Sexual Reproduction

All organisms create offspring, or reproduce, either through asexual or sexual reproduction. Asexual reproduction does not involve the fusion of gametes. Offspring are genetically identical to the parent and are usually formed when one organism replicates its genetic material and then divides itself in half. In sexual reproduction, male and female cells are formed and combine to form a new offspring. Each of these cells is called a gamete (egg or sperm) and is produced from cells called germ cells.

Sexual reproduction involves the fusion of two haploid (n) gametes. Each haploid cell has half the total chromosomes of the parent. When these haploid gametes join, they form a diploid (2n) zygote, having twice as many chromosomes as each gamete. The zygote has the entire complement of chromosomes the organism needs. In humans each gamete has 23 chromosomes. When these two cells come together, the resulting zygote has 46 chromosomes. The zygote will divide, copying each of those chromosomes every time, so that the full-grown person has 46 chromosomes in every one of his or her somatic, or body, cells. The germ cells, however, will again give rise to haploid gametes.

Germ cells produce male sperm cells or female egg cells through spermatogenesis and oogenesis, respectively. Spermatogenesis is the formation of sperm. Oogenesis is the process of egg cell formation. Spermatogenesis results in four individual sperm cells being produced from each existing cell. In all sexually reproducing organisms, far more sperm are produced (and subsequently released) than are needed to fertilize an egg. This happens because there are many obstacles sperm face in reaching the egg. In all species, sperm first have to find the egg (which may be far away, such as in some plants, or may be difficult to reach, such as in mammals) and then penetrate it, sometimes competing with other sperm to get there. By producing far more sperm than are needed, organisms increase the chances that at least one sperm cell will make it to the egg and fertilize it. In contrast, oogenesis results in only a single viable egg cell. More than one egg cell is produced, however all but one are inert, meaning they cannot be combined with a sperm to form a new organism.

Sperm and egg production happen throughout the reproductive lives of the organism. In humans, both sexes reach reproductive age around 12 years old. Males then continue producing sperm for the rest of their lives. While viability and mobility of the sperm does decrease with age (in all mammals), most males can have children up until death. Females, on the other hand, have a finite reproductive life. In humans, reproductive capability of females usually lasts until about 50 years old. Human females are born with all the eggs they will have. The human female will release one egg per month until she reaches a stage of life called menopause, when her body stops the cycle that releases viable eggs. This indicates the end of her reproductive life. Other mammals, such as chimpanzees and whales, go through similar changes.