The advantage of sexual reproduction is that it generates genetic diversity, which can make a population of organism better able to survive harsh environmental conditions. Sexual reproduction is possible because of meiosis, which is the shuffling of genes within a cell before it divides into four sperm or eggs. However, mitosis is required in order for a multicellular organism to have the organs that sustain meiosis and sexual reproduction. Mitosis is one cell dividing its nucleus and DNA into two cells that have the same amount of DNA as the original cell. Meiosis is one cell dividing into four cells that each has half the amount of DNA that as in the original cell.
Meiosis Produces Gametes
Meiosis is what produces the gametes, sperm or eggs that fuse after two organisms have sex. Gametes only have half the normal number of chromosomes, or strands of DNA, that a normal cell does. So, two of them must fuse in order to form a new cell that will develop into a new organism. In sexually reproducing organisms, gametes are only produced by meiosis, not mitosis. One advantage of meiosis is that the gametes of some organisms can be released into the environment, such as into the water or into the air. External fertilization is the process in which a sperm or pollen fertilizes an egg while both are outside of the mother.
Mitosis Builds Reproductive Organs
In multicellular organisms that have reproductive organs that do meiosis to produce gametes, the organs were built by cells that underwent mitosis. Thus, in these organisms meiosis is only possible because mitosis made organs that nurture the cells to undergo meiosis. In human females, this organ is called the ovary; in males, it is called the testicles. Organs are made of different layers or chambers of cells. Each layer or chamber has a different function. Within this hierarchy of tissues within an organ, meiosis is controlled and protected.
The Reproductive Endocrine System
The human reproductive system is controlled by the brain. The sperm are produced in the testicles, and the eggs are produced in the ovaries, but both of these organs receive commands from the brain. They also talk back to the brain in a process called feedback. The brain and the reproductive organs talk to each other by releasing endocrine hormones into the blood. Just as with the reproductive organs, the brain was formed by cells that underwent mitosis. In fact, the cells that produce the hormones in each organ were the result of mitosis, not meiosis.
Spermatogonia and Oogonia
Another importance of mitosis in sustaining meiosis is that the cells that undergo meiosis to produce gametes can also under mitosis. These cells undergo mitosis before meiosis so that they can make more copies of themselves. The more copies there are of them, the more gametes can be produced later on. In men, these cells are called spermatogonia. In women, they are called oogonia (oh-oh-go-knee-uh). Mitosis of spermatogonia is how a man can produce sperm even in old age. It is also how a woman has 400,000 eggs by the time she is born.