query: when does crossing over take place in meiosis

pos: Chromosomal crossover Chromosomal crossover Chromosomal crossover (or crossing over) is the exchange of genetic material between 2 homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the "pachytene" stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. Crossing over was described, in theory, by Thomas Hunt Morgan. He relied

pos: Synapsis the purging of the detrimental. Following synapsis, a type of recombination referred to as synthesis dependent strand annealing (SDSA) occurs frequently. SDSA recombination involves information exchange between paired non-sister homologous chromatids, but not physical exchange. SDSA recombination does not cause crossing-over. Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double-strand breaks (see Genetic recombination). The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis. The processes of DNA repair and chiasma formation that take place following synapsis have consequences at many levels,

pos: Pseudoautosomal region they allow the X and Y chromosomes to pair and properly segregate during meiosis in males. Pseudoautosomal genes are found in two different locations: PAR1 and PAR2. These are believed to have evolved independently. in mice, some PAR1 genes have transferred to autosomes. Pairing (synapsis) of the X and Y chromosomes and crossing over (recombination) between their pseudoautosomal regions appear to be necessary for the normal progression of male meiosis. Thus, those cells in which X-Y recombination does not occur will fail to complete meiosis. Structural and/or genetic dissimilarity (due to hybridization or mutation) between the pseudoautosomal regions of the

pos: Homologous chromosome separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis II. The process of meiosis I is generally longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. During meiosis, genetic recombination (by random segregation) and crossing over produces daughter cells that each contain different combinations of maternally and paternally coded genes. This recombination of genes allows for the introduction of new allele pairings and genetic variation. Genetic variation among organisms

pos: Synapsis Synapsis Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex. Autosomes undergo synapsis during meiosis, and are held together

pos: Chromosome segregation the chromosomes initially present is now composed of two copies called chromatids. These chromosomes (paired chromatids) then pair with the homologous chromosome (also paired chromatids) present in the same nucleus (see prophase I in the meiosis diagram). The process of alignment of paired homologous chromosomes is called synapsis (see Synapsis). During synapsis, genetic recombination usually occurs. Some of the recombination events occur by crossing over (involving physical exchange between two chromatids), but most recombination events involve information exchange but not physical exchange between two chromatids (see Synthesis-dependent strand annealing (SDSA)). Following recombination, chromosome segregation occurs as indicated by the stages

neg: Unequal crossing over During meiosis, the duplicated chromosomes (chromatids) in eukaryotic organisms are attached to each other in the centromere region and are thus paired. The maternal and paternal chromosomes then align alongside each other. During this time, recombination can take place via crossing over of sections of the paternal and maternal chromatids and leads to reciprocal recombination or non-reciprocal recombination. Unequal crossing over requires a measure of similarity between the sequences for misalignment to occur. The more similarity within the sequences, the more likely unequal crossing over will occur. One of the sequences is thus lost and replaced with the duplication of

neg: Goniaea australasiae Goniaea australasiae Goniaea australasiae is a species of grasshopper in the family Acrididae. Experiments have been carried out with "G. australasiae" to study the time of genetic recombination in relation to the sequence of stages in meiosis, the relationship of chiasmata to crossing over, and the mechanism of recombination. Combined cytological and autoradiographic analyses of meiosis showed that crossing over is achieved by breakage and exchange of segments of nonsister, homologous chromatids, and each such exchange event results in the formation of a cytologically visible chiasma. The meiotic stage at which this form of recombination takes place has been identified

neg: Homologous chromosome the same in mitosis as they do in meiosis. Prior to every single mitotic division a cell undergoes, the chromosomes in the parent cell replicate themselves. The homologous chromosomes within the cell will ordinarily not pair up and undergo genetic recombination with each other. Instead, the replicants, or sister chromatids, will line up along the metaphase plate and then separate in the same way as meiosis II - by being pulled apart at their centromeres by nuclear mitotic spindles. If any crossing over does occur between sister chromatids during mitosis, it does not produce any new recombinant genotypes. Homologous pairing

neg: MSH4 viability patterns suggesting that segregation of non-exchange chromosomes occurred efficiently. Thus, in "S. cerevisiae", proper segregation apparently does not entirely depend on crossovers between homologous pairs. The "him-14" gene of the worm "Caenorhabditis elegans" encodes an ortholog of "MSH4". Formation of crossovers during "C. elegans" meiosis requires the "him-14(MSH4)" gene. Loss of him-14(MSH-4) function severely reduces crossing over, resulting in lack of chiasmata between homologs and consequent missegregation. Thus, in "C. elegans", segregation apparently does depend on crossovers between homologous pairs. Him-14(MSH4) functions during the pachytene stage of meiosis, indicating that it is not needed for establishing the preceding stages

neg: Chromosomal crossover a linkage structure (chromosome) tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic maps. There are two popular and overlapping theories that explain the origins of crossing-over, coming from the different theories on the origin of meiosis. The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing-over is a novel way to replace possibly damaged sections of DNA. The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity. In

neg: RecLOH Rather, one homologous segment "writes over" the other. The mechanism is presumed to be different from RecLOH events in autosomal chromosomes, since the target is the very same chromosome instead of the homologous one. During the mutation one of these copies overwrites the other. Thus the differences between the two are lost. Because differences are lost, heterozygosity is lost. Recombination on the Y-chromosome does not only take place during meiosis, but virtually at every mitosis when the Y chromosome condenses, because it doesn't require pairing between chromosomes. Recombination frequency even exceeds the frame shift mutation frequency (slipped strand mispairing) of

neg: Sordaria fimicola for an introductory biology lab class is to cross one of the mutant types with a wild type and observe the ratio of coloring in the offspring. This experiment illustrates the concepts of genetic inheritance in a haploid organism. The eight ascospores are produced inside an ascus. Sordaria squashes can give us information about crossing over during meiosis. If no crossing over then there is a 4:4 pattern. 4 black spores, and 4 tan spores all lined up. If crossing over does occur there is a 2:2:2:2 pattern visible, or a 2:4:2 pattern. Another common lab use is to observe

neg: Cytogenetics were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms. The lily is a favored organism for the cytological examination of meiosis since the chromosomes are large and each morphological stage of meiosis can be easily identified microscopically. Hotta et al. presented evidence for a common pattern of DNA nicking and repair synthesis in male meiotic cells of lilies and rodents during the zygotene–pachytene stages of meiosis when crossing over was presumed to occur. The presence of a common pattern between organisms as phylogenetically distant as lily and mouse led the authors to

neg: Haplodiploidy evolution of eusocial behavior in hymenopterans. A eusocial worker helping her mother birth more sisters propagates more of her own genes than had she reproduced herself. In normal sexual reproduction, the father has two sets of chromosomes, and crossing over takes place between the chromatids of each pair during the meiosis which produces the sperm. Therefore, "the sperms are not identical", because in each chromosome of a pair there will be different alleles at many of the loci. But when the father is haploid "all the sperms are identical" (except for a small number where gene mutations have taken place

neg: Mosaic (genetics) to mark the wild-type chromosome as above, GAL80 serves this purpose, so that when it is removed by mitotic recombination, GAL4 is allowed to function, and GFP turns on. This results in the cells of interest being marked brightly in a dark background. In 1929, Alfred Sturtevant studied mosaicism in "Drosophila". A few years later, In the 1930s, Curt Stern demonstrated that genetic recombination, normal in meiosis, can also take place in mitosis. When it does, it results in somatic (body) mosaics. These are organisms which contain two or more genetically distinct types of tissue. The term "somatic mosaicism" was

neg: Goniaea australasiae as "early pachytene," and it is well removed from premeiotic chromosome duplication. Goniaea australasiae Goniaea australasiae is a species of grasshopper in the family Acrididae. Experiments have been carried out with "G. australasiae" to study the time of genetic recombination in relation to the sequence of stages in meiosis, the relationship of chiasmata to crossing over, and the mechanism of recombination. Combined cytological and autoradiographic analyses of meiosis showed that crossing over is achieved by breakage and exchange of segments of nonsister, homologous chromatids, and each such exchange event results in the formation of a cytologically visible chiasma. The meiotic

neg: Parthenogenesis because at anaphase II the sister chromatids are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis after meiosis the offspring is completely homozygous and has only half the mother's genetic material. This can result in parthenogenetic offspring being unique from each other and from their mother. In apomictic parthenogenesis, the offspring are clones of the mother and hence (except for aphids) are usually female. In the case of aphids, parthenogenetically produced males and females are clones of their mother except that the males lack one of the X chromosomes (XO). When meiosis

neg: Gene family or polyploidization can be either autopolyploidization or alloploidization. Autopolyploidization is the duplication of the same genome and allopolyploidization is the duplication of two closely related genomes or hybridized genomes from different species. Duplication occurs primarily through uneven crossing over events in meiosis of germ cells. (1,2) When two chromosomes misalign, crossing over - the exchange of gene alleles - results in one chromosome expanding or increasing in gene number and the other contracting or decreasing in gene number. The expansion of a gene cluster is the duplication of genes that leads to larger gene families. Gene members of a multigene

neg: Meiosis meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Chromosomes condense further during 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 the meiotic spindle begins to form. During these stages, two centrosomes, containing a pair of centrioles in animal

neg: Unequal crossing over Unequal crossing over Unequal crossing over is a type of gene duplication or deletion event that deletes a sequence in one strand and replaces it with a duplication from its sister chromatid in mitosis or from its homologous chromosome during meiosis. It is a type of chromosomal crossover between homologous sequences that are not paired precisely. Normally genes are responsible for occurrence of crossing over. It exchanges sequences of different links between chromosomes. Along with gene conversion, it is believed to be the main driver for the generation of gene duplications and is a source of mutation in the genome.

neg: Mating of yeast type switching”, above). The relative rarity in nature of meiotic events that result from out-crossing appears to be inconsistent with the idea that production of genetic variation is the primary selective force maintaining mating capability in this organism. However this finding is consistent with the alternative idea that the primary selective force maintaining mating capability is enhanced recombinational repair of DNA damage during meiosis, since this benefit is realized during each meiosis subsequent to a mating, whether or not out-crossing occurs. "Schizosaccharomyces pombe" is a facultative sexual yeast that can undergo mating when nutrients are limiting. Exposure of "S. pombe"

neg: Haplogroup Y chromosome does not recombine. Therefore, the Y chromosome and any mutations that arise in it are passed on from father to son in a direct male line of descent. This means the Y chromosome and mtDNA share specific properties. Other chromosomes, autosomes and X chromosomes in women, share their genetic material (called crossing over leading to recombination) during meiosis (a special type of cell division that occurs for the purposes of sexual reproduction). Effectively this means that the genetic material from these chromosomes gets mixed up in every generation, and so any new mutations are passed down randomly from

neg: DMC1 (gene) the absence of Rad51 strand exchange activity, Dmc1 appears to be able to repair all meiotic DNA breaks, and this absence does not affect meiotic crossing over rates. DMC1 (gene) Meiotic recombination protein DMC1/LIM15 homolog is a protein that in humans is encoded by the "DMC1" gene. Meiotic recombination protein Dmc1 is a homolog of the bacterial strand exchange protein RecA. Dmc1 plays the central role in homologous recombination in meiosis by assembling at the sites of programmed DNA double strand breaks and carrying out a search for allelic DNA sequences located on homologous chromatids. The name "Dmc" stands for

neg: Cdc14 abnormally. The abnormality arises due to delay in dissembling of spindle during Anaphase I. However, the segregation of chromosomes continue and both the phases of meiotic segregations take place on prolonged meiosis I spindle. Cdc14 along with SPO12 and SLK19 play a critical role in ensuring that the two phases of chromosomal segregation take place consecutively during meiosis. Cdc14 is widely distributed, being found in most eukaryote kingdoms. However, it is not found in all species based on searches of sequenced genomes. One or more Cdc14 genes are found in alveolates, animals, fungi, trypanosomes, and lower plants. However, Cdc14 genes

neg: Oogenesis occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body. Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur. If the egg is not fertilized, it is disintegrated and released (menstruation) and the secondary oocyte does not complete meiosis II (and doesn't become an ovum). When meiosis II has completed, an ootid and another polar body have now been created.

neg: Shofetim (parsha) bribe is not necessarily monetary was be derived from the fact that does not say: “And you shall take no profit.” The Gemara illustrated this by telling how Samuel was once crossing a river on a ferry and a certain man gave him a hand to help him out of the ferryboat. Samuel asked him what he was doing in the place, and when the man told Samuel that he had a case to present before Samuel, Samuel told him that he was disqualified from presiding over the case, as the man did Samuel a favor, and although no money

neg: MLH1 first division of meiosis. In humans, a common variant of the "MLH1" gene is associated with increased risk of sperm damage and male infertility. MLH1 protein appears to localize to sites of crossing over in meiotic chromosomes. Recombination during meiosis is often initiated by a DNA double-strand break (DSB) as illustrated in the accompanying diagram. During recombination, sections of DNA at the 5' ends of the break are cut away in a process called "resection". In the "strand invasion" step that follows, an overhanging 3' end of the broken DNA molecule then "invades" the DNA of an homologous chromosome that

neg: Ascus of spores – the phenomenon called "puffing". This is an example of positive feedback. A faint hissing sound can also be heard for species of "Peziza" and other cup fungi. Asci, notably those of "Neurospora crassa", have been used in laboratories for studying the process of meiosis, because the four cells produced by meiosis line up in regular order. By modifying genes coding for spore color and nutritional requirements, the biologist can study crossing over and other phenomena. The formation of asci and their use in genetic analysis are described in detail in "Neurospora crassa". Asci of most Pezizomycotina develop

neg: MLH3 nature of only two transcript variants has been determined. Orthologs of human MLH3 have also been studied in other organisms including mouse and the budding yeast "Saccharomyces cerevisiae". In addition to its role in DNA mismatch repair, MLH3 protein is also involved in meiotic crossing over. MLH3 forms a heterodimer with MLH1 that appears to be necessary for mouse oocytes to progress through metaphase II of meiosis. The MLH1-MLH3 heterodimers promote crossovers. Recombination during meiosis is often initiated by a DNA double-strand break (DSB) as illustrated in the accompanying diagram. During recombination, sections of DNA at the 5' ends of

neg: Meiosis chromosome pairs along the metaphase plate during metaphase I & orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I & 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 DNA within chromosomes. 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

neg: Parthenogenesis fraction of the mother's alleles since crossing over of DNA takes place during meiosis, creating variation. Parthenogenetic offspring in species that use either the XY or the X0 sex-determination system have two X chromosomes and are female. In species that use the ZW sex-determination system, they have either two Z chromosomes (male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female). Some species reproduce exclusively by parthenogenesis (such as the Bdelloid rotifers), while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis (other terms

neg: Parthenogenesis mother's genetic material and heterozygosity is mostly preserved (if the mother has two alleles for a locus, it is likely that the offspring will get both). This is because in anaphase I the homologous chromosomes are separated. Heterozygosity is not completely preserved when crossing over occurs in central fusion. In the case of pre-meiotic doubling, recombination -if it happens- occurs between identical sister chromatids. If "terminal fusion" (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous. This is

neg: Meiosis and may start at the centromere (procentric), at the chromosome ends (proterminal), or at any other portion (intermediate). Individuals of a pair are equal in length and in position of the centromere. Thus pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes. The "pachytene" stage ( ), also known as "pachynema", from Greek words meaning "thick threads". At this point a tetrad of the chromosomes has formed known as a bivalent. This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Nonsister chromatids of homologous chromosomes may exchange segments over regions

neg: Sister chromatids Compare sister chromatids to homologous chromosomes, which are the two "different" copies of a chromosome that diploid organisms (like humans) inherit, one from each parent. Sister chromatids are by and large identical (since they carry the same alleles, also called variants or versions, of genes) because they derive from one original chromosome. An exception is towards the end of meiosis, after crossing over has occurred, because sections of each sister chromatid may have been exchanged with corresponding sections of the homologous chromatids with which they are paired during meiosis. Homologous chromosomes might or might not be the same as each

neg: Ectopic recombination Ectopic recombination Ectopic recombination is an atypical form of recombination in which crossing over occurs at non-homologous, rather than along homologous, loci. Such recombination often results in dramatic chromosomal rearrangement, which is generally harmful to the organism. Some research, however, has suggested that ectopic recombination can result in mutated chromosomes that benefit the organism. Ectopic recombination can occur during both meiosis and mitosis, although it is more likely occur during meiosis. It occurs relatively frequently—in at least one yeast species (Saccharomyces cerevisiae) the frequency of ectopic recombination is roughly on par with that of allelic (or traditional) recombination. If the

neg: Homologous chromosome crossing over occurs and throughout the process of chromosomal segregation during meiosis. Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double-strand breaks. At the diplotene stage of prophase I the synaptonemal complex disassembles before which will allow the homologous chromosomes to separate, while the sister chromatids stay associated by their centromeres. In metaphase I of meiosis I, the pairs of homologous chromosomes, also known as bivalents or tetrads, line up in a random order along the metaphase plate. The random orientation is another way for cells to introduce genetic variation. Meiotic spindles

neg: Cell (biology) the S phase of the cell cycle. In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II. Replication, like all cellular activities, requires specialized proteins for carrying out the job. Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and

neg: Chromosomal crossover 1931, Barbara McClintock discovered a triploid maize plant. She made key findings regarding corn's karyotype, including the size and shape of the chromosomes. McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, and later showed the first ever cytological demonstration of crossing over in meiosis. Working with student Harriet Creighton, McClintock also made significant contributions to the early understanding of codependency of linked genes. Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes. In her report, “The Significance of Responses of the Genome to Challenge”,

neg: Parthenogenesis of anaphase II are joined together. The criterion for "sexuality" varies from all cases of restitutional meiosis, to those where the nuclei fuse or to only those where gametes are mature at the time of fusion. Those cases of automixis that are classified as sexual reproduction are compared to self-fertilization in their mechanism and consequences. The genetic composition of the offspring depends on what type of apomixis takes place. When endomitosis occurs before meiosis or when "central fusion" occurs (restitutional meiosis of anaphase I or the fusion of its products), the offspring get all to more than half of the

neg: Meiosis cellular reproduction, or during mitosis. Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to: The probability of nondisjunction in human oocytes increases with increasing maternal age, presumably due to loss of cohesin over time. Alongside with the variations of meiosis related to the moment when meiosis occur in life cycles, resulting in post-zygotic, pre-gametic and intermediate meiosis (see above), the number of nuclear divisions in meiosis

neg: Keratella cochlearis very short or non-existent. During the remaining months of the year it gradually lengthens once more. These changes take place over several generations as each individual rotifer has a lifespan of just a few days or weeks. Periodically sexual reproduction takes place. It is not known why this occurs at some times and not others. The female undergoes meiosis and produces eggs with half the usual number of chromosomes. Some of these develop into male rotifers. Each of these is able to inseminate another of these eggs to restore the full complement of chromosomes in the offspring. Keratella cochlearis Keratella

neg: Mongolian gerbil living areas keeps the growth of bacteria and fungi down and keeps odours down too. This means they are very easy animals to keep in a research setting. About 50% of Mongolian gerbils also display an epileptic faint when under stress which might serve as a way to play dead in the wild. This characteristic has proven very useful to researchers of epilepsy and related illnesses. During meiosis male Mongolian gerbils do not interchange reciprocal alleles between pairs of homologous chromosomes ("chiasmata" or "Crossing Over"), a trait very rare in eutherian mammals and indeed animals in general. Meiotic recombination occurs

neg: Aspergillus nidulans Aspergillus nidulans Aspergillus nidulans (also called Emericella nidulans when referring to its sexual form, or teleomorph) is one of many species of filamentous fungi in the phylum Ascomycota. It has been an important research organism for studying eukaryotic cell biology for over 50 years, being used to study a wide range of subjects including recombination, DNA repair, mutation, cell cycle control, tubulin, chromatin, nucleokinesis, pathogenesis, metabolism, and experimental evolution. It is one of the few species in its genus able to form sexual spores through meiosis, allowing crossing of strains in the laboratory. "A. nidulans" is a homothallic fungus, meaning

neg: BRCA1 sister chromatid. In the nucleus of many types of normal cells, the BRCA1 protein interacts with RAD51 during repair of DNA double-strand breaks. These breaks can be caused by natural radiation or other exposures, but also occur when chromosomes exchange genetic material (homologous recombination, e.g., "crossing over" during meiosis). The BRCA2 protein, which has a function similar to that of BRCA1, also interacts with the RAD51 protein. By influencing DNA damage repair, these three proteins play a role in maintaining the stability of the human genome. BRCA1 is also involved in another type of DNA repair, termed mismatch repair. BRCA1

neg: Balancer chromosome Balancer chromosome Balancer chromosomes are special, modified chromosomes used for genetically screening a population of organisms to select for heterozygotes. Balancer chromosomes can be used as a genetic tool to prevent crossing over (genetic recombination) between homologous chromosomes during meiosis. Balancers are most often used in "Drosophila melanogaster" (fruit fly) genetics to allow populations of flies carrying heterozygous mutations to be maintained without constantly screening for the mutations but can also be used in mice. Balancer chromosomes have three important properties: they suppress recombination with their homologs, carry dominant markers, and negatively affect reproductive fitness when carried homozygously. Balancer chromosomes

neg: Synthesis-dependent strand annealing nonreciprocal genetic transfer takes place between two homologous sequences. Assembly of a nucleoprotein filament comprising single-stranded DNA (ssDNA) and the RecA homolog, Rad51, is a key step necessary for homology search during recombination. In the budding yeast "Saccharomyces cerevisiae", Srs2 translocase dismantles Rad51 filaments during meiosis. By directly interacting with Rad51, Srs2 dislodges Rad51 from nucleoprotein filaments thereby inhibiting Rad51-dependent formation of joint molecules and D-loop structures. This dismantling activity is specific for Rad51 since Srs2 does not dismantle DMC1 (a meiosis-specific Rad51 homolog), Rad52 (a Rad 51 mediator) or replication protein A (RPA, a single-stranded DNA binding protein). Srs2

neg: Genetic linkage of a genetic linkage map. Recombination frequency ("θ") is the frequency with which a single chromosomal crossover will take place between two genes during meiosis. A centimorgan (cM) is a unit that describes a recombination frequency of 1%. In this way we can measure the genetic distance between two loci, based upon their recombination frequency. This is a good estimate of the real distance. Double crossovers would turn into no recombination. In this case we cannot tell if crossovers took place. If the loci we're analysing are very close (less than 7 cM) a double crossover is very unlikely. When

neg: Saccharomyces cerevisiae type, upon cell division, often produce cells of the opposite mating type with which they can mate. The relative rarity in nature of meiotic events that result from outcrossing is inconsistent with the idea that production of genetic variation is the main selective force maintaining meiosis in this organism. However, this finding is consistent with the alternative idea that the main selective force maintaining meiosis is enhanced recombinational repair of DNA damage, since this benefit is realized during each meiosis, whether or not out-crossing occurs. "S. cerevisiae" was the first eukaryotic genome to be completely sequenced. The genome sequence was

neg: Meiosis 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 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

neg: Phycomyces blakesleeanus in which the diploid zygote is formed and meiosis is presumed to take place. The data from this cross provided supporting evidence for meiosis during zygospore development. Phycomyces blakesleeanus Phycomyces blakesleeanus is a filamentous fungus in the Order Mucorales of the phylum Zygomycota or subphylum Mucoromycotina. The spore-bearing sporangiophores of "Phycomyces" are very sensitive to different environmental signals including light, gravity, wind, chemicals and adjacent objects. They exhibit phototropic growth: most "Phycomyces" research has focused on sporangiophore photobiology, such as phototropism and photomecism ('light growth response'). Metabolic, developmental, and photoresponse mutants have been isolated, some of which have been genetically

neg: XX male syndrome XX male syndrome XX male syndrome is a rare congenital condition where an individual with a female genotype has phenotypically male characteristics that can vary between cases. In 90% of these individuals the syndrome is caused by unequal crossing over between X and Y chromosomes during meiosis in the father, and results in the X chromosome containing the SRY gene, as opposed to the Y chromosome where it is normally found. When the X with the SRY gene combines with a normal X from the mother during fertilization, the result is an XX male. Less common are SRY-negative XX males

neg: Heteroduplex Heteroduplex A heteroduplex is a double-stranded (duplex) molecule of nucleic acid originated through the genetic recombination of single complementary strands derived from "different" sources, such as from different homologous chromosomes or even from different organisms. One such example is the heteroduplex DNA strand formed in hybridization processes, usually for biochemistry-based phylogenetic analyses. Another is the heteroduplexes formed when non-natural analogs of nucleic acids are used to bind with nucleic acids; these heteroduplexes result from performing antisense techniques using single-stranded peptide nucleic acid, 2'-O-methyl phosphorothioate or Morpholino oligos to bind with RNA. In meiosis, the process of crossing-over occurs between non-sister

neg: Non-allelic homologous recombination syndrome or 3q29 microdeletion syndrome. Non-allelic homologous recombination Non-allelic homologous recombination (NAHR) is a form of homologous recombination that occurs between two lengths of DNA that have high sequence similarity, but are not alleles. It usually occurs between sequences of DNA that have been previously duplicated through evolution, and therefore have low copy repeats (LCRs). These repeat elements typically range from 10–300 kb in length and share 95-97% sequence identity. During meiosis or mitosis, LCRs can misalign and subsequent crossing-over can result in genetic rearrangement. When non-allelic homologous recombination occurs between different LCRs, deletions or further duplications of the DNA

neg: Non-allelic homologous recombination Non-allelic homologous recombination Non-allelic homologous recombination (NAHR) is a form of homologous recombination that occurs between two lengths of DNA that have high sequence similarity, but are not alleles. It usually occurs between sequences of DNA that have been previously duplicated through evolution, and therefore have low copy repeats (LCRs). These repeat elements typically range from 10–300 kb in length and share 95-97% sequence identity. During meiosis or mitosis, LCRs can misalign and subsequent crossing-over can result in genetic rearrangement. When non-allelic homologous recombination occurs between different LCRs, deletions or further duplications of the DNA can occur. This can give

neg: Chiasma (genetics) two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage. Sister chromatids also form chiasmata between each other (also known as a chi structure), but because their genetic material is identical, it does not cause any noticeable change in the resulting daughter cells. In humans, there seems to be one chiasma per chromosome arm, and in mammals, the number of chromosome arms is a

neg: Hixon rail crash and slowed to walking pace as it approached the crossing. It stopped for a moment while the police car went over the crossing to check where the entrance to the airfield was; on its return, one of the police officers told Groves that "this is the place" and proceeded back over the level crossing. The trailer needed to be raised by the crew to negotiate the track, but, in addition, it needed to be low enough to clear the overhead lines. While this was taking place the transporter slowed to around . At this speed, it would take approximately one

neg: Caenorhabditis elegans "C. elegans" does. Research into meiosis has been considerably simplified since every germ cell nucleus is at the same given position as it moves down the gonad, so is at the same stage in meiosis. In an early phase of meiosis, the oocytes become extremely resistant to radiation and this resistance depends on expression of genes "rad51" and "atm" that have key roles in recombinational repair. Gene "mre-11" also plays a crucial role in recombinational repair of DNA damage during meiosis. A study of the frequency of outcrossing in natural populations showed that selfing is the predominant mode of reproduction

neg: Autogamy plant. While the latter method does blur the lines between autogamous self-fertilization and normal sexual reproduction, it is still considered autogamous self-fertilization. Self-pollination can lead to inbreeding depression due to expression of deleterious recessive mutations. Meiosis followed by self-pollination results in little genetic variation, raising the question of how meiosis in self-pollinating plants is adaptively maintained over an extended period in preference to a less complicated and less costly asexual ameiotic process for producing progeny. For instance, "Arabidopsis thaliana" is a predominantly self-pollinating plant that has an outcrossing rate in the wild estimated at less than 0.3%, and self-pollination appears

neg: Meiosis of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed "disjunction". When the segregation is not normal, it is called "nondisjunction". This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of

neg: Mishpatim say that precludes bribery by means of money, and even verbal bribery is also prohibited. The law that a bribe is not necessarily monetary was be derived from the fact that does not say: “And you shall take no profit.” The Gemara illustrated this by telling how Samuel was once crossing a river on a ferry and a certain man gave him a hand to help him out of the ferryboat. Samuel asked him what he was doing in the place, and when the man told Samuel that he had a case to present before Samuel, Samuel told him that

neg: Salzburg Cathedral long and 33 meters high at the crossing/dome. The baroque style of St. Rupert's can be seen in the choir and the nave. The Salzburg Cathedral was damaged during World War II when a single bomb crashed through the central dome over the crossing. Repairs were somewhat slow to take place, but restoration was complete by 1959. Wolfgang Amadeus Mozart was baptized here on January 28, 1756, the day after his birth. Salzburg Cathedral is located adjacent to Residenzplatz and Domplatz in the Altstadt (Old Town) area of the city. The Domplatz is accessed by three open arcade arches in

neg: Boveri–Sutton chromosome theory all genetic inheritance. Over a period of time random mutation creates changes in the DNA sequence of a gene. Genes are located on chromosomes. The chromosome theory of inheritance is credited to papers by Walter Sutton in 1902 and 1903, as well as to independent work by Theodor Boveri during roughly the same period. Boveri was studying sea urchins, in which he found that all the chromosomes had to be present for proper embryonic development to take place. Sutton's work with grasshoppers showed that chromosomes occur in matched pairs of maternal and paternal chromosomes which separate during meiosis and "may

neg: Fungus meiosis. The most commonly known basidiocarps are mushrooms, but they may also take other forms (see Morphology section). In glomeromycetes (formerly zygomycetes), haploid hyphae of two individuals fuse, forming a gametangium, a specialized cell structure that becomes a fertile gamete-producing cell. The gametangium develops into a zygospore, a thick-walled spore formed by the union of gametes. When the zygospore germinates, it undergoes meiosis, generating new haploid hyphae, which may then form asexual sporangiospores. These sporangiospores allow the fungus to rapidly disperse and germinate into new genetically identical haploid fungal mycelia. Both asexual and sexual spores or sporangiospores are often actively

neg: Meiosis of homology. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology. At the sites where exchange happens, chiasmata form. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope, and chiasmata are not visible until the next stage. During the "diplotene" stage, also

neg: Chromosomal crossover that meiosis evolved from bacterial transformation, with the function of propagating genetic diversity. . Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive. It is likely that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes. Meiotic recombination may be initiated by double-stranded breaks that are introduced into the DNA by exposure to DNA damaging agents or the Spo11 protein. One or more exonucleases

neg: Trypanosoma brucei and the chromosomes do not condense during mitosis. The basal body, unlike the centrosome of most eukaryotic cells, does not play a role in the organisation of the spindle and instead is involved in division of the kinetoplast. The events of reproduction are: In the 1980s, DNA analyses of the developmental stages of "T. brucei" started to indicate that the trypomastigote in the tsetse fly undergoes meiosis, i.e. a sexual reproduction stage. But it is not always necessary for a complete life cycle. The existence of meiosis-specific proteins was reported in 2011. The haploid gametes (daughter cells produced after meiosis)

neg: Panama disease thus attempts at meiosis in the plant's ovules cannot produce a viable gamete. Only rarely does the first reduction division in meiosis in the plants' flowers tidily fail completely, resulting in a euploid triploid ovule, which can be fertilized by normal haploid pollen from a diploid banana variety; a whole stem of bananas would contain only a few seeds and sometimes none. As a result, the resulting new banana variety is tetraploid, and thus contains seeds; the market for bananas is not accustomed to bananas with seeds. Experience showed that where both meiosis steps failed, causing a heptaploid seedling, or

neg: Gametocyte Gametocyte A gametocyte is a eukaryotic germ cell that divides by mitosis into other gametocytes or by meiosis into gametids during gametogenesis. Male gametocytes are called "spermatocytes", and female gametocytes are called "oocytes". The development of gametogonia to primary gametocytes is called "gametocytogenesis". The further development of primary gametocytes to secondary gametocytes is a part of gametidogenesis. Gametogenesis is the formation or production of gametes (taking place during meiosis). The development and maturation of sex cells also takes place during meiosis. Gametogenesis is also the process of formation in male and female gametes that occur in the gonads (ovary and

neg: Homologous recombination that certain genes associated with physical traits can be inherited together, or genetically linked. In 1911, after observing that linked traits could on occasion be inherited separately, Thomas Hunt Morgan suggested that "crossovers" can occur between linked genes, where one of the linked genes physically crosses over to a different chromosome. Two decades later, Barbara McClintock and Harriet Creighton demonstrated that chromosomal crossover occurs during meiosis, the process of cell division by which sperm and egg cells are made. Within the same year as McClintock's discovery, Curt Stern showed that crossing over—later called "recombination"—could also occur in somatic cells like

neg: Shibuya a common meeting place and almost always crowded. Three large TV screens mounted on nearby buildings overlook the crossing, as well as many advertising signs. The Starbucks store overlooking the crossing is also one of the busiest in the world. Its heavy traffic and inundation of advertising have led to it being compared to the Times Square intersection in New York City and Dundas Square intersection in Toronto. Tokyo-based architecture professor Julian Worrall has said Shibuya Crossing is "a great example of what Tokyo does best when it's not trying." Shibuya Crossing is often featured in movies and television shows

neg: The Crossing (Star Trek: Enterprise) The Crossing (Star Trek: Enterprise) "The Crossing" is the forty-fourth episode of the television series "", the eighteenth of the second season. Incorporeal beings attempt to take over the Earth starship "Enterprise". At the start of the episode Enterprise is taken inside an enormous alien spacecraft of unknown origin, and ghostly aliens take over the crew's bodies. Later in the episode the crew takes shelter in the set seen in The Catwalk (S2E12), which is a long tube with enhanced shielding. While at warp speed, a huge alien vessel overtakes and swallows "Enterprise", disabling the ship's engines and weapons. When

neg: Tetrahymena Rad 51 protein levels. These findings suggest that ciliates such as "T. thermophila" utilize a Rad51-dependent recombinational pathway to repair damaged DNA. The Rad51 recombinase of "T. thermophila" is a homolog of the "Escherichia coli" RecA recombinase. In "T. thermophila", Rad51 participates in homologous recombination during mitosis, meiosis and in the repair of double-strand breaks. During conjugation, Rad51 is necessary for completion of meiosis. Meiosis in "T. thermophila" appears to employ a Mus81-dependent pathway that does not use a synaptonemal complex and is considered secondary in most other model eukaryotes. This pathway includes the Mus81 resolvase and the Sgs1 helicase.

neg: Meiosis of the chromosomes and the disassembly of the spindle. Nuclear envelopes reform and cleavage or cell plate formation eventually produces 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 occurs in eukaryotic life cycles involving sexual reproduction, consisting 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. At certain stages of the life cycle, germ cells produce

neg: Saccharomyces cerevisiae a mother cell undergoes meiosis and gametogenesis, lifespan is reset. The replicative potential of gametes (spores) formed by aged cells is the same as gametes formed by young cells, indicating that age-associated damage is removed by meiosis from aged mother cells. This observation suggests that during meiosis removal of age-associated damages leads to rejuvenation. However, the nature of these damages remains to be established. "S. cerevisiae" reproduces by mitosis as diploid cells when nutrients are abundant. However, when starved, these cells undergo meiosis to form haploid spores. Evidence from studies of "S. cerevisiae" bear on the adaptive function of meiosis

neg: Meiosis to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. It is retinoic acid, derived from the primitive kidney (mesonephros) that stimulates meiosis in ovarian oogonia. Tissues

neg: Oogonium oogonium refers to a female gametangium if the union of the male (motile or non-motile) and the female gamete takes place within this structure. In "Oomycota" and some other organisms, the female oogonia, and the male equivalent antheridia, are a result of sexual sporulation, i.e. the development of structures within which meiosis will occur. The haploid nuclei (gametes) are formed by meiosis within the antheridia and oogonia, and when fertilization occurs, a diploid oospore is produced which will eventually germinate into the diploid somatic stage of the thallophyte life cycle. In many algae (e.g., "Chara"), the main plant is haploid;

neg: Pollination year two sperm cells are produced by mitosis of the body cell of the male gametophyte. The pollen tube elongates and pierces and grows through the megasporangium wall and delivers the sperm cells to the female gametophyte inside. Fertilisation takes place when the nucleus of one of the sperm cells enters the egg cell in the megagametophyte’s archegonium. In flowering plants, the anthers of the flower produce microspores by meiosis. These undergo mitosis to form male gametophytes, each of which contains two haploid cells. Meanwhile, the ovules produce megaspores by meiosis, further division of these form the female gametophytes, which

neg: Homologous recombination homologous recombination occurs during meiosis. Whether recombination in the DSBR pathway results in chromosomal crossover is determined by how the double Holliday junction is cut, or "resolved". Chromosomal crossover will occur if one Holliday junction is cut on the crossing strand and the other Holliday junction is cut on the non-crossing strand (in Figure 4, along the horizontal purple arrowheads at one Holliday junction and along the vertical orange arrowheads at the other). Alternatively, if the two Holliday junctions are cut on the crossing strands (along the horizontal purple arrowheads at both Holliday junctions in Figure 4), then chromosomes without

neg: MLH1 of multiple DNA repair genes are often found in cancers, and may contribute to the thousands of mutations usually found in cancers (see Mutation frequencies in cancers). In addition to its role in DNA mismatch repair, MLH1 protein is also involved in meiotic crossing over. MLH1 forms a heterodimer with MLH3 that appears to be necessary for oocytes to progress through metaphase II of meiosis. Female and male "MLH1"(-/-) mutant mice are infertile, and sterility is associated with a reduced level of chiasmata. During spermatogenesis in "MLH1"(-/-) mutant mice chromosomes often separate prematurely and there is frequent arrest in the

neg: Messiah (TV series) a serial killer who sets out to commit twelve murders in the same vein as the Apostles. The screenplay was written by Lizzie Mickery, who also wrote "Messiah II" and "Messiah III". The series was broadcast over a May bank holiday weekend in 2001, on 26 (Saturday) and 27 (Sunday) May respectively. The series does have some major differences to the novel it was adapted from. For example; the final scenes in the novel take place at Easter, traditionally the time when Judas Iscariot hanged himself. However, the events of the final scenes in the series take place on New

neg: Phymatopus stage of meiosis occurs as normal, but there is no crossing over or chiasma formation in the females at the prophase I stage. The males on the other hand show conventional meiotic recombination. In many hepialids, the female releases attractants to draw in males. This is the ancestral condition whereas in "Phymatopus", the opposite occurs, with males attracting females Host plant: Food plants: Mobility is defined as the ability to migrate. According to the metapopulation theory, migration is important for individuals to colonize new areas and recolonize old ones to avoid extinction. Moths, like other insects, are only mobile in

neg: Parthenogenesis in amphibians found to be tetraploid. Most "Ambystoma" hybrids are described by how many haploid sets of chromosomes they contain from each of their parent species, reflecting their level of ploidy. Embryonic mortality in parthenogenic amphibians is high. Hatching rates for North American salamander species have ranged from 19.5% to 30.5%. It is speculated that intergenomic exchanges, like crossing over during meiosis, may play a role. Intergenomic exchanges are often lethal due to the fact that chromosomes in unisexual species are homeologous. Homologous chromosomes are largely identical, in this case as a result of the chromosome replication. Gynogenesis is a form of

neg: Homologous chromosome a single parent, the alleles of genes near to one another along the length of the chromosome move together. Using this logic he concluded that the two genes he was studying were located on homologous chromosomes. Later on during the 1930s Harriet Creighton and Barbara McClintock were studying meiosis in corn cells and examining gene loci on corn chromosomes. Creighton and McClintock discovered that the new allele combinations present in the offspring and the event of crossing over were directly related. This proved interchromosomal genetic recombination. Homologous chromosomes are chromosomes which contain the same genes in the same order along

neg: Balancer chromosome it cannot be transcribed into virus proteins. The mechanism of maintaining double stranded RNA is not known. Balancer chromosome Balancer chromosomes are special, modified chromosomes used for genetically screening a population of organisms to select for heterozygotes. Balancer chromosomes can be used as a genetic tool to prevent crossing over (genetic recombination) between homologous chromosomes during meiosis. Balancers are most often used in "Drosophila melanogaster" (fruit fly) genetics to allow populations of flies carrying heterozygous mutations to be maintained without constantly screening for the mutations but can also be used in mice. Balancer chromosomes have three important properties: they suppress

neg: Meiosis known as "diplonema", from Greek words meaning "two threads", the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. 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 the transition to anaphase I. 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

neg: Genetic variation wherein organisms have three or more sets of genetic variation (3n or more). Crossing over (genetic recombination) and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Furthermore, random fertilization also contributes to variation. Variation and recombination can be facilitated by transposable genetic elements, endogenous retroviruses, LINEs, SINEs, etc. For a given genome of a multicellular organism, genetic variation may be acquired in somatic cells or inherited through the germline. Genetic variation can be divided into different forms according to the size and type of genomic variation underpinning genetic change. Small-scale sequence

neg: Homologous chromosome of each homologous chromosome pair. The alleles on the homologous chromosomes may be different, resulting in different phenotypes of the same genes. This mixing of maternal and paternal traits is enhanced by crossing over during meiosis, wherein lengths of chromosomal arms and the DNA they contain within a homologous chromosome pair are exchanged with one another. Early in the 1900s William Bateson and Reginald Punnett were studying genetic inheritance and they noted that some combinations of alleles appeared more frequently than others. That data and information was further explored by Thomas Morgan. Using test cross experiments, he revealed that, for

neg: Genetics chromosome would theoretically never recombine. However, they do, via the cellular process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. This process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells. The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in 1931. Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other. The probability of chromosomal crossover

neg: Centimorgan Centimorgan In genetics, a centimorgan (abbreviated cM) or map unit (m.u.) is a unit for measuring genetic linkage. It is defined as the distance between chromosome positions (also termed loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a chromosome. However, it is not a true physical distance. The number of base-pairs to which it corresponds varies widely across the genome (different regions of a chromosome have different propensities towards crossover) and it also depends on if the meiosis where the crossing-over

neg: Gene duplication Gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. Gene duplications can arise as products of several types of errors in DNA replication and repair machinery as well as through fortuitous capture by selfish genetic elements. Common sources of gene duplications include ectopic recombination, retrotransposition event, aneuploidy, polyploidy, and replication slippage. Duplications arise from an event termed unequal crossing-over that occurs during meiosis between misaligned homologous chromosomes.The chance of

neg: Homologous chromosome reactive oxygen species. Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same genetic sequence. Once the base pairs have been matched and oriented correctly between the two strands, the homologous chromosomes perform a process that is very similar to recombination, or crossing over as seen in meiosis. Part of the intact DNA sequence overlaps with that of the damaged chromosome's sequence. Replication proteins and complexes are then recruited to the site of damage, allowing for repair and proper replication to occur. Through this functioning, double-strand breaks can be repaired and DNA can function normally. Current

neg: Interference (genetic) a larger fraction of events occurring within closer proximity to each other than would be expected under simple models of crossover interference. Interference (genetic) Crossover interference is the term used to refer to the non-random placement of crossovers with respect to each other during meiosis. The term is attributed to Muller, who observed that one crossover "interferes with the coincident occurrence of another crossing over in the same pair of chromosomes, and I have accordingly termed this phenomenon ‘interference’." Meiotic crossovers (COs) appear to be regulated to ensure that COs on the same chromosome are distributed far apart (crossover interference).

neg: Meiosis chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis. Although the process of meiosis is related to the more general cell division process of mitosis, it differs in two important respects: Meiosis begins with a

neg: New Zealand mud snail able to infect any member of the population. Sexual reproduction mixes up resistance genes through crossing over and the random assortment of gametes in meiosis, meaning the members of a sexual population will all have subtly different combinations of resistance genes. This variation in resistance genes means no one parasite strain is able to sweep through the whole population. New Zealand mudsnails are commonly infected with trematode parasites, which are particularly abundant in shallow water, but scarce in deeper water. As predicted, sexual reproduction dominates in shallow water, due to its advantages in parasite resistance. Asexual reproduction is dominant in

neg: C. D. Darlington and J.B.S. Haldane came to the Innes. Although neither an experimentalist nor cytologist, Haldane formed a close friendship with Darlington, whose self-confidence grew. He began to make significant contributions to the understanding of the relationship of genetic crossing-over and the microscopically observed events that the chromosome passed through during meiosis. In February 1929 he made a study trip with fellow botanist John Macqueen Cowan to the Near East. In 1931 he began writing the book that would establish his reputation, "Recent Advances in Cytology." It was published in 1932, and created a firestorm of controversy at first, then nearly universal

neg: Genetic recombination the CO/DHJ type. The NCO/SDSA pathway contributes little to genetic variation, since 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 is well documented in male "Drosophila melanogaster". Heterochiasmy is the term used to describe recombination rates which differ between the sexes of a species. This sexual dimorphic pattern in recombination rate has been

neg: Chromosomal crossover junction. The contact between two chromatids that will soon undergo crossing-over is known as a "chiasma". The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure. The MSH4 and MSH5 proteins form a hetero-oligomeric structure (heterodimer) in yeast and humans. In the yeast "Saccharomyces cerevisiae" MSH4 and MSH5 act specifically to facilitate crossovers between homologous chromosomes during meiosis. The MSH4/MSH5 complex binds and stabilizes double Holliday junctions and promotes their resolution into crossover products. An MSH4 hypomorphic (partially functional) mutant of "S. cerevisiae" showed a 30% genome wide reduction in

neg: Barbara McClintock ideas was the notion of genetic recombination by crossing-over during meiosis—a mechanism by which chromosomes exchange information. She produced the first genetic map for maize, linking regions of the chromosome to physical traits. She demonstrated the role of the telomere and centromere, regions of the chromosome that are important in the conservation of genetic information. She was recognized as among the best in the field, awarded prestigious fellowships, and elected a member of the National Academy of Sciences in 1944. During the 1940s and 1950s, McClintock discovered transposition and used it to demonstrate that genes are responsible for turning physical

neg: Evolution of sexual reproduction viral centromeres would incite the grouping of both sets into tetrads. It is speculated that this grouping may be the origin of crossing over, characteristic of the first division in modern meiosis. The partitioning apparatus of the mitotic-like cell cycle the cells used to replicate independently would then pull each set of chromosomes to one side of the cell, still bound by centromeres. These centromeres would prevent their replication in subsequent division, resulting in four daughter cells with one copy of one of the two original pox-like viruses. The process resulting from combination of two similar pox viruses within the