chromosomes begin moving toward the equator of the cell. During
metaphase II, the centromeres of the paired chromatids align along the
equatorial plate in both cells. Then in anaphase II, the chromosomes separate
at the centromeres. The spindle fibers pull the separated chromosomes toward
each pole of the cell. Finally, during telophase II, the chromosomes are
enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of
the two cells. At the conclusion of meiosis, there are four haploid daughter
cells that go on to develop into either sperm or egg cells.
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 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 have illuminated novel mechanisms of cell cycle control. The first regulatory point in oocyte is in the stage of the first meiotic division ( ).


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 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 m 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, 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 ). Oocytes of different species vary as to when resumes and fertilization takes place. In some animals, oocytes remain arrested at the 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 or decondensed its. Most vertebrate oocytes are then arrested again at II, where they remain until fertilization. Like the of somatic cells, the of oocytes is controlled by. The regulation of MPF during oocyte meiosis, however, displays unique features that are responsible for II arrest ( ). Hormonal stimulation of -arrested oocytes initially triggers the resumption of meiosis by activating MPF, as at the G to M transition of somatic cells.

As in, MPF then induces chromosome condensation, breakdown, and formation of the spindle. Activation of the B then leads to the metaphase to anaphase transition of meiosis I, accompanied by a decrease in the activity of MPF. Following, 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 during a normal M phase. The factor responsible for 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. 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 ( ). 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, it was called cytostatic factor ( CSF ). More recent experiments have identified a known as Mos as an essential component of CSF. Mos is specifically synthesized in oocytes around the time of completion of I and is then required both for the increase in activity during meiosis II and for the maintenance of MPF activity during 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 called Rsk, which inhibits action of the and arrests meiosis at metaphase II ( ). Oocytes can remain arrested at this point in the meiotic cell cycle for several days, awaiting fertilization.

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