The main event of conception is the uniting of the mother and father's chromosomes to form a new, unique human being. Bear with me for a moment while we delve into the fascinating world of your chromosomes.
Chromosomes: Your Genetic Blueprint
All of your body's cells contain chromosomes, which are packages of DNA strands; DNA holds the map of your genes. If you're a human, 46 is the magic number: we have 46 chromosomes, in 23 matched pairs. One chromosome in each pair was contributed by your father, and one by your mother. Every cell in your body contains your complete genetic blueprint, or your genome, in the 46 chromosomes in its nucleus.
A sex cell (egg or sperm), however, is an exception. Rather than a complete set of 23 pairs of chromosomes, an egg or sperm has only 23 single chromosomes. When the two unite, the chromosomes combine, giving the new cell the proper number of 46 chromosomes.
23 Chromosome Pairs
Take a look at the 46 chromosomes in one of your normal cells (not an egg or sperm). What a chromosome actually "looks like" is a complicated question, but we can represent them kind of like this:
Some chromosomes are larger than others, because they contain more DNA. All chromosomes are part of a matched pair, one from your mother, and one from your father (which I illustrated here by coloring them pink and blue). The pair is "matched" because they contain the same genes -- for example, both of your parents contributed a gene for eye color, and of the color of your eyes depends on which gene is dominant.
The Sex Chromosome: XX or XY
The last chromosome is different. It's called the sex chromosome, and it determines whether you are female or male. If you're female, we call this chromosome XX, and as you can see, it is a nicely matched pair like the rest of the chromosomes.
If you're male, the last pair of chromosomes is called XY, and they're not a matched pair. The X chromosome, contributed by the mother, matches up with a much smaller Y chromosome contributed by the father. For reasons that aren't fully understood, the X chromosome contains far more genetic material than the Y, and thus it is larger in size. In 2003, the Y chromosome made headlines around the world when it was mapped by the Human Genome Project.
By the way, the sex chromosome isn't "last" in a biological sense, but is just designated as #23 by convention. Chromosomes actually float around the cell nucleus haphazardly (or at least it seems that way to us), and are simply numbered largest-to-smallest.
Chromosomes in the Egg
23 Single Chromosomes
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 the 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, depending on whether the father's sperm contributes an X or a Y to pair with the mother's X.
Chromosomes in the Sperm
23 Single Chromosomes
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 spermatazoa" and "Y-chromosome-bearing spermatazoa", but I always just say X-sperm and Y-sperm.
If all this talk has put you into a chromosomal coma, wake up! This next part will be on the quiz.
Spermatogenesis: Equal X's and Y's For All!
It seems reasonable to think that if some men only have sons, they may only have Y sperm, or that fathers with all girls might have only X sperm. But in fact, years of testing have shown that virtually all men have a nearly equal number of X and Y sperm -- even men who have fathered only boys or girls.
The explanation for this is that the process by which sperm are formed -- spermatogenesis -- 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. Remember seeing cells divide in biology class?
This process of mitosis won't work for creating sperm cells,though, because sperm cells are special in two ways. First, they have only half the usual number of chromosomes. And second, they're each genetically unique, not exact copies like normal cells. (If sperm and egg cells were just identical copies, 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 (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 (this is the good part):
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.