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:''For the article on the [[figure of speech]], see [[meiosis (figure of speech)]].'' |
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[[Image:Mitosis-flourescent.jpg|thumb|300px|Light micrograph of a [[newt]] [[kidney]] cell in early [[anaphase]] of mitosis.]] |
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In [[biology]], '''meiosis''' is the process that transforms one [[diploid]] [[cell (biology)|cell]] into four [[haploid]] cells in [[eukaryote]]s in order to redistribute the diploid's cell's [[genome]]. |
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Meiosis forms the basis of [[sexual reproduction]] and can only occur in [[eukaryote]]s. In meiosis, the diploid cell's [[genome]], which is composed of ordered structures of coiled [[DNA]] called [[chromosome]]s, is replicated once and separated twice, producing four haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will [[fertilization|fertilize]] with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of separation by meiosis and [[genetic recombination]] through fertilization is called the ''life cycle''. The result is that the offspring produced during [[germination]] after meiosis will have a slightly different ''blueprint'' which has instructions for the cells to work, contained in the DNA. This allows sexual reproduction to occur. |
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In [[biology]], '''mitosis''' is the process by which a cell separates its duplicated [[genome]] into two identical halves. It is generally followed immediately by [[cytokinesis]] which divides the [[cytoplasm]] and [[cell membrane]]. This results in two identical daughter cells with a roughly equal distribution of [[organelle]]s and other cellular components. Mitosis and cytokinesis together is defined as the '''mitotic (M) phase''' of the [[cell cycle]], the [[cell division|division]] of the mother cell into two daughter cells, each the genetic equivalent of the parent cell. |
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Meiosis uses many biochemical processes that are similar to those used in [[mitosis]] in order to distribute chromosomes among the resulting cells; however the outcome is very different. |
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Mitosis occurs exclusively in [[eukaryote|eukaryotic]] cells. In multicellular organisms, the [[somatic]] cells undergo mitosis, while [[germ cell]]s — cells destined to become [[sperm]] in males or [[ovum|ova]] in females — divide by a related process called [[meiosis]]. [[Prokaryote|Prokaryotic]] cells, which lack a nucleus, divide by a process called [[binary fission]]. |
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==History== |
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Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "mitotic phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the [[fungus|fungi]] and [[slime mould]]s, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of [[Drosophila melanogaster|fruit fly]] embryonic development. |
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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 [[Germans|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 [[cross-over]] in [[Drosophila melanogaster]] meiosis and provided the first true genetic interpretation of meiosis. |
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==Occurrence of meiosis in eukaryotic life cycles== |
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==Overview== |
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[[Image:gametic_meiosis.png|thumb|right|150px|Gametic life cycle.]] |
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[[Image:MajorEventsInMitosis.jpg|right|thumb|350px|Mitosis divides genetic information during cell division]] |
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[[Image:zygotic_meiosis.png|thumb|right|150px|Zygotic life cycle.]] |
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[[Image:sporic_meiosis.png|thumb|right|150px|Sporic life cycle.]] |
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{{main|Biological life cycle}} |
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Meiosis occurs in all 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. |
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The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the [[zygote]]. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the [[gamete]]. Humans, for example, are diploid creatures. Human primordial germ cells (PGCs, a type of barely-pluripotent stem cell) undergo meiosis to create haploid gametes, which are [[sperm cell]]s for males or [[ova]] for females. These gametes then fertilize in the [[fallopian tube]] of the female before implantation in the uterus, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and [[differentiation]] to create an [[embryo]], the early stage of human life. |
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The primary result of mitosis is the division of the parent cell's genome into two daughter cells. The genome is comprised of a number of [[chromosome]]s, complexes of tightly-coiled [[DNA]] that contain [[DNA sequence|genetic information]] vital for proper cell function. Because each resultant daughter cell should be [[clone (genetics)|genetically identical]] to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the middle of [[interphase]], the period that precedes the mitotic phase in the cell cycle where preparation for mitosis occurs. |
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There are three types of life cycles that utilise sexual reproduction, differentiated by the location of the organisms stage. |
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Each chromosome now contains two identical copies of itself, called ''[[chromatid|sister chromatid]]s'', attached together in a specialized region of the chromosome known as the ''[[centromere]]''. Each sister chromatid is not considered a chromosome in itself. |
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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. |
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In animals and plants, the [[nuclear envelope]] that separates the DNA from the [[cytoplasm]] degrades, and its fluid spills out into the cytoplasm. The chromosomes align themselves in an imaginary diameter line spanning the cell. [[Microtubule]]s, essentially miniature strings, splay out from opposite ends of the cell and shorten, pulling apart the sister chromatids of each chromosome. As a matter of convention, each sister chromatid is now considered a chromosome, so they are renamed to ''sister chromosomes''. As the cell elongates, corresponding sister chromosomes are pulled toward opposite ends. A new nuclear envelope forms around the separated sister chromosomes. |
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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. [[Fungus|Fungi]] and many [[protozoa]] are members of the zygotic life cycle. |
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As mitosis completes cytokinesis is well underway. In [[animal cell]]s, the cell pinches inward where the imaginary line used to be, separating the two developing nuclei. In [[plant cell]]s, the daughter cells will construct a new dividing cell wall between each other. Eventually, the mother cell will be split in half, giving rise to two daughter cells, each with an equivalent and complete copy of the original genome. |
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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, and indeed its diagram supports this conclusion. |
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Recall that prokaryotic cells undergo a similar process to mitosis called binary fission. Prokaryotes cannot be properly said to undergo mitosis because they lack a nucleus and only have a single chromosome with no centromere. |
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==Chromosome segregation in meiosis == |
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==How mitosis distributes genetic information== |
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[[Image: |
[[Image:MajorEventsInMeiosis.jpg|thumb|350px|Meiosis produces four genetically varied gametes]] |
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A diploid cell contains a full set of chromosome pairs, each pair containing one chromosome from each parent. These chromosome pairs are called ''[[homologous chromosome]]s''. Homologous chromosomes need not be genetically identical. For example, one particular [[locus]] (location) on one of the father's chromosomes may code for blue eyes, while the same locus on the mother's chromosome may code for brown eyes. This [[genetic variation|genetic variety]] produced by [[sexual reproduction]] is the key to its power. |
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Before division the genome is [[DNA replication|replicated]]. Each chromosome now contains two identical sister chromatids joined together by a region of DNA called the [[centromere]]. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. There are four haploid cells produced at the conclusion of meiosis. |
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When DNA is replicated, each chromosome will make an identical copy of itself. The copies are called ''sister chromatids'', and together they are considered one chromosome. After separation, however, each sister chromatid is considered a full-fledged chromosome by itself. The two copies of the original chromosome are then called ''sister chromosomes''. |
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This description suggests that two out of four gametes will contain the maternal set of chromosomes, while the other two will contain the paternal set. In practice, however, the gametes are genetically varied, containing a mix of both paternal and maternal genetic information. This is accomplished in two processes. During meiosis I, genetic information is distributed through [[independent assortment]]. Homologous chromosomes will eventually part ways into separate cells. However, homologous chromosomes are oriented independently of their companions. That means that each daughter cell has a fifty-fifty chance of receiving the maternal chromosome or the paternal chromosome. At the same time during meiosis I, when the chromosomes are pairing up together for a short time before being separated during [[synapsis]], [[chromosomal crossover]] occurs. During this time, nonsister chromatids of homologous chromosomes may exchange segments at random locations called [[chiasma|chiasmata]]. The chromosome that is subjected to crossing over is then called a ''recombinant chromosome''. |
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Mitosis allocates one copy, and only one copy, of each sister chromosome to a daughter cell. Consider the diagram above, which traces the distribution of chromosomes during mitosis. The blue and red chromosomes are homologous chromosomes. After DNA replication during S phase, each homologous chromosome contains two sister chromatids. After mitosis, the sister chromatids become sister chromosomes and part ways, going to separate daughter cells. Homologous chromosomes are therefore kept together, resulting in the complete transfer of the parent's genome. |
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The diagram shown above summarizes the segregation of the meiotic chromosomes. Chromosomes which are the same size (one light blue and one red to parentage) are homologous to each other. They are replicated before meiosis so that each chromosome contains two genetically identical sister chromatids (the vertical bars of the H-like structure). Crossing over occurs between nonsister chromatids of the two homologous chromosomes. Homologous chromosomes are separated in meiosis I. In this case, each daughter cell receives one recombinant mother chromosome and recombinant father chromosome. Meiosis II separates the sister chromatids. At conclusion, four genetically varied gametes are produced. |
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"Sister chromosomes", "sister chromatids", "mother cells", "daughter cells", and "parent cells" have no actual gender. The use of feminine or masculine terminology is by scientific convention only. |
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==Phases of mitosis== |
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[[Image:Cell cycle.jpg|thumb|right|The cell cycle.]] |
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The mitotic phase is a relatively short action-packed period of the [[cell cycle]]. It alternates with the much longer ''[[interphase]]'', where the cell prepares itself for division. Interphase is divided into three phases, G1 (first gap), S (synthesis), and G2 (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and divides (M). |
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==Process== |
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Mitosis is a continual and dynamic process. For purposes of description, however, mitosis is conventionally broken down into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase. |
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Because meiosis is a "one-way" process, it cannot be said to engage in a [[cell cycle]] that 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. |
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[[Interphase]] is divided into three phases: |
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===Prophase=== |
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*'''[[G1 phase|Growth 1 (G<sub>1</sub>) phase]]''': Characterized by increasing cell size from accelerated manufacture of [[organelle]]s, [[protein]]s, and other cellular matter. |
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[[Image:Prophase.jpg|right|frame|'''Prophase:''' The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.]] |
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*'''[[S phase|Synthesis (S) phase]]''': The genetic material is replicated. |
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{{main|Prophase}} |
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*'''[[G2 phase|Growth 2 (G<sub>2</sub>) phase]]''': The cell continues to grow. |
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Normally, the genetic material in the nucleus is in a loosely bundled coil called [[chromatin]]. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the chromosomes have two sister chromatids, bound together at the [[centromere]] by the protein cohesin. Chromosomes are visible at high magnification through a light microscope. |
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It is immediately followed by meiosis I, which divides one diploid cell into two haploid cells by the separation of homologous chromosomes, and meiosis II, which divides two haploid cells into four haploid cells by the separation of sister chromatids. 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). |
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Just outside the nucleus are two [[centrosome]]s. Each centrosome, which was replicated earlier independent of mitosis, acts as a coordinating center for the cell's [[microtubule]]s. The two centrosomes sprout microtubules (which may be thought of as cellular ropes or poles) by polymerizing free-floating tubulin protein. By repulsive interaction of these microtubules with each other, the centrosomes push themselves to opposite ends of the cell (although new research has shown that there might be a mechanism inside the centromeres that also grab the microtubules and pull the chromatids apart). The network of microtubules is the beginning of the [[mitotic spindle]]. |
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===Meiosis I=== |
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Some centrosomes contain a pair of [[centriole]]s that may help organize microtubule assembly, but they are not essential to formation of the mitotic spindle. Plant cells that lack centrioles have no trouble undergoing mitosis. |
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====Prophase I==== |
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In the ''leptotene'' stage, the cell's genetic material, which is normally in a loosely arranged pile known as [[chromatin]], condenses into visible threadlike structures. Along the thread, [[centromere]]s are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called [[synapsis]], a protein structure called the [[synaptonemal complex]] attaches the homologous chromosomes tightly together all along their lengths. |
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The ''zygotene'' stage sees the completion of synapsis. The paired homologous chromosomes are said to be ''bivalent''. They may also be referred to as a ''tetrad'', a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear. |
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===Prometaphase=== |
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[[Image:Prometaphase.jpg|right|frame|'''Prometaphase:''' The nuclear membrane has degraded, and microtubules have invaded the nuclear space. These microtubules can attach to kinetochores or they can interact with opposing microtubules.]] |
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{{main|Prometaphase}} |
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The nuclear envelope dissolves and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Some [[protist]]s, such as [[algae]], undergo a variation called closed mitosis where the microtubules are able to penetrate an intact nuclear envelope. |
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The ''pachytene'' stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope. |
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Each chromosome forms two [[kinetochore]]s at the centromere, one attached at each chromatid. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome. Although the kinetochore is not fully understood, it is known that it contains a [[List_of_gene_families#Motor_proteins|molecular motor]]. When a microtubule connects with the kinetochore, the motor activates, using energy from [[Adenosine triphosphate|ATP]] to "crawl" up the tube toward the originating centrosome. The kinetochore provides the pulling force necessary to later separate the chromosome's two chromatids. |
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During the ''diplotene'' stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some [[transcription (genetics)|transcription]] of DNA. They are held together by virtue of ''recombination nodules'', betraying the sites of previous crossing over, the chiasmata. |
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When the spindle grows to sufficient length, ''kinetochore microtubules'' begin searching for kinetochores to attach to. A number of ''nonkinetochore microtubules'' find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle.CXV |
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Chromosomes recondense during the ''diakinesis'' stage. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles. |
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Prometaphase is sometimes considered part of prophase. |
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During these stages, [[centrioles]] are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout 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 two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis. |
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===Metaphase=== |
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[[Image:Metaphase.jpg|frame|right|'''Metaphase:''' The chromosomes have aligned at the metaphase plate.]] |
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{{main|Metaphase}} |
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As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene themselves on the ''metaphase plate'' or ''equatorial plane'', an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug of war between equally strong people. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. |
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Microtubules that attach to the kinetochores are known as ''kinetochore microtubules''. Other microtubules will interact with microtubules from the opposite centriole. These are called ''nonkinetochore microtubules''. |
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Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules, it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the ''mitotic spindle checkpoint''. |
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====Metaphase I==== |
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As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment, the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to either pole. |
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[[Image:Anaphase.jpg|frame|right|'''Early anaphase:''' Kinetochore microtubules shorten.]] |
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{{main|Anaphase}} |
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When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase. Two events occur in order: |
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====Anaphase I==== |
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# The proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids turned sister chromosomes are pulled apart by shortening kinetochore microtubules and toward the respective centrosomes to which they are attached. |
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Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two diploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in preparation for division down the middle. |
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# The nonkinetochore microtubules elongate, pushing the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. |
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====Telophase I==== |
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These two stages are sometimes called early and late anaphase. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations. |
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The first 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. 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. |
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Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II. |
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===Telophase=== |
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[[Image:Telophase.jpg|frame|right|'''Telophase:''' The pinching is known as the ''cleavage furrow''. Note the decondensing chromosomes.]] |
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{{main|Telophase}} |
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Telophase is a reversal of prophase and prometaphase events. It "cleans up" the aftereffects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. |
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=== |
===Meiosis II=== |
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'''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 chromatins. Centrioles move to the polar regions and are arranged by spindle fibers. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane. |
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Often (mistakenly) thought to be the same process as telophase, cytokinesis, if slated to occur, is usually well under way by this time. In animal cells, a [[cleavage furrow]] develops where the metaphase plate used to be, pinching off the separated nuclei. In plant cells, vesicles derived from the [[Golgi apparatus]] move along microtubules to the middle of the cell, coalescing into a cell plate that develops into a cell wall, separating the two nuclei. Each daughter cell has a complete copy of the genome of its parent cell. Mitosis is complete. |
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In '''metaphase II''', the centromeres contain two kinetochores, organizing fibers from the centrosomes on each side. This is followed by '''anaphase II''', where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles. |
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===Errors in mitosis=== |
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Although errors in mitosis are rare, the process may go wrong, especially during early cellular divisions in the [[zygote]]. Mitotic errors can be especially dangerous to the organism because future offspring from this parent cell will carry the same disorder. |
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The process ends with '''telophase II''', which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes. Meiosis is complete. |
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In ''non-disjunction'', a chromosome may fail to separate during anaphase. One daughter cell will receive both sister chromosomes and the other will receive none. This results in the former cell having three chromosomes coding for the same thing (two sisters and a homologous), a condition known as ''trisomy'', and the latter cell having only one chromosome (the homologous chromosome), a condition known as ''monosomy''. These cells are considered [[aneuploidy|aneuploidic]] cells. Aneuploidy can cause [[cancer]]. |
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==Significance of meiosis== |
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Mitosis is a traumatic process. The cell goes through dramatic changes in ultrastructure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Occasionally, chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing [[genetic deletion|deletion]]. The fragment may incorrectly reattach to another, non-homologous chromosome, causing [[translocation]]. It may reattach back to the original chromosome, but in reverse orientation, causing [[chromosomal inversion|inversion]]. Or, it may be treated erroneously as a separate chromosome, causing [[chromosomal duplication]]. The effect of these genetic abnormalities depend on the specific nature of the error. It may range from no noticeable effect at all to organism death. |
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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, may also results in developmental abnormalities sterility or lethality. However [[Polyploidy]] is a prominent feature of many crop plant genomes and is illustrated to have increased their robustness |
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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. A system of creating diversity (meiosis) allows a species to maintain stability under environmental change. |
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==Endomitosis== |
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Endomitosis is a variant of mitosis without nuclear or cellular division, resulting in cells with many copies of the same chromosome occupying a single nucleus. This process may also be referred to as [[endoreduplication]] and the cells as [[Ploidy | endoploid]]. |
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==Nondisjunction== |
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==Light micrographs of mitosis== |
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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]]. |
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Real mitotic cells can be visualized through the microscope by [[staining (biology)|staining]] them with [[fluorescent]] antibodies and [[dyes]]. These light micrographs are included below. |
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This is a cause of several medical conditions in humans, including: |
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<gallery> |
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*[[Down Syndrome]] - trisomy of chromosome 21 |
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*[[Patau Syndrome]] - trisomy of chromosome 13 |
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*[[Edward Syndrome]] - trisomy of chromosome 18 |
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*[[Klinefelter Syndrome]] - an extra X chromosome in males |
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*[[Turner Syndrome]] - only one X chromosome present |
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*[[XYY syndrome]] - an extra Y chromosome in males |
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==Meiosis in humans== |
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Image:Prophase-flourescent.jpg|'''Early prophase:''' Nonkinetochore microtubules, shown as green strands, have established a matrix around the degrading nucleus, in blue. The green nodules are the centrosomes. |
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In [[female|females]], meiosis occurs in precursor cells known as [[oogonia]] that divide twice into [[oocytes]]. These stem 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 is called [[oogenesis]]. |
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Image:Prometaphase-flourescent.jpg|'''Early prometaphase:''' The nuclear membrane has just degraded, allowing the microtubules to quickly interact with the kinetochores on the chromosomes, which have just condensed. |
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Image:Mitosis-flourescent.jpg|'''Early anaphase:''' The centrosomes have moved to the poles of the cell and have established the mitotic spindle. The chromosomes, in light blue, have been split by shortening kinetochore microtubules. |
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Image:Anaphase-flourescent.jpg|'''Anaphase:''' Lengthening nonkinetochore microtubules push the two sets of chromosomes further apart. |
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In [[male|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 is called [[spermatogenesis]]. |
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</gallery> |
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== |
==See also== |
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*[[Mitosis]] |
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* [http://www.scienceaid.co.uk/celldivision.html Science aid: Cell division, mitosis and meiosis]: A simple account of the process aimed at teens |
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*[[Spermatogenesis]] |
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*[[Oogenesis]] |
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{{chromo}} |
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==References== |
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*{{cite web |
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| author= Alberts B, Johnson A, Lewis J, Raff M, Roberts K, and Walter P |
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| year= 2002 |
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| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=mitosis+AND+mboc4%5Bbook%5D+AND+374238%5Buid%5D&rid=mboc4.section.3349 |
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| title= Mitosis |
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| format= |
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| work= Molecular Biology of the Cell |
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| publisher=Garland Science |
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| accessdate=January 22 |
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| accessyear=2006 |
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}} |
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* <cite |
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[[Category:Cell biology]] |
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style="font-style:normal" |
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id="{{{Ref|Reference-{{{Last|Title}}}-2001}}}"><span |
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class{{{Last|}}}Campbell N and Reece J="hiddenStructure" |
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>{{{Last|Campbell N and Reece J}}}</span><span |
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>]]</span><span |
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class="hiddenStructure" |
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>, </span><span |
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class{{{Date|}}}="hiddenStructure" |
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> ({{{Date|}}})</span><span |
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class2001="hiddenStructure" |
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> (<span |
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>December </span |
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>2001)</span><span |
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classCampbell N and Reece J{{{Last|}}}{{{Date|}}}2001="hiddenStructure" |
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>.</span><span |
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classThe Cell Cycle.="hiddenStructure" |
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> "The Cell Cycle."</span><span |
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class="hiddenStructure" |
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><i><span |
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class="hiddenStructure" |
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class6th ed.="hiddenStructure" |
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>, 6th ed.</span><span |
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classpp. 213-233="hiddenStructure" |
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>, pp. 213-233</span><span |
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classBenjamin Cummings/Addison-Wesley="hiddenStructure" |
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>, <span |
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classUpper Saddle River, NJ="hiddenStructure" |
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>Upper Saddle River, NJ: </span |
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>Benjamin Cummings/Addison-Wesley.</span><span |
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classISBN 0805366245="hiddenStructure" |
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> ISBN 0805366245.</span><span |
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<!-- if Title set (which is required) and Publisher and ID both unset --> |
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class{{{TiBenjamin Cummings/Addison-WesleyISBN 0805366245tle|}}}="hiddenStructure" |
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>.</span></cite> |
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*{{cite web |
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| author= Cooper G |
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| year= 2000 |
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| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=mitosis+AND+cooper%5Bbook%5D+AND+166086%5Buid%5D&rid=cooper.section.2470 |
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| title= The Events of M Phase |
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| format= |
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| work= The Cell: A Molecular Approach |
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| publisher=Sinaeur Associates, Inc |
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| accessdate=January 22 |
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| accessyear=2006 |
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}} |
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* <cite |
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style="font-style:normal" |
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id="{{{Ref|Reference-{{{Last|Title}}}-2002}}}"><span |
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class="hiddenStructure" |
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>[[|</span><span |
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class{{{Last|}}}Freeman S="hiddenStructure" |
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classFreeman S{{{Last|}}}{{{Date|}}}2002="hiddenStructure" |
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>.</span><span |
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classCell Division.="hiddenStructure" |
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> "Cell Division."</span><span |
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class="hiddenStructure" |
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> </span |
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><i><span |
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classhttp://www.prenhall.com/freeman/biology="hiddenStructure" |
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> [http://www.prenhall.com/freeman/biology </span |
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>Biological Science<span |
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classhttp://www.prenhall.com/freeman/biology="hiddenStructure" |
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class="hiddenStructure" |
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class="hiddenStructure" |
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classpp. 155-174="hiddenStructure" |
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>, pp. 155-174</span><span |
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classPrentice Hall="hiddenStructure" |
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>, <span |
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classUpper Saddle River, NJ="hiddenStructure" |
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>Upper Saddle River, NJ: </span |
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>Prentice Hall.</span><span |
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classISBN 0130819239="hiddenStructure" |
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> ISBN 0130819239.</span><span |
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<!-- if Title set (which is required) and Publisher and ID both unset --> |
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class{{{TiPrentice HallISBN 0130819239tle|}}}="hiddenStructure" |
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>.</span></cite> |
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*{{cite web |
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| author= Lodish H, Berk A, Zipursky L, Matsudaira P, Baltimore D, Darnell J |
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| year= 2000 |
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| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=Overview+of+the+Cell+Cycle+and+Its+Control+AND+106176%5Buid%5D&rid=mcb.section.3463 |
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| title= Overview of the Cell Cycle and Its Control |
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| format= |
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| work= Molecular Cell Biology |
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| publisher=W.H. Freeman |
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| accessdate=January 22 |
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| accessyear=2006 |
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}} |
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[[Category:Cell cycle|mitosis]] |
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[[Category:Mitosis|*]] |
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Revision as of 17:40, 27 May 2006
- For the article on the figure of speech, see meiosis (figure of speech).
In biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid's cell's genome.
Meiosis forms the basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell's genome, which is composed of ordered structures of coiled DNA called chromosomes, is replicated once and separated twice, producing four haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will fertilize with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of separation by meiosis and genetic recombination through fertilization is called the life cycle. The result is that the offspring produced during germination after meiosis will have a slightly different blueprint which has instructions for the cells to work, contained in the DNA. This allows sexual reproduction to occur.
Meiosis uses many biochemical processes that are similar to those used in mitosis in order to distribute chromosomes among the resulting cells; however the outcome is very different.
History
Meiosis was discovered and described for the first time in sea urchin eggs 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 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 American geneticist Thomas Hunt Morgan (1866-1945) observed cross-over in Drosophila melanogaster meiosis and provided the first true genetic interpretation of meiosis.
Occurrence of meiosis in eukaryotic life cycles
Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal 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 cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it.
The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the zygote. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human primordial germ cells (PGCs, a type of barely-pluripotent stem cell) undergo meiosis to create haploid gametes, which are sperm cells for males or ova for females. These gametes then fertilize in the fallopian tube of the female before implantation in the uterus, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation to create an embryo, the early stage of human life.
There are three types of life cycles that utilise sexual reproduction, differentiated by the location of the organisms stage.
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. 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, and indeed its diagram supports this conclusion.
Chromosome segregation in meiosis
A diploid cell contains a full set of chromosome pairs, each pair containing one chromosome from each parent. These chromosome pairs are called homologous chromosomes. Homologous chromosomes need not be genetically identical. For example, one particular locus (location) on one of the father's chromosomes may code for blue eyes, while the same locus on the mother's chromosome may code for brown eyes. This genetic variety produced by sexual reproduction is the key to its power.
Before division the genome is replicated. Each chromosome now contains two identical sister chromatids joined together by a region of DNA called the centromere. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. There are four haploid cells produced at the conclusion of meiosis.
This description suggests that two out of four gametes will contain the maternal set of chromosomes, while the other two will contain the paternal set. In practice, however, the gametes are genetically varied, containing a mix of both paternal and maternal genetic information. This is accomplished in two processes. During meiosis I, genetic information is distributed through independent assortment. Homologous chromosomes will eventually part ways into separate cells. However, homologous chromosomes are oriented independently of their companions. That means that each daughter cell has a fifty-fifty chance of receiving the maternal chromosome or the paternal chromosome. At the same time during meiosis I, when the chromosomes are pairing up together for a short time before being separated during synapsis, chromosomal crossover occurs. During this time, nonsister chromatids of homologous chromosomes may exchange segments at random locations called chiasmata. The chromosome that is subjected to crossing over is then called a recombinant chromosome.
The diagram shown above summarizes the segregation of the meiotic chromosomes. Chromosomes which are the same size (one light blue and one red to parentage) are homologous to each other. They are replicated before meiosis so that each chromosome contains two genetically identical sister chromatids (the vertical bars of the H-like structure). Crossing over occurs between nonsister chromatids of the two homologous chromosomes. Homologous chromosomes are separated in meiosis I. In this case, each daughter cell receives one recombinant mother chromosome and recombinant father chromosome. Meiosis II separates the sister chromatids. At conclusion, four genetically varied gametes are produced.
Process
Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle that 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:
- Growth 1 (G1) phase: Characterized by increasing cell size from accelerated manufacture of organelles, proteins, and other cellular matter.
- Synthesis (S) phase: The genetic material is replicated.
- Growth 2 (G2) phase: The cell continues to grow.
It is immediately followed by meiosis I, which divides one diploid cell into two haploid cells by the separation of homologous chromosomes, and meiosis II, which divides two haploid cells into four haploid cells by the separation of sister chromatids. 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 (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis I
Prophase I
In the leptotene stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called synapsis, a protein structure called the synaptonemal complex attaches the homologous chromosomes tightly together all along their lengths.
The zygotene stage sees the completion of synapsis. The paired homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.
The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata.
Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout 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 two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules.
Metaphase I
As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment, the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to either pole.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two diploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in preparation for division down the middle.
Telophase I
The first 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. 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. Note that many plants skip telophase I and interphase II, going immediately into prophase II.
Meiosis II
Prophase II takes an 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 chromatins. Centrioles move to the polar regions and are arranged by spindle fibers. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane.
In metaphase II, the centromeres contain two kinetochores, organizing fibers from the centrosomes on each side. This is followed by anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles.
The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes. Meiosis is complete.
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, may also results in developmental abnormalities sterility or lethality. However Polyploidy is a prominent feature of many crop plant genomes and is illustrated to have increased their robustness
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. A system of creating diversity (meiosis) allows a species to maintain stability under environmental change.
Nondisjunction
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, including:
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - an extra X chromosome in males
- Turner Syndrome - only one X chromosome present
- XYY syndrome - an extra Y chromosome in males
Meiosis in humans
In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the 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 is called oogenesis.
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 tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called spermatogenesis.