Meiosis
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Reverted edits by [[Special:Contributions/121.97.162.18|121.97.162.18]] ([[User talk:121.97.162.18|talk]]) to last version by ClueBot
{{for|the [[figure of speech]]|meiosis (figure of speech)}}
{{distinguish|miosis}}
In [[biology]] or life science, '''meiosis''' (pronounced my-oh-sis) is a process of reduction division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of [[gamete]]s. The word "meiosis" comes from the Greek verb '''meioun''', meaning "to make small," since it results in a reduction in chromosome number in the gamete cell.
Meiosis is essential for [[sexual reproduction]] and therefore occurs in all [[eukaryote]]s (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the [[Bdelloid]] [[rotifers]], have lost the ability to carry out meiosis and have acquired the ability to reproduce by [[parthenogenesis]]. Meiosis does not occur in [[archaea]] or [[bacteria]], which reproduce via asexual processes such as [[mitosis]] or [[binary fission]]. Each cell has half the number of chromosomes as the parent cell.
During meiosis, the [[genome]] of a [[diploid]] [[germ cell]], which is composed of long segments of [[DNA]] packaged into [[chromosome]]s, undergoes DNA replication followed by two rounds of division, resulting in four [[haploid]] cells. Each of these cells contain one complete set of [[chromosomes]], or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during [[fertilization]] to create a new diploid cell, or [[zygote]] before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo [[genetic recombination]] during meiosis, each gamete, and thus each zygote, will have a unique genetic ''blueprint'' encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.
In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. In these groups, gametes are produced by mitosis.
Meiosis uses many of the same biochemical mechanisms employed during [[mitosis]] to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and genetic recombination between [[Homologous_chromosome|homologous chromosomes]].
==History==
Meiosis was discovered and described for the first time in [[sea urchin]] [[egg (biology)|egg]]s in [[1876]], by noted German biologist [[Oscar Hertwig]] (1849-1922). It was described again in [[1883]], at the level of chromosomes, by [[Belgian]] zoologist [[Edouard Van Beneden]] (1846-1910), in ''[[Ascaris]]'' worms' eggs. The significance of meiosis for reproduction and inheritance, however, was described only in [[1890]] by [[Germany|German]] biologist [[August Weismann]] (1834-1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In [[1911]] the [[United States|American]] geneticist [[Thomas Hunt Morgan]] (1866-1945) observed [[chromosomal crossover|crossover]] in ''[[Drosophila melanogaster]]'' meiosis and provided the first true genetics.
==Evolution==
Meiosis is thought to have appeared 1.4 billion years ago. The only supergroup of [[eukaryotes]] which does not have meiosis in all organisms is [[excavata]]. The other five major supergroups, [[opisthokonts]], [[amoebozoa]], [[rhizaria]], [[archaeplastida]] and [[chromalveolates]] all seem to have genes for meiosis universally present, even if not always functional. Some [[excavate|excavata]] species do have meiosis which is consistent with the hypothesis that [[excavata]] is an ancient, [[paraphyletic]] grade. An example of eukaryotic organism in which meiosis does not exist is [[euglenoid]].
==Occurrence of meiosis in eukaryotic life cycles==
[[Image:gametic_meiosis.png|thumb|right|150px|Gametic life cycle.]]
[[Image:zygotic_meiosis.png|thumb|right|150px|Zygotic life cycle.]]
[[Image:sporic_meiosis.png|thumb|right|150px|Sporic life cycle.]]
{{main|Biological life cycle}}
Meiosis occur in eukaryotic life cycles involving [[sexual reproduction]], comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal [[mitosis|mitotic]] cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the [[germ cell]]s that continue in the life cycle. The rest of the cells, called [[somatic cell]]s, function within the organism and will [[death|die]] with it.
Cycling meiosis and fertilisation events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (''gametic life cycle''), or during the haploid state (''zygotic life cycle''), or both (''sporic life cycle'', in which there two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phase(s). In the ''gametic life cycle'', the species is diploid, grown from a diploid cell called the [[zygote]]. In the ''zygotic life cycle'' the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the [[gamete]]. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are [[spermatozoa]] for males or [[ova]] for females. These gametes then fertilize in the [[Fallopian tubes]] of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and [[differentiation]], turns into a [[blastocyst]] and then gets implanted in the [[uterus]] [[endometrium]] to create an [[embryo]].
In the ''gametic life cycle'', of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by [[mitosis]] to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells.
In the ''zygotic life cycle'', the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo [[mitosis]] to create the organism. Many [[Fungus|fungi]] and many [[protozoa]] are members of the zygotic life cycle.
Finally, in the ''sporic life cycle'', the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the [[alternation of generations]]. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.
==Process==
Because meiosis is a "one-way" process, it cannot be said to engage in a [[cell cycle]] as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle.
Interphase is divided into three phases:
*'''[[G1 phase|Growth 1 (G<sub>1</sub>) phase]]''': Immediately follows cytokinesis. This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1 stage each of the 46 human chromosomes consists of a single (very long) molecule of DNA. At this point cells are '''46,2N''', identical to somatic cells.
*'''[[S phase|Synthesis (S) phase]]''': The genetic material is replicated: each of its chromosomes duplicates ('''46,2N'''). The cell is still diploid, however, because it still contains the same number of [[centromere]]s. However, the identical sister chromatids are in the chromatin form because spiralisation and condensation into denser chromosomes have not taken place yet. It will take place in prophase I in meiosis.
*'''[[G2 phase|Growth 2 (G<sub>2</sub>) phase]]''': G2 phase is absent in Meiosis
Interphase is immediately followed by meiosis I and meiosis II. Meiosis I consists of '''segregating''' the [[homologous chromosome]]s from each other, then dividing the diploid cell into two haploid cells each containing one of the segregates. Meiosis II consists of decoupling each chromosome's sister strands ([[chromatid]]s), segregating the DNA into two sets of strands (each set containing one of each homologue), and dividing both haploid, duplicated cells to produce four haploid, unduplicated cells. Meiosis I and II are both divided into [[prophase]], [[metaphase]], [[anaphase]], and [[telophase]] subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G<sub>1</sub>, S, G<sub>2</sub>), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis generates genetic diversity in two ways: (1) independent assortment of chromosomes at both of the meiotic divisions allows genetic differences among gametes; and (2) physical exchange of chromosomal regions by homologous recombination during prophase I results in new genetic combinations within chromosomes.
===Meiosis I===
In meiosis I, the homologous pairs in a diploid cell separate , producing two haploid cells ('''46, N'''). The 46 chromosomes number is significant. A regular diploid cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromosomes it is only considered N because later in anaphase I the identical sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, a process similar to mitosis will occur whereby the sister chromatids are finally split, creating 2 haploid cells ('''23, N''').
====Prophase I====
Homologous chromosomes pair and crossing over, or recombination, occurs--a step unique to meiosis. Chromosomes form structures called synapses. The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata.
=====Leptotene=====
The first stage of prophase I is the ''leptotene'' stage, also known as ''leptonema'', from Greek words meaning "thin threads."<ref name="Snustad-Simmons">Principles of Genetics, Fourth Edition, John Wiley and Sons, Inc., 2006.</ref> During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane. Hence, this stage is called "bouquet stage".
=====Zygotene=====
The ''zygotene'' stage, also known as ''zygonema'', from Greek words meaning "paired threads,"<ref name="Snustad-Simmons"/> occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be ''[[Bivalent (genetics)|bivalent]]''. They may also be referred to as a ''tetrad'', a reference to the four sister chromatids. The two homologous chromosomes become "zipped" together, forming the [[synaptonemal complex]], in a process known as [[synapsis]].
=====Pachytene=====
The ''pachytene'' stage, also known as ''pachynema'', from Greek words meaning "thick threads,"<ref name="Snustad-Simmons"/> contains the following [[chromosomal crossover]]. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. ([[Sex chromosomes]], however, are not identical, and only exchange information over a small region of homology.) Exchange takes place at sites where ''recombination nodules'' or chiasmata (singular: [[chiasma (genetics)|chiasma]]) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
=====Diplotene=====
During the ''diplotene'' stage, also known as ''diplonema'', from Greek words meaning "two threads,"<ref name="Snustad-Simmons"/> the [[synaptonemal complex]] degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some [[transcription (genetics)|transcription]] of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at [[chiasma (genetics)|chiasmata]], the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in Anaphase I.
In fetal [[oogenesis]] all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the [[dictyotene|''dictyotene stage'']] and remains so until [[puberty]]. In males, only [[spermatogonia]] exist until meiosis begins at puberty.
=====Diakinesis=====
Chromosomes condense further during the ''diakinesis'' stage, from Greek words meaning "moving through."<ref name="Snustad-Simmons"/> This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles [[prometaphase]] of mitosis; the [[nucleoli]] disappear, the [[nuclear membrane]] disintegrates into vesicles, and the meiotic spindle begins to form.
=====Synchronous processes=====
During these stages, [[centrosomes]], each containing a pair of [[centrioles]] are migrating to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as [[microtubule]] organizing centers nucleating microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the [[kinetochore]]. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. <ref name="Raven-Johnson-Mason-Losos-Singer"> Raven, Peter H.; Johnson, George B.; Mason, Kenneth A.; Losos, Jonathan & Singer, Susan. Biology, Eighth Edition, McGraw-Hill, 2007.</ref><ref name="Petronczki-Siomos-Nasmyth"> Petronczki, Mark; Siomos, Maria F. & Nasmyth, Kim (2003-02-21). "Un Ménage à Quatre The Molecular Biology of Chromosome Segregation in Meiosis", ''Cell'' 112 (4): 423-40. [http://dx.doi.org/10.1016/S0092-8674(03)00083-7 doi:10.1016/S0092-8674(03)00083-7.]</ref>
Microtubules that attach to the kinetochores are known as ''kinetochore microtubules''. Other microtubules will interact with microtubules from the opposite centriole. These are also ''nonkinetochore microtubules''.
==Meiosis-phases==
====Metaphase I====
Homologous pairs move together along the phase plate:
as '''kinetochore microtubules''' from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate.
====Anaphase I====
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one functional unit of a pair of kinetochores<ref name="Petronczki-Siomos-Nasmyth"/>, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further a part. The cell elongates in preparation for division down the middle.
====Telophase I====
The last meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. This effect produces a variety of responses from the neuro-synchromatic enzyme, also known as NSE. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.
Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
telophase I contains no nucleus, two daughter cells, and chromatids remain attached.
===Meiosis II===
Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I. End result is production of four haploid cells ('''23,1N''') from the two haploid cells ('''46,1N''') produced in meiosis I.
'''Prophase II''' takes an [[inverse proportion|inversely proportional]] time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the [[nuclear envelope]] again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
In '''metaphase II''', the centromeres contain two kinetochores, that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
This is followed by '''anaphase II''', where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
The process ends with '''telophase II''', which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the microtubules. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete.
==The Significance of Meiosis==
Meiosis facilitates stable sexual reproduction. Without the halving of [[ploidy]], or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. [[Polyploidy]], the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality <ref>[http://www.bio.miami.edu/dana/104/104F02_15.html BIL 104 - Lecture 15<!-- Bot generated title -->]</ref>. Polyploidy is poorly tolerated in animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. Recombination and independent assortment allow for a greater diversity of genotypes in the population. As a system of creating diversity, meiosis allows a species to maintain stability under environmental changes.
==Nondisjunction==<!-- This section is linked from [[Klinefelter's syndrome]] -->
The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed ''disjunction''. When the separation is not normal, it is called '''nondisjunction'''. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for [[trisomy]] or [[monosomy]]. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during [[mitosis]].
This is a cause of several medical conditions in humans:
* [[Down Syndrome]] - trisomy of chromosome 21
* [[Patau Syndrome]] - trisomy of chromosome 13
* [[Edward Syndrome]] - trisomy of chromosome 18
* [[Klinefelter Syndrome]] - extra X chromosomes in males - ie XXY, XXXY, XXXXY
* [[Turner Syndrome]] - atypical X chromosome dosage in females - ie XO, XXX, XXXX
* [[XYY syndrome|XYY Syndrome]] - an extra Y chromosome in males
==Meiosis in humans==
In females, meiosis occurs in cells known as [[oogonia]] (singular: oogonium). Each oogonium that initiates meiosis will divide twice to form a single [[oocyte]] and three [[Polar body|polar bodies]]. However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the [[ovarian follicle|follicle]]. Follicles begin growth at a steady pace in a process known as [[folliculogenesis]], and a small number enter the [[menstrual cycle]]. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females occurs during [[oogenesis]], and differs from the typical meiosis in that it features a long period of meiotic arrest known as the [[Dictyate]] stage and lacks the assistance of [[centrosomes]].
In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the [[seminiferous tubule]]s of the [[testicles]]. Sperm is produced at a steady pace. The process of meiosis in males occurs during [[spermatogenesis]].
==References==
<references/>
==See also==
*[[Mitosis]]
*[[Ploidy]]
*[[Spermatogenesis]]
*[[Oogenesis]]
*[[Multigene family]]
*[[Allele]]
== External links ==
* [http://www.johnkyrk.com/meiosis.html Meiosis Flash Animation]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Meiosis.html Meiosis at Kimball's Biology Pages]
* [http://www.cellcycleontology.org CCO] The Cell-Cycle Ontology
[[Category:Cellular processes]]
[[Category:Molecular genetics]]
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