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75-507: [REDACTED] Look up translocation in Wiktionary, the free dictionary. Translocation may refer to: Chromosomal translocation , a chromosome abnormality caused by rearrangement of parts Robertsonian translocation , a chromosomal rearrangement in pairs 13, 14, 15, 21, and 22 Nonreciprocal translocation , transfer of genes from one chromosome to another PEP group translocation ,
150-433: 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. 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. Treatment of
225-514: A DNA double-strand break by reconnecting the originally broken ends, but when it acts inappropriately it may join ends incorrectly resulting in genomic rearrangements including translocations. In order for the illegitimate joining of broken ends to occur, the exchange partners DNAs need to be physically close to each other in the 3D genome . Meiotic Meiosis ( / m aɪ ˈ oʊ s ɪ s / ; from Ancient Greek μείωσις ( meíōsis ) 'lessening', (since it
300-493: A barrel shaped spindle. In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores,
375-465: 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. Also evidence for meiotic recombination , indicative of sexual reproduction , was found in G. intestinalis . Another example of organisms previously thought to be asexual are parasitic protozoa of
450-433: A diploid cell called the zygote . The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (the spermatozoa in males and ova in females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of
525-406: A diploid cell, which contains two copies of each chromosome, termed homologs . First, the cell undergoes DNA replication , so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections ( crossovers ) between the homologs. In the first meiotic division,
600-420: A gain or loss of genetic material, though they may be detected in prenatal diagnosis . However, carriers of balanced reciprocal translocations may create gametes with unbalanced chromosome translocations during meiotic chromosomal segregation . This can lead to infertility, miscarriages or children with abnormalities. Genetic counseling and genetic testing are often offered to families that may carry
675-419: A method used by bacteria for sugar uptake Twin-arginine translocation pathway , a protein export pathway found in plants, bacteria, and archaea Translocation (botany) , transport of nutrients through phloem Protein translocation , also called protein targeting, a process in protein biosynthesis Species translocation , movement of a species, by people, from one area to another Topics referred to by
750-407: A new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act. Meiosis uses many of
825-400: A new diploid organism. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. Meiosis occurs in all animals and plants. The result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there
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#1732891339180900-402: A pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents
975-420: A period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The result is the production of four haploid cells (n chromosomes; 23 in humans) from
1050-423: A prolonged G 2 -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
1125-399: A single haploid cell called the gamete . Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle. In the haplodiplontic life cycle (with sporic or intermediate meiosis),
1200-411: A slight risk of uniparental disomy 14 due to trisomy rescue . Some human diseases caused by translocations are: Chromosomal translocations between the sex chromosomes can also result in a number of genetic conditions, such as The International System for Human Cytogenetic Nomenclature (ISCN) is used to denote a translocation between chromosomes . The designation t(A;B)(p1;q2) is used to denote
1275-448: A total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells. 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 ,
1350-421: A translocation between chromosome A and chromosome B. The information in the second set of parentheses, when given, gives the precise location within the chromosome for chromosomes A and B respectively—with p indicating the short arm of the chromosome, q indicating the long arm, and the numbers after p or q refers to regions, bands and sub-bands seen when staining the chromosome with a staining dye . See also
1425-463: A translocation. Most balanced translocation carriers are healthy and do not have any symptoms. It is important to distinguish between chromosomal translocations that occur in germ cells , due to errors in meiosis (i.e. during gametogenesis ), and those that occur in somatic cells , due to errors in mitosis . The former results in a chromosomal abnormality featured in all cells of the offspring, as in translocation carriers. Somatic translocations, on
1500-479: Is crossed over , creating new combinations of code on each chromosome. 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. 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 . In meiosis, DNA replication
1575-456: Is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes . Two detached fragments of two different chromosomes are switched. Robertsonian translocation occurs when two non-homologous chromosomes get attached, meaning that given two healthy pairs of chromosomes, one of each pair "sticks" and blends together homogeneously. A gene fusion may be created when the translocation joins two otherwise-separated genes. It
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#17328913391801650-416: Is a reductional division) 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
1725-471: Is a risk of unbalanced gametes that lead to miscarriages or abnormal offspring. For example, carriers of Robertsonian translocations involving chromosome 21 have a higher risk of having a child with Down syndrome . This is known as a 'translocation Downs'. This is due to a mis-segregation ( nondisjunction ) during gametogenesis. The mother has a higher (10%) risk of transmission than the father (1%). Robertsonian translocations involving chromosome 14 also carry
1800-400: Is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores instead of gametes. When they germinate, these spores undergo repeated cell division by mitosis, developing into a multicellular haploid gametophyte generation, which then produces gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for
1875-423: Is cleaved, allowing the sister chromatids to segregate. 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 decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces
1950-536: Is detected on cytogenetics or a karyotype of affected cells . Translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes ). Reciprocal translocations are usually an exchange of material between non-homologous chromosomes and occur in about 1 in 491 live births. Such translocations are usually harmless, as they do not result in
2025-399: Is different from Wikidata All article disambiguation pages All disambiguation pages Chromosomal translocation In genetics , chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal , and Robertsonian translocation. Reciprocal translocation
2100-520: Is divided into meiosis I and meiosis II which are further divided into Karyokinesis I, Cytokinesis I, Karyokinesis II, and Cytokinesis II, respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is divided into three phases: Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing
2175-621: 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. 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
2250-578: Is likely mediated by oxidative stress leading to increased DNA damage. Meiosis occurs in eukaryotic life cycles involving sexual reproduction , consisting of the cyclical process of growth and development by mitotic cell division, production of gametes by meiosis and fertilization. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production. Cycling meiosis and fertilization events results in alternation between haploid and diploid states. The organism phase of
2325-449: Is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene. Most breaks are repaired without forming crossovers resulting in gene conversion . However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in
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2400-433: Is thought to mark the beginning of the field of radiation cytology, and led him to be called "the father of radiation cytology". The initiating event in the formation of a translocation is generally a double-strand break in chromosomal DNA . A type of DNA repair that has a major role in generating chromosomal translocations is the non-homologous end joining pathway. When this pathway functions appropriately it restores
2475-440: The diakinesis stage, from Greek words meaning "moving through". 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
2550-474: The meiotic spindle begins to form. Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore . Over time, the MTOCs merge until two poles have formed, generating
2625-449: The DNA of the germline . The repair process used appears to involve homologous recombinational repair Prophase I arrested oocytes have a high capability for efficient repair of DNA damage , particularly exogenously induced double-strand breaks. DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility . Genetic recombination can be viewed as fundamentally
2700-498: 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
2775-409: The ability to reproduce by parthenogenesis . Meiosis does not occur in archaea or bacteria , which generally reproduce asexually via binary fission . However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin. Meiosis was discovered and described for
2850-411: The centromeres of two acrocentric chromosomes. The reciprocal exchange of parts gives rise to one large metacentric chromosome and one extremely small chromosome that may be lost from the organism with little effect because it contains few genes. The resulting karyotype in humans leaves only 45 chromosomes, since two chromosomes have fused together. This has no direct effect on the phenotype, since
2925-535: The chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II. Meiosis I and II are each divided into prophase , metaphase , anaphase , and telophase stages, similar in purpose to their analogous subphases in
3000-596: The chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids ), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes. The first stage of prophase I is the leptotene stage, also known as leptonema , from Greek words meaning "thin threads". In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within
3075-493: The definition of a genetic locus . The translocation is the mechanism that can cause a gene to move from one linkage group to another. In 1938, Karl Sax , at the Harvard University Biological Laboratories, published a paper entitled "Chromosome Aberrations Induced by X-rays", which demonstrated that radiation could induce major genetic changes by affecting chromosomal translocations. The paper
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3150-469: The exchange of genetic information. The exchange of information between the homologous 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 an ordinary light microscope, and chiasmata are not visible until
3225-618: The first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig . It was described again in 1883, at the level of chromosomes , by the Belgian zoologist Edouard Van Beneden , in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann , who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if
3300-443: The genus Leishmania , which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process. 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. Dacks and Rogers proposed, based on a phylogenetic analysis, that facultative sex
3375-518: The homologous chromosomes become much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex . Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes. The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen ), also known as pachynema , from Greek words meaning "thick threads".
3450-414: The homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of ~400 nm in mice). Leptotene is followed by the zygotene stage, also known as zygonema , from Greek words meaning "paired threads", which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. In this stage
3525-460: The homologs are segregated to separate daughter cells by the spindle apparatus . The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and three small polar bodies. Because of recombination, an individual chromatid can consist of
3600-402: The idiosyncratic rendering "maiosis": We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming . The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek . Meiosis
3675-417: The life cycle can occur either during the diploid state ( diplontic life cycle), during the haploid state ( haplontic life cycle), or both ( haplodiplontic life cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells. In the diplontic life cycle (with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from
3750-458: 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 spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to produce
3825-556: 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
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#17328913391803900-577: 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); 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. However, such physical exchange does not always occur during meiosis. In
3975-704: The metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation ). Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of
4050-431: The mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II). During meiosis, specific genes are more highly transcribed . In addition to strong meiotic stage-specific expression of mRNA , there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating
4125-417: The next stage. During the diplotene stage, also known as diplonema , from Greek words meaning "two threads", the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at
4200-452: The nucleus. The chromosomes each form a linear array of loops mediated by cohesin , and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice). This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade
4275-474: The number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster , which helped to establish that genetic traits are transmitted on chromosomes. The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using
4350-466: The only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number (nucleolar organiser genes). Robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. The most common translocation in humans involves chromosomes 13 and 14 and is seen in about 0.97 / 1000 newborns. Carriers of Robertsonian translocations are not associated with any phenotypic abnormalities, but there
4425-417: The oocytes of the silkworm Bombyx mori , meiosis is completely achiasmate (lacking crossovers). Although synaptonemal complexes are present during the pachytene stage of meiosis in B. mori , crossing-over homologous recombination is absent between the paired chromosomes . Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at
4500-453: The other hand, result in abnormalities featured only in the affected cell and its progenitors, as in chronic myelogenous leukemia with the Philadelphia chromosome translocation. Nonreciprocal translocation involves the one-way transfer of genes from one chromosome to another nonhomologous chromosome. Robertsonian translocation is a type of translocation caused by breaks at or near
4575-417: The paired 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. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along with
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#17328913391804650-399: 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. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. Sister chromatids remain attached during telophase I. Cells may enter
4725-730: The ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division . Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice ). During prophase I, homologous maternal and paternal chromosomes pair, synapse , and exchange genetic information (by homologous recombination ), forming at least one crossover per chromosome. These crossovers become visible as chiasmata (plural; singular chiasma ). This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of
4800-447: The prophase I stage of meiosis. 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
4875-401: The same mechanisms as mitosis , the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores : haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers , do not have the ability to carry out meiosis and have acquired
4950-561: 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 . It is an essential process for oogenesis and spermatogenesis . Although the process of meiosis is related to the more general cell division process of mitosis , it differs in two important respects: usually occurs between identical sister chromatids and does not result in genetic changes Meiosis begins with
5025-465: 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
5100-423: The same term [REDACTED] This disambiguation page lists articles associated with the title Translocation . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Translocation&oldid=1063793768 " Category : Disambiguation pages Hidden categories: Short description
5175-405: The second meiotic division. In metaphase II , the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. 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 , in which the remaining centromeric cohesin, not protected by Shugoshin anymore,
5250-452: The sister chromatids from separating. This allows the sister chromatids to remain together while homologs are segregated. The first meiotic division effectively ends when the chromosomes 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. Cytokinesis,
5325-446: The transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell. In human fetal oogenesis , all developing oocytes develop to this stage and are arrested in prophase I before birth. This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Chromosomes condense further during
5400-428: The two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II. In prophase II , we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for
5475-413: The ultimate meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis. Meiosis I segregates homologous chromosomes , which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because
5550-401: The yeast Schizosaccharomyces pombe with hydrogen peroxide increased the frequency of mating and the formation of meiotic spores by 4 to 18-fold. Volvox carteri , a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex. This induction can be inhibited by antioxidants indicating that the induction of meiotic sex by heat shock
5625-404: Was likely present in the common ancestor of eukaryotes. 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
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