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Meiosis and Gametogenesis

Meiosis is the process of cell division that allows haploid cells to be created from diploid parent cells. It occurs only in sexual reproduction, only in the primary sex organs, and only for the purpose of gamete production, or gametogenesis. Meiosis is similar to mitosis (see Chapter 12), but two complete division cycles take place instead of one, and four haploid cells are produced as opposed to two diploid cells.

 

Meiosis consists of the following stages and events:
  • Prophase I: During this phase, chromosomes condense, the nuclear envelope breaks down, and spindle formation begins.
  • Metaphase I: At this stage, homologous chromosome pairs line up on the cell’s equator; this is the major difference between meiosis and mitosis!
  • Anaphase I: The homologous chromosomes separate and begin to move towards the opposite poles of the cell.
  • Telophase I: Nuclear membranes may or may not form and cytokinesis may or may not occur; regardless, the haploid sets of chromosomes have been separated, and are now ready to proceed through meiosis II.
  • Prophase II: During this phase, chromosomes condense, the nuclear envelope breaks down, and spindle formation begins again.
  • Metaphase II: At this stage, chromosomes (which still consist of two chromatids) line up randomly on the cell’s equator.
  • Anaphase II: The chromatids separate and begin to move towards the opposite poles of the cell.
  • Telophase II: As meiosis draws to a close, sets of haploid genomes become enclosed by new nuclear membranes, the chromosomes begin to decondense, and the spindle apparatus is disassembled.
As noted earlier, the result of meiotic cell division is four haploid cells from each parental diploid cell. The process of meiosis itself is virtually identical in the two sexes, except for the timing of the divisions (as mentioned previously). Gametogenesis proceeds differently, however, in males and females. In males, each of the newly created haploid cells, referred to as spermatids, will eventually mature and become a functional sperm cell. In females, on the other hand, only one of the four haploid cells produced by meiosis will become a mature egg. The other three cells do not grow and are pushed to the side of the follicle. They are visible microscopically as dark polar bodies, and play no further role in the reproductive process. This is probably because the presence of four functional eggs at one time would virtually guarantee multiple births, which might not be desirable depending on the species.


The stages of meiosis





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