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.
But some spermatagonia will undergo meiosis, in which a single spermatagonium divides into not two, but four sperm cells. Here's an overview of what happens:
At first, the cell has the normal 46 chromosomes, scattered around the cell nucleus. (Pretend that the 6 chromosomes I've shown here are actually all 46. Pink chromosomes come from mom, blue from dad.)
The chromosome's DNA replicates itself (shown by the 2 black lines inside each blob in the picture). The cell still has the normal number of 46 chromosomes, but twice the DNA. So far, this is just like what happens during normal mitosis, but something new is about to happen.
The chromosomes next match themselves up in corresponding pairs, the only occasion they do so. Because this is a man, of course chromosome pair #23 is XY. (Again, you can pretend that pairs 3 through 22 are also shown here.)
Chromosome pairs exchange sections of DNA between themselves, in a process called crossover, thus mixing up genes from both parents. We now have new, unique chromosomes.
The cell divides! The new cells have 23 chromosomes each, and the genetic code is further shuffled by mixing and matching chromosomes into the new cells. One cell must get the X, and the other the Y, from the 23rd chromosome of the parent cell.
The new cells have the right number of chromosomes for a sperm cell, but still twice the DNA (from the replication at the beginning). So...
Each cell divides yet again, yeilding two X's and two Y's. Called spermatids, these little round cells will develop a midpiece and flagellum on their way to becoming mature sperm cells: 2 X-sperms, and 2 Y-sperms.
This whole process takes about 74 days, but don't that there won't be any sperm ready on the big day -- thousands of sperm mature every second.
When we last left the egg, it had been nurtured in its follicle in the ovary, and escorted into the fallopian tube to await the sperm. The sperm's life story, as we'll see, is dramatically different.
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