Misplaced Pages

#800199

98-519: In the schematic above, the rectangular blocks represent each of the repeated DNA sequences at a particular VNTR location. The repeats are in tandem – i.e. they are clustered together and oriented in the same direction. Individual repeats can be removed from (or added to) the VNTR via recombination or replication errors, leading to alleles with different numbers of repeats. Flanking regions are segments of repetitive sequence (shown here as thin lines), allowing

196-433: 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

294-493: 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,

392-519: A benefit to pathogenic bacteria by allowing repair of DNA damage, particularly damages that occur in the inflammatory, oxidizing environment associated with infection of a host. When two or more viruses, each containing lethal genomic damages, infect the same host cell, the virus genomes can often pair with each other and undergo HRR to produce viable progeny. This process, referred to as multiplicity reactivation, has been studied in lambda and T4 bacteriophages , as well as in several pathogenic viruses. In

490-449: A chromosome if they know the frequency of the crossovers. Geneticists can also use this method to infer the presence of certain genes. Genes that typically stay together during recombination are said to be linked . One gene in a linked pair can sometimes be used as a marker to deduce the presence of the other gene. This is typically used to detect the presence of a disease-causing gene. The recombination frequency between two loci observed

588-465: 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

686-433: 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

784-406: 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,

882-478: A form of recombination. Recombination also occurs in the reoviridae (dsRNA)(e.g. reovirus), orthomyxoviridae ((-)ssRNA)(e.g. influenza virus ) and coronaviridae ((+)ssRNA) (e.g. SARS ). Recombination in RNA viruses appears to be an adaptation for coping with genome damage. Switching between template strands during genome replication, referred to as copy-choice recombination, was originally proposed to explain

980-407: 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

1078-400: 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

SECTION 10

#1732858311801

1176-402: 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

1274-420: 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

1372-423: 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

1470-495: A repeat sequence of about ten to one hundred nucleotides, and the number of times the sequence repeats varies from about five to fifty times. The sequences of minisatellites are larger than those of microsatellites , in which the repeat sequence is generally 1 to 6 nucleotides. The two types of repeat sequences are both tandem but are specified by the length of the repeat sequence. VNTRs, therefore, because they have repeat sequences of ten to one hundred nucleotides in which every repeat

1568-399: 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),

1666-448: 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 ,

1764-542: A variety of exogenous agents (e.g. UV light , X-rays , chemical cross-linking agents) can be repaired by homologous recombinational repair (HRR). These findings suggest that DNA damages arising from natural processes , such as exposure to reactive oxygen species that are byproducts of normal metabolism, are also repaired by HRR. In humans, deficiencies in the gene products necessary for HRR during meiosis likely cause infertility In humans, deficiencies in gene products necessary for HRR, such as BRCA1 and BRCA2 , increase

1862-479: 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

1960-571: Is a common mechanism used in DNA repair . Gene conversion – the process during which homologous sequences are made identical also falls under genetic recombination. Genetic recombination and recombinational DNA repair also occurs in bacteria and archaea , which use asexual reproduction . Recombination can be artificially induced in laboratory ( in vitro ) settings, producing recombinant DNA for purposes including vaccine development. V(D)J recombination in organisms with an adaptive immune system

2058-416: 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

SECTION 20

#1732858311801

2156-596: Is a small probability of recombination at any location along a chromosome, the frequency of recombination between two locations depends on the distance separating them. Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between alleles. Tracking the movement of genes resulting from crossovers has proven quite useful to geneticists. Because two genes that are close together are less likely to become separated than genes that are farther apart, geneticists can deduce roughly how far apart two genes are on

2254-411: Is a type of site-specific genetic recombination that helps immune cells rapidly diversify to recognize and adapt to new pathogens . During meiosis, synapsis (the pairing of homologous chromosomes) ordinarily precedes genetic recombination. Genetic recombination is catalyzed by many different enzymes . Recombinases are key enzymes that catalyse the strand transfer step during recombination. RecA ,

2352-461: Is altered. Gene conversion has often been studied in fungal crosses where the 4 products of individual meioses can be conveniently observed. Gene conversion events can be distinguished as deviations in an individual meiosis from the normal 2:2 segregation pattern (e.g. a 3:1 pattern). Recombination can occur between DNA sequences that contain no sequence homology . This can cause chromosomal translocations , sometimes leading to cancer. B cells of

2450-400: 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

2548-555: Is called recombinant DNA . A prime example of such a use of genetic recombination is gene targeting , which can be used to add, delete or otherwise change an organism's genes. This technique is important to biomedical researchers as it allows them to study the effects of specific genes. Techniques based on genetic recombination are also applied in protein engineering to develop new proteins of biological interest. Examples include Restriction enzyme mediated integration , Gibson assembly and Golden Gate Cloning . DNA damages caused by

2646-423: 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

2744-520: 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

2842-457: Is exactly the same, are considered minisatellites. However, while all VNTRs are minisatellites, not all minisatellites are VNTRs. VNTRs can vary in number of repeats from individual to individual, as where some non-VNTR minisatellites have repeat sequences that repeat the same number of times in all individuals containing the tandem repeats in their genomes. Genetic recombination Genetic recombination (also known as genetic reshuffling )

2940-621: 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

3038-578: 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

Variable number tandem repeat - Misplaced Pages Continue

3136-406: Is the crossing-over value . It is the frequency of crossing over between two linked gene loci ( markers ), and depends on the distance between the genetic loci observed. For any fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure ( chromosome ) tends to be constant, and the same is then true for the crossing-over value which is used in

3234-785: Is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes , genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) int er chromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes (random orientation of pairs of homologous chromosomes in meiosis I); & (2) int ra chromosomal recombination, occurring through crossing over. During meiosis in eukaryotes , genetic recombination involves

3332-449: 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

3430-601: Is well documented in male Drosophila melanogaster . The "Haldane-Huxley rule" states that achiasmy usually occurs in the heterogametic sex . Heterochiasmy occurs when recombination rates differ between the sexes of a species. In humans, each oocyte has on average 41.6 ± 11.3 recombinations, 1.63-fold higher than sperms. This sexual dimorphic pattern in recombination rate has been observed in many species. In mammals, females most often have higher rates of recombination. Numerous RNA viruses are capable of genetic recombination when at least two viral genomes are present in

3528-859: The Diamond Princess cruise, two mutations, 29736G > T and 29751G > T (G13 and G28) were located in Coronavirus 3′ stem-loop II-like motif (s2m) of SARS-CoV-2. Although s2m is considered an RNA motif highly conserved in 3' untranslated region among many coronavirus species, this result also suggests that s2m of SARS-CoV-2 is RNA recombination /mutation hotspot. SARS-CoV-2's entire receptor binding motif appeared, based on preliminary observations, to have been introduced through recombination from coronaviruses of pangolins . However, more comprehensive analyses later refuted this suggestion and showed that SARS-CoV-2 likely evolved solely within bats and with little or no recombination. Nowak and Ohtsuki noted that

3626-890: The CODIS database. When removed from surrounding DNA by the PCR or RFLP methods, and their size determined by gel electrophoresis or Southern blotting , they produce a pattern of bands unique to each individual. When tested with a group of independent VNTR markers, the likelihood of two unrelated individuals' having the same allelic pattern is extremely low. VNTR analysis is also being used to study genetic diversity and breeding patterns in populations of wild or domesticated animals. As such, VNTRs can be used to distinguish strains of bacterial pathogens. In this microbial forensics context, such assays are usually called Multiple Loci VNTR Analysis or MLVA . In analyzing VNTR data, two basic genetic principles can be used: Repetitive DNA , representing over 40% of

3724-440: 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

3822-560: The genomes of an asexual population tend to accumulate more deleterious mutations over time than beneficial or reversing mutations. Chromosomal crossover involves recombination between the paired chromosomes inherited from each of one's parents, generally occurring during meiosis . During prophase I (pachytene stage) the four available chromatids are in tight formation with one another. While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information. Because there

3920-420: The immune system perform genetic recombination, called immunoglobulin class switching . It is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG . In genetic engineering , recombination can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what

4018-474: 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

Variable number tandem repeat - Misplaced Pages Continue

4116-508: The poliovirus RNA-dependent RNA polymerase (RdRp) is able to carry out recombination. Recombination appears to occur by a copy choice mechanism in which the RdRp switches (+)ssRNA templates during negative strand synthesis. Recombination by RdRp strand switching also occurs in the (+)ssRNA plant carmoviruses and tombusviruses . Recombination appears to be a major driving force in determining genetic variability within coronaviruses, as well as

4214-449: 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

4312-498: 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

4410-453: The VNTR blocks to be extracted with restriction enzymes and analyzed by RFLP , or amplified by the polymerase chain reaction (PCR) technique and their size determined by gel electrophoresis . VNTRs were an important source of RFLP genetic markers used in linkage analysis (mapping) of diploid genomes. Now that many genomes have been sequenced , VNTRs have become essential to forensic crime investigations, via DNA fingerprinting and

4508-512: The ability of coronavirus species to jump from one host to another and, infrequently, for the emergence of novel species, although the mechanism of recombination in is unclear. In early 2020, many genomic sequences of Australian SARS‐CoV‐2 isolates have deletions or mutations (29742G>A or 29742G>U; "G19A" or "G19U") in the s2m, suggesting that RNA recombination may have occurred in this RNA element. 29742G("G19"), 29744G("G21"), and 29751G("G28") were predicted as recombination hotspots. During

4606-409: 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

4704-403: The arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events. Achiasmy is the phenomenon where autosomal recombination is completely absent in one sex of a species. Achiasmatic chromosomal segregation

4802-701: The bacterial RecA protein is RadA. Bacteria regularly undergo genetic recombination in three main ways: Sometimes a strand of DNA is transferred into the target cell but fails to be copied as the target divides. This is called an abortive transfer . In eukaryotes , recombination during meiosis is facilitated by chromosomal crossover . The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric alleles . The shuffling of genes brought about by genetic recombination produces increased genetic variation . It also allows sexually reproducing organisms to avoid Muller's ratchet , in which

4900-400: The case of pathogenic viruses, multiplicity reactivation may be an adaptive benefit to the virus since it allows the repair of DNA damages caused by exposure to the oxidizing environment produced during host infection. See also reassortment . A molecular model for the mechanism of meiotic recombination presented by Anderson and Sekelsky is outlined in the first figure in this article. Two of

4998-453: The chief recombinase found in Escherichia coli , is responsible for the repair of DNA double strand breaks (DSBs). In yeast and other eukaryotic organisms there are two recombinases required for repairing DSBs. The RAD51 protein is required for mitotic and meiotic recombination, whereas the DNA repair protein, DMC1 , is specific to meiotic recombination. In the archaea, the ortholog of

SECTION 50

#1732858311801

5096-535: 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

5194-596: 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

5292-412: The class of clustered tandem repeats that exhibit allelic variation in their lengths. VNTRs are a type of minisatellite in which the size of the repeat sequence is generally ten to one hundred base pairs. Minisatellites are a type of DNA tandem repeat sequence , meaning that the sequences repeat one after another without other sequences or nucleotides in between them. Minisatellites are characterized by

5390-469: 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

5488-619: The first months of the COVID-19 pandemic, such a recombination event was suggested to have been a critical step in the evolution of SARS-CoV-2's ability to infect humans. Linkage disequilibrium analysis confirmed that RNA recombination with the 11083G > T mutation also contributed to the increase of mutations among the viral progeny. The findings indicate that the 11083G > T mutation of SARS-CoV-2 spread during Diamond Princess shipboard quarantine and arose through de novo RNA recombination under positive selection pressure. In three patients on

5586-618: 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

5684-543: The four chromatids present early in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this model, is initiated by a double-strand break (or gap) shown in the DNA molecule (chromatid) at the top of the figure. Other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (caused by exposure to a cross-linking agent such as mitomycin C) can be repaired by HRR. Two types of recombinant product are produced. Indicated on

5782-443: 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

5880-518: 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".

5978-414: 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

SECTION 60

#1732858311801

6076-460: 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

6174-494: The human genome, is arranged in a bewildering array of patterns. Repeats were first identified by the extraction of Satellite DNA , which does not reveal how they are organized. The use of restriction enzymes showed that some repeat blocks were interspersed throughout the genome. DNA sequencing later showed that other repeats are clustered at specific locations, with tandem repeats being more common than inverted repeats (which may interfere with DNA replication). VNTRs are

6272-402: 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

6370-417: 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

6468-458: 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

6566-556: 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

6664-577: 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

6762-704: 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

6860-431: 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

6958-417: 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

7056-452: 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

7154-474: 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

7252-417: 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

7350-420: The origin of life ( abiogenesis ) is also the origin of biological evolution . They pointed out that all known life on earth is based on biopolymers and proposed that any theory for the origin of life must involve biological polymers that act as information carriers and catalysts. Lehman argued that recombination was an evolutionary development as ancient as the origins of life. Smail et al. proposed that in

7448-417: 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

7546-613: The pairing of homologous chromosomes . This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed) (see SDSA – Synthesis Dependent Strand Annealing pathway in Figure); or by the breaking and rejoining of DNA strands, which forms new molecules of DNA (see DHJ pathway in Figure). Recombination may also occur during mitosis in eukaryotes where it ordinarily involves

7644-399: 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

7742-730: 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

7840-626: The positive correlation of recombination events over short distances in organisms with a DNA genome (see first Figure, SDSA pathway). Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans. Especially in coronaviruses, recombination may also occur even among distantly related evolutionary groups (subgenera), due to their characteristic transcription mechanism, that involves subgenomic mRNAs that are formed by template switching. When replicating its (+)ssRNA genome ,

7938-542: The primordial Earth, recombination played a key role in the expansion of the initially short informational polymers (presumed to be RNA ) that were the precursors to life. [REDACTED]  This article incorporates public domain material from Science Primer . NCBI . Archived from the original on 2009-12-08. Meiosis This is an accepted version of this page Meiosis ( / m aɪ ˈ oʊ s ɪ s / ; from Ancient Greek μείωσις ( meíōsis )  'lessening', (since it

8036-413: The production of genetic maps . In gene conversion, a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed. Gene conversion occurs at high frequency at the actual site of the recombination event during meiosis . It is a process by which a DNA sequence is copied from one DNA helix (which remains unchanged) to another DNA helix, whose sequence

8134-447: 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

8232-427: The right side is a "crossover" (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a "non-crossover" (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two "Holliday junctions" indicated in the lower right of the figure by two X-shaped structures in each of which there is an exchange of single strands between

8330-426: The risk of cancer (see DNA repair-deficiency disorder ). In bacteria, transformation is a process of gene transfer that ordinarily occurs between individual cells of the same bacterial species. Transformation involves integration of donor DNA into the recipient chromosome by recombination. This process appears to be an adaptation for repairing DNA damages in the recipient chromosome by HRR. Transformation may provide

8428-430: The same host cell. Recombination is largely responsible for RNA virus diversity and immune evasion. RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among picornaviridae ( (+)ssRNA ) (e.g. poliovirus ). In the retroviridae ((+)ssRNA)(e.g. HIV ), damage in the RNA genome appears to be avoided during reverse transcription by strand switching,

8526-401: 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

8624-561: 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

8722-465: 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

8820-405: 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,

8918-452: 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,

9016-446: 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

9114-428: 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

9212-573: The two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway. The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to as "synthesis dependent strand annealing" (SDSA). Recombination events of the NCO/SDSA type appear to be more common than the CO/DHJ type. The NCO/SDSA pathway contributes little to genetic variation, since

9310-509: The two sister chromosomes formed after chromosomal replication. In this case, new combinations of alleles are not produced since the sister chromosomes are usually identical. In meiosis and mitosis, recombination occurs between similar molecules of DNA ( homologous sequences ). In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes

9408-413: 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

9506-401: 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

9604-404: 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

#800199