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What is the difference between male meiosis and female meiosis for humans?


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2009-09-13 19:46:04
2009-09-13 19:46:04

There are three differences between male and female meiosis. The first difference should be obvious. Male meiosis creates sperm, while female meiosis creates eggs. The second difference is related to the first. Male meiosis takes place in the testicles, while female meiosis takes place in the ovaries.
The last difference is more subtle and requires an understanding of sex chromosomes. For the most part, each human's DNA is made up of forty-six chromosomes, twenty three from each parent (victims of Down's Syndrome are exceptions). Of these forty-six chromosomes, two are known as sex chromosomes because they determine the sex of the individual. A male will generally have one X and one Y chromosome, while a female will in all but rare incidences have two X chromosomes, however only one of the two is active. A process of masking of the second X chromosome results in production at approximately the 1000 cell stage of embryological development of what is known as a Barr body which has been assumed to be entirely inactive, although recent research suggests Barr bodies may have a greater influence than previously supposed. During meiosis I, the sex chromosomes separate and enter different sperm or egg cells (gametes). Males will end up with one half X sperm and the other half Y sperm, while females will all have X eggs because they had no Y chromosome in the first place. There are more subtle differences though. At the end of meiosis I females have two daughter cells and meiosis II only occurs if and when fertilization occurs by a sperm cell. At that time both daughter cells divide to form 4 cells and of the 4 cells formed, 3 are discarded as polar bodies and the 4th cell having an enhanced cytoplasmic component combines its nuclear component with the sperm cell's nuclear component and crossing over occurs to form the embryo which then begins to divide via mitosis to become two cells, then four and so on. An egg cell that is not fertilized is ovulated as a pair of daughter cells and there is no formation of polar bodies, hence, the eggs that are ultimately discarded at menstruation are not "finished" eggs. They have not undergone meiosis II. Yet another distinction is that meiosis in females is interrupted. Of the 7 million potential oocytes that form during the 5th month of pregnancy, 99.9% will be eliminated prior to being ovulated and these are resorbed by the body. Potential oocytes remain in a latency phase until hormones released at puberty result in oocytes going through meiosis I and being ripened individually and released on a monthly basis. Without fertilization by a sperm making its way to the fallopian tube, meiosis II will not occur and the spent egg released will have two identical daughter cells. In males, meiosis does not begin until puberty and once initiated, a 74 hour cycle occurs with several hundred million sperm cells being produced daily with no similar selection process involving polar bodies or daughter cells. Meiosis in females ends at menopause when all of the primary oocytes have been either ovulated or absorbed. Meiosis in males is a lifetime endeavor and while sperm production decreases after reaching a peak in the mid 20's and the percentage of sperm that swim erratically increases with age, a healthy human male will continue manufacturing sperm from puberty until death. When a sperm and an egg fuse, the chromosomes pair and recombine. If the combination is XY, a male is produced. If the combination is XX, a female is created but again, there are complications. When the 22 autosomal chromosomes (non sex chromosomes) cross-over, a good deal of recombination occurs, but the Y chromosome has 95% of its information conserved that fails to recombine with the X chromosome. It is believed that the Y chromosome "hijacked" the sex-determining region or SRY locus some 300 million years ago, although Dawkins suggests the ancestral "Adam" goes back only 60,000 years in humans and was predated (sic) by an ancestral "Eve" 140,000 years ago (see "population bottleneck", also a wikipedia contribution having related links). As a result, males produce both X and Y factors in their testes which are intitiated when the SRY locus received from their father results in the expression of a protein called TDF (testis determining factor) resulting in a developing embryo manufacturing seminiferous tubules, testes and a penis. Hence, all embryos start out females and only become males with the expression of genes on the SRY locus occurs (with the SRY and 18 other genes not being found on the X or any of the other autosomal chromosomes). All mammals except the platypus have the SRY locus form of sex determination (whereas birds have the WZ sex inheritance system). In cases where the SRY locus gets translocated to an X chromosome, a female (xx) can develop seminiferous tubules, testes and a penis with these individuals said to have xx male syndrome, however, such individuals are sterile although could possibly become pregnant given in-vitro fertilization and hormonal supplements.

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