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Gamete


Gametes (in Greek: γαμέτες) —also known as sex cells, germ cells, or spores—are the specialized cells that come together during fertilization (conception) in organisms that reproduce sexually. In those species that produce two morphologically distinct types of gametes, and in which a particular individual produces only one type, "females" of the species produce the larger gamete called an ovum (or egg) and "males" produce the smaller gamete termed (in animals) a spermatozoon (or sperm). The equivalent "male" structure in higher plants is called a pollen grain. Organs that produce gametes are called gonads in animals, and archegonia or antheridia in plants.

Gametes are haploid cells; that is, they contain one complete set of chromosomes (the actual number varies from species to species). When two gametes unite (typically in animals, involving a sperm and an egg), they form a zygote—a cell having two complete sets of chromosomes and therefore diploid. The zygote cell receives one set of chromosomes from each of the two gametes involved in the union. After fusion of the two gamete nuclei, and after multiple cell divisions and cellular differentiation, a zygote develops, first into an embryo, and ultimately into a mature individual capable of producing gametes. Gametes from a mature diploid individual will be produced in the gonadal tissue through meiosis—a process of cellular division that reduces the number of sets of chromosomes from two to one (i.e., produces haploid gametes).

The diploid somatic cells of an individual will contain one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg—that is, the cells of the offspring will have genes expressing characteristics of both the "father" and the "mother". A gamete's chromosomes are not exact duplicates of either of the sets of chromosomes carried in the somatic cells of the individual that produced the gametes. They can be "hybrids" produced through crossover (a form of genetic recombination) of chromosomes, something that takes place in meiosis. This hybridization has a random element and the chromosomes tend to be a little different in every gamete an individual produces. This recombination, and the fact that the two chromosome sets ultimately have come from either a grandmother or a grandfather on each parental side, account for the genetic dissimilarity of siblings.

Gamete size and evolution

Isogamy occurs when gametes from both sexes are the same size. Anisogamy or heterogamy is the condition where females and males produce gametes of different sizes. Isogamy is considered to be the ancestral condition, the evolution of gametes of unequal size is a current area of evolutionary research.

Reference

  • Randerson, J.P., and Hurst, L.D. 2001. The uncertain evolution of the sexes. Trends in Ecology & Evolution 16(10):571-579

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