Now take a look at the chromosomes in an egg. We see only half of the usual number of chromosomes. Why didn't I color these chromosomes pink, since they all come from the mother? The answer is that the mother doesn't just pass along a copy of the chromosomes she received from her mother, but new, unique chromosomes that contain a mixture of the genes from both of her parents, ensuring that each oocyte, and each child, is genetically unique.
Notice that the last chromosome, #23, is a big X chromosome. Because the mother's own 23rd chromosome is XX, the egg's 23rd chromosome must always also be an X, because the mother only has X's to contribute. So any egg can become a baby boy or a baby girl |
Here are our friends the chromosomes again, this time from a sperm. And again, these chromosomes aren't a copy of any of the man's original chromosomes, but a unique mixture derived from each pair, bestowing traits from both of his parents to each sperm, and thus to each child.
Notice that the last chromosome, the sex chromosome, is either an X or a Y in a sperm cell. This is possible because a man has an XY pair as his own 23rd chromosome, so a choice is possible when a sperm cell is formed.
Technically, I should call the two types of sperm "X-chromosome-bearing sperm" and "Y-chromosome-bearing sperm", but I always just say X-sperm and Y-sperm.
Spermatogenesis: Equal X's and Y's
The process by which sperm are formed guarantees that an equal number of X and Y-sperm are produced; this is because X-sperm and Y-sperm aren't manufactured separately, but result from the division of an XY parent cell.
Normal cells in your body are reproducing all the time, by a process called mitosis: the cell's DNA replicates (makes an exact copy of itself), then divides down the middle, resulting in two cells that are identical to the original cell.
Obviously, mitosis won't work for creating sperm cells, because a sperm cell somehow has to wind up with only half the usual number of chromosomes. Further, even if you could somehow start out with such a cell, if it just made copies of itself, they would all be genetically identical; all of a couple's offspring would be clones of each other.
The answer is a specialized form of cell division called meiosis, used only in the formation of sperm cells (which is called spermatogenesis) and oocytes (oogenesis).
In the testes, sperm is produced by cells called spermatagonium. These cells reproduce themselves in the usual way, by mitosis, so that a man doesn't run out of them; after all, he'll be producing sperm his entire life, starting from puberty.
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Sperm ~ An Army on a Mission
After spending 74 days being formed in spermatogenesis, sperm spend another two weeks in "boot camp" -- becoming proficient swimmers as they wind their way through twisted, coiled tubes in the testis, and at last reach a holding area around the prostate gland.
The journey ahead of them is a long and dangerous one; a sperm must not only find the egg and survive to reach it, but beat out millions of other sperm to get there first!
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