Basic Human Genetics 2
Say you need a new cell in your Big toe. A cell's 46 chromosomes will each make copies of themselves, they will line up, and the pairs will be divided, each set going to its own part of the original cell, where they will each form their own nucleus. The cell then divides into two, which each new cell having one of the new nuclei. The new cells will be genetic clones of each other. So mitosis is the kind of cell division that happens in your TOE, and most other parts of your body.
Sex cell division - "Meiosis"
Time to propagate the species!
In Women: In the ovaries, the precursor cell to the eggs divide, but instead of each chromosome making a copy of itself, each of the 22 pairs of autosomal chromosomes lines up with its mate. So, the Chromosome 1 you got from your father will line up with the Chr 1 you got from your mother. Then they will "cross-over", meaning that they will exchange pieces with each other. There will be at least one cross-over, sometimes 2 or more, so you end up with a Chromosome 1 that has some from your Dad, then your Mom, then your Dad, etc. The same goes for all the other Autosomal Chromosomes.
X chromosomes in women: there are 2, and they also cross-over.
Because they divided with out making copies of themselves, the egg only has 23 chromosomes.
Mitochondria: mitochondria also go into the egg.
In Men: In the testes, the precursor to the sperm does the same lining up and crossing over for the 22 autosomes
X and Y chromosomes in men: because they don't match, the X and the Y don't cross-over in men. They do hold onto each other at one end during this process though, which I think is kind of sweet.
Mitochondria: no mitochondria go into the sperm.
Sperm + Egg = Baby!! well not most of the time. Either the egg is not there at the right time, or the sperm, or if you get fertilization the uterus is often not receptive to implantation, or something went wrong in meiosis or...but that's why they call it "The miracle of life"!
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