Gamete
Gametes, from the
ancient Greek γαμετης (spouse), are the specialized
germ cells that come together during
fertilization (conception) in
organisms that reproduce
sexually. The creation of gametes is called
gametogenesis, in which
gametocytes divide by
meiosis into various gametes. 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 cell). 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 that 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.
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Gametogenesis (spermatogenesis, oogenesis) & fertilization
