Naina B. answered • 11/27/14
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Hannah,
In fact, crossing over occurs during meiosis, a cell division that produces germ cells and gametes: sperms and eggs. Both sperms and eggs have half the number of chromosomes than present in somatic cells: skin cells, brain cells or neurons etc.
In contrast to mitosis, a cell divides into four daughter cells after meiosis and each daughter cell carries half the number of chromosomes than what were present in mother cell/parent cell.
Meiosis occurs in germ cells and germ cell precursors to produce respective immature germ cells such as spermatogonia, spermatids, oocytes etc.
During fertilization mature sperm penetrates mature egg and becomes one cell known as zygote; unicellular zygote now carries the same number of chromosomes as present in other body cells of its two parents.
In zygote, chromosomes from sperm and egg pair: for example chromosome 4 from sperm of the father would pair with chromosome 4 of egg of the mother.
This zygote would grow into an embryo primarily by cell-proliferation and differentiation leading to a complex organism such as you and I or your favorite pet or plant!
Cell-proliferation uses mitosis, however, the zygote, embryo or fetus (humans, primates) would form future precursor germ cells of the organism. These are formed by meiosis where paired chromosome might undergo crossover to exchange genetic information.
Hope it helps, if anything is unclear then post again or email me. I shall explain or may send you an attachment with diagram.
Dr. Naina B.
Naina B.
Fertilization is random, so far there is no evidence of genotype affecting fertilization in singe pool of sperms.
Independent assortments of homologous chromosomes implies to the fact that homologous chromosomes do not affect each other during cell division, each chromosome would go to one pole of mitotic spindle but their assortment is an independent event.
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11/28/14
Hannah K.
could you please send me diagrams to support the explanation and the last two points are not very clear so could you explain further please.
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11/29/14
Naina B.
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11/29/14
Naina B.
Fertilization occurs when sperm penetrates egg, sperms swim around the egg and randomly one sperm is able to penetrate. There is no particular that would promote one sperm over other. All sperms would have same number of chromosomes
but they may carry different alleles of genes. These genes or genotype of the sperm has no effect on its ability to fertilize an egg. Any sperm can fertilize the egg and that is known as random fertilization.
I could not paste diagram here, you can look at cell division pictures at the following link:
publications.nigms.nih.gov/insidethecell/ch4_phases_allbig.html
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11/29/14
Muniba P.
If meisis occur before fertilization then how crossing over occur after fertilization
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07/04/21
Hannah K.
11/28/14