Gametogenesis is a process in which an organism makes gametes, cells which are used in sexual reproduction. Each gamete contains half the genetic material of the parent organism. When it fuses with another gamete, it creates a complete set of DNA. This merger of two gametes can develop into a new organism in a series of multiplications and divisions. Organisms from blue whales to almond trees engage in gametogenesis so that they can reproduce.
Female organisms produce eggs or oocytes in a process known as oogenesis, while male organisms make sperm in spermatogenesis. Both processes rely on meiosis, a cell division technique which ends with four cells containing half the DNA of the parent cell. Such cells are known as haploid, in contrast with their diploid parent cells which contain a complete set of DNA. The two different gametogenesis techniques are very different, however.
The process starts in the gonads, organs specifically used for gametogenesis. Unique cells inside the gonads known as germ cells propagate themselves with meiosis. First the cells split in two, creating two smaller cells with half the DNA of the parent cell. Each of these daughter cells duplicates. At the end of meiosis, four haploid cells have been produced and these cells can mature into gametes and fertilize or be fertilized, depending on whether they are sperm or oocytes.
In males, gametogenesis happens on a constant basis. The germ cells steadily renew the supply of sperm to keep the host organism fertile. In females, gametogenesis is a somewhat more complicated process. Some of the work happens before birth as the body builds up a supply of immature oocytes which are held in a state of stasis. Once the organism reaches sexual maturity, individual oocytes mature one at a time. Fertilization is required to complete the maturation process. In humans and some other animals, unfertilized cells are periodically shed along with the uterine lining.
Something else interesting happens with gametogensis in females. The meiotic division results in a single gamete and three so-called "polar cells." This is the result of conservation of energy. Producing a fertile egg cell requires enough energy that it would not be possible to make four at a time. As a result, when the germ cell divides for the first time, it produces an immature oocyte and a polar cell. Both of these cells in turn replicate, with the polar cell making two more polar cells and the immature oocyte making a polar cell and another immature oocyte which will mature if it is fertilized. The polar cells, meanwhile, are reabsorbed by the body.