Meiosis
Meiosis (/maɪˈoʊsɪs/ ; from Ancient Greek μείωσις (meíōsis) 'lessening', (since it is a reductional division)[1][2] is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome.[3] Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome again.
For the figure of speech, see Meiosis (figure of speech). For the process whereby cell nuclei divide to produce two copies of themselves, see Mitosis. For excessive constriction of the pupils, see Miosis. For the parasitic infestation, see Myiasis. For muscle inflammation, see Myositis.
Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.[4]
In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell.[3] The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi (Χ)) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.
During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human
cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.
Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi.[5][6][7] It is an essential process for oogenesis and spermatogenesis.
Origin and function[edit]
Origin of meiosis[edit]
Meiosis appears to be a fundamental characteristic of eukaryotic organisms and to have been present early in eukaryotic evolution. Eukaryotes that were once thought to lack meiotic sex have recently been shown to likely have, or once have had, this capability. As one example, Giardia intestinalis, a common intestinal parasite, was previously considered to have descended from a lineage that predated the emergence of meiosis and sex. However, G. intestinalis has now been found to possess a core set of meiotic genes, including five meiosis specific genes.[33] Also evidence for meiotic recombination, indicative of sexual reproduction, was found in G. intestinalis.[34] Another example of organisms previously thought to be asexual are parasitic protozoa of the genus Leishmania, which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process.[35] Although amoeba were once generally regarded as asexual, evidence has been presented that most lineages are anciently sexual and that the majority of asexual groups probably arose recently and independently.[36] Dacks and Rogers[37] proposed, based on a phylogenetic analysis, that facultative sex was likely present in the common ancestor of eukaryotes.
Genetic variation[edit]
The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles, which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes);[38] and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes.[39]
Prophase I arrest[edit]
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.[40] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline.[40] The repair process used appears to involve homologous recombinational repair[40][41] Prophase I arrested oocytes have a high capability for efficient repair of DNA damage, particularly exogenously induced double-strand breaks.[41] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.[41]
Meiosis as an adaptation for repairing germline DNA[edit]
Genetic recombination can be viewed as fundamentally a DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA that is passed on to progeny.[42][43] Experimental findings indicate that a substantial benefit of meiosis is recombinational repair of DNA damage in the germline, as indicated by the following examples. Hydrogen peroxide is an agent that causes oxidative stress leading to oxidative DNA damage.[44] Treatment of the yeast Schizosaccharomyces pombe with hydrogen peroxide increased the frequency of mating and the formation of meiotic spores by 4 to 18-fold.[45] Volvox carteri, a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex.[46] This induction can be inhibited by antioxidants indicating that the induction of meiotic sex by heat shock is likely mediated by oxidative stress leading to increased DNA damage.[47]