Sex and reproduction

	Image: David M. Phillips, Visuals Unlimited, Inc. Human sperm and egg.

That's right, sex - it is the key to the human life cycle!

Sexual reproduction involves the merging of genetic material from different parents. It depends on reproductive cells (egg and sperm cells), each containing one half of the parent's genome. Such reproduction underlies evolution and genetic variation.

Certain species, including humans, need sexual reproduction to survive, and so not surprisingly, it is one of the most powerful of all drives.

For humans, each reproductive cell has 23 chromosomes (half the full set). Because these chromosomes come in pairs, when the cells combine genetic material, each chromosome from one parent must match up with the same chromosome from the other.

But before this combination takes place, the father and mother’s genes each line-up on their respective sides in a process called meiosis. This process guarantees variation in the reproductive cells. The chromosomes unite to scramble the genetic information before dividing in two again. They then form four reproductive cells, each with different genetic compositions. This is the process that makes you different from your sister or brother.