The oocytes will reenter meiosis only when they are ovulated in response to hormones. Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase. Those germ cells that enter meiosis become oocytes, the source of future eggs. Female germ cells, or oogonia, stop dividing and enter meiosis within the fetal ovary. On the other hand, meiosis occurs with quite different kinetics in mammalian females. Even then, only limited numbers of spermatogonia enter meiosis at any one time, such that adult males retain a pool of actively dividing spermatogonia that acts as a stem cell population. Male germ cells, or spermatogonia, do not enter meiosis until after puberty. In mammals, the timing of meiosis differs greatly between males and females (Figure 2). Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis. The germ cells reside in specialized environments provided by the gonads, or sex organs. In most multicellular organisms, meiosis is restricted to germ cells that are set aside in early development. These controls have been strongly conserved during evolution, so such yeast experiments have provided valuable insight into meiosis in multicellular organisms as well. By analyzing yeast mutants that are unable to complete the various events of meiosis, investigators have been able to identify some of the molecules involved in this complex process. The entry of yeast into meiosis is a highly regulated process that involves significant changes in gene transcription (Lopez-Maury et al., 2008). The commitment to meiosis enhances the probability that the next generation will survive, because genetic recombination during meiosis generates four reproductive spores per cell, each of which has a novel genotype. When nutrients become limited, however, yeast enter meiosis. When conditions are favorable, yeast reproduce asexually by mitosis.

Meiosis represents a survival mechanism for some simple eukaryotes such as yeast. More recently, researchers have been able to identify some of the molecular players in meiosis from biochemical analyses of meiotic chromosomes and from genetic studies of meiosis-specific mutants. Then, in the 1950s, electron microscopy provided scientists with a glimpse of the intricate structures formed when chromosomes recombine. Researchers' initial understanding of meiosis was based upon careful observations of chromosome behavior using light microscopes. As a result, the gametes produced during meiosis are genetically unique. Meiosis also differs from mitosis in that it involves a process known as recombination, during which chromosomes exchange segments with one another. To accomplish this feat, meiosis, unlike mitosis, involves a single round of DNA replication followed by two rounds of cell division (Figure 1). Because the chromosome number of a species remains the same from one generation to the next, the chromosome number of germ cells must be reduced by half during meiosis. Meiosis, from the Greek word meioun, meaning 'to make small,' refers to the specialized process by which germ cells divide to produce gametes. We have meiosis to thank for this variety. Furthermore, with few exceptions, each individual in a population of sexually reproducing organisms has a distinct genetic composition. Furthermore, chromatids from a maternal chromosome are said to be non-sister chromatids to the homologous paternal chromosome.Organisms that reproduce sexually are thought to have an advantage over organisms that reproduce asexually, because novel combinations of genes are possible in each generation. In other words, unreplicated and replicated chromosomes are considered single chromosomes, even though replicated chromosomes have two copies of DNA strands attached. Each pair of connected sister chromatids are called chromosomes: one maternal chromosome (with two sister chromatids) and one paternal chromosome (also with two sister chromatids). Each homologous pair of chromosomes has two pairs of sister chromatids. The replicated strands are called sister chromatids, which are identical copies of each other connected at the centromere. During the S phase, the unreplicated chromosomes replicate. These are known as unreplicated chromosomes. Prior to the S phase, each homologous pair of chromosomes (one paternal and one maternal chromosome of the same type) are unreplicated. It is important to be able to understand chromosomal terminology to fully understand meiosis. During interphase before meiosis begins, chromosomes duplicate and produce sister chromatids of each chromosome.

Prophase 1 meiosis