![]() ![]() 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. 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. ![]()
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