Meiosis

  

Meiosis (/maɪˈoʊsɨs/) is a special type of cell division necessary for sexual reproduction in eukaryotes. The cells produced by meiosis are gametes or spores. In many organisms, including all animals and land plants (but not some other groups such as fungi), gametes are called sperm and egg cells.
   Whilst the process of meiosis bears a number of similarities with the 'life-cycle' cell division process of mitosis, it differs in two important respects: the chromosomes in meiosis undergo a recombination which shuffles the genes producing a different genetic combination in each gamete, compared with the co-existence of each of the two separate pairs of each chromosome (one received from each parent) in each cell which results from mitosis. 
   The outcome of meiosis is potentially four (genetically unique) haploid cells, compared with the two (genetically identical) diploid cells produced from mitosis. Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father. The cell divides twice, potentially producing up to four haploid cells containing one copy of each chromosome. ("Potentially" because in some cases, such as the formation of oocytes in mammals, only one of the possible four haploid cells survives.) In animals the haploid cell resulting from meiosis is a male or female gamete. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, resulting in offspring that are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations. This genetic diversity can provide the variation of physical and behavioural attributes (phenotypes) upon which natural selection can act, but, as described below in Section 6, Origin and function of meiosis, the genetic diversity may be largely a by-product of the homologous recombination that is primarily employed for its DNA repair function during meiosis.
    It is also noteworthy that during meiosis, specific genes are more highly transcribed, and these are called the meiome, the term used in functional genomics for the meiotic transcriptome. Meiosis is a key feature for all sexually reproducing eukaryotes in which homologous chromosome pairing, synapse and recombination occur. In addition to strong meiotic stage-specific expression of mRNA (the meiome), however, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis.[3] Thus, both the meiome and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.
   Prior to the meiosis process the cell's chromosomes are duplicated by a round of DNA replication, creating from the maternal and paternal versions of each chromosome (homologs) two exact copies, sister chromatids, attached at the centromere region. In the beginning of meiosis the maternal and paternal homologs pair to each other. Then they typically exchange parts by homologous recombination leading to crossovers of DNA between the maternal and paternal versions of the chromosome. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids.
    After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. Upon fertilization, for example when a sperm enters an egg cell, two gamete cells produced by meiosis fuse. The gamete from the mother and the gamete from the father each contribute one half of the set of chromosomes that make up the new offspring's genome.
   Meiosis uses many of the same mechanisms as mitosis, a type of cell division used by eukaryotes like plants and animals to split one cell into two identical daughter cells. In all plants and in many protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like Bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which generally reproduce via asexual processes such as binary fission. However, a similar "sexual" process, known as bacterial transformation, involves transfer of DNA from one bacterium to another and recombination of these DNA molecules of different parental origin.