query: when do homologous chromosomes pair up in meiosis

pos: Homologous chromosome Homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area. Chromosomes are linear arrangements of condensed deoxyribonucleic acid (DNA)

pos: Polysomy -"somy" is used to name aneuploid karyotypes. This is not to be confused with the suffix -"ploidy", referring to the number of complete sets of chromosomes. Polysomy is usually caused by non-disjunction (the failure of a pair of homologous chromosomes to separate) during meiosis, but may also be due to a translocation mutation (a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes). Polysomy is found in many diseases, including Down syndrome in humans where affected individuals possess three copies (trisomy) of chromosome 21. Polysomic inheritance occurs during meiosis when chiasmata form between more than two homologous partners, producing

pos: Evolution two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already. An example of such a speciation event is when the plant species "Arabidopsis thaliana" and "Arabidopsis arenosa" crossbred

pos: Ploidy cells. The haploid gametes produced by most organisms combine to form a zygote with "n" pairs of chromosomes, i.e. 2"n" chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be homologous. Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During meiosis, sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ

pos: Meiosis 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 the Greek letter Chi (X)) between the homologous chromosomes. In most organisms, these links are essential to 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

pos: Homologous chromosome this capability for regenerative medicine. This medicine could be very prevalent in relation to cancer, as DNA damage is thought to be contributor to carcinogenesis. Manipulating the repair function of homologous chromosomes might allow for bettering a cell’s damage response system. While research has not yet confirmed the effectiveness of such treatment, it may become a useful therapy for cancer. Homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same loci where

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: Nondisjunction Nondisjunction Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy). Calvin Bridges and Thomas Hunt Morgan are credited with discovering nondisjunction in "Drosophila melanogaster" sex chromosomes in the spring of 1910, while working in the Zoological Laboratory of Columbia University. In general, nondisjunction can occur in any

pos: Sequence homology the new environment is vastly different for the horizontally moving gene. In general, though, xenologs typically have similar function in both organisms. The term was coined by Walter Fitch. Homoeologous (also spelled homeologous) chromosomes or parts of chromosomes are those brought together following inter-species hybridization and allopolyploidization, and whose relationship was completely homologous in an ancestral species. In allopolyploids, the homologous chromosomes within each parental sub-genome should pair faithfully during meiosis, leading to disomic inheritance; however in some allopolyploids, the homoeologous chromosomes of the parental genomes may be nearly as similar to one another as the homologous chromosomes, leading to

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: Complete linkage Trait Loci (QTL) maps can be produced using two separate methods. One way uses the frequency of marker alleles and compares them to individuals selected from the two tails of the trait distribution. This is called the Trait-Based approach and strictly uses phenotypic information only to select the individuals for a sample. The other approach is called the Marker-Base approach (MB), and uses both the difference in marker allele frequencies and the phenotypic values of each marker genotype when selecting samples. In diploid eukaryotic cells, recombination can occur during the process of Meiosis. Homologous chromosomes pair up during meiosis before

pos: Mendelian inheritance The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles. An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent. Molecular proof of this principle was subsequently found through observation of meiosis by two scientists independently, the German botanist Oscar Hertwig in 1876, and the Belgian zoologist Edouard Van Beneden in 1883. Paternal and maternal chromosomes get separated in meiosis

pos: Parthenogenesis in squamata sexual species, identical duplicate sister chromosomes, produced through premeiotic replication, pair and separate during meiosis I in true parthenotes. Pairing of identical sister chromosomes, in comparison to the alternative of pairing homologous chromosomes, maintains heterozygosity in obligate parthenotes. Meiosis II involves the separation of sister chromatids in both sexual and parthenogenetic species. This method of parthenogenesis is observed in obligate parthenotes, such as lizards in the genus "Cnemidophorus" and "Lacerta", and also in certain facultative parthenotes like the Burmese python. Another mechanism typically observed in facultative parthenote reptiles is terminal fusion, in which a haploid polar body produced as a

pos: Meiosis further divided into Karyokinesis I and Cytokinesis I and 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 homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing 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

pos: 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

pos: Cytodeme Cytodeme The Cytodeme is the total assembly of organisms that use an identical suite of chromosomes to carry their genes. The term was first printed in the 1950s in a book by Heslop-Harrison. Discussing the "Deme Terminology" - he continued ""cytodeme, a population differing in some distinctive cytological feature from others."" In most cases the suite is composed of several pairs of homologous chromosomes with or without a pair of sex chromosomes. Since the only acceptable proof of the identity (homology) of chromosomes lies in their ability to pair fully from end to end during meiosis it follows that: As

pos: 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

pos: Rice rice have been characterized. Studies of rice gene OsRAD51C showed that this gene is necessary for homologous recombinational repair of DNA, particularly the accurate repair of DNA double-strand breaks during meiosis. Rice gene OsDMC1 was found to be essential for pairing of homologous chromosomes during meiosis, and rice gene OsMRE11 was found to be required for both synapsis of homologous chromosomes and repair of double-strand breaks during meiosis. Rice plays an important role in certain religions and popular beliefs. In many cultures relatives will scatter rice during or towards the end of a wedding ceremony in front of the bride

pos: Gametogenesis of the pollen tube (pollen bicellular in the anther and in the stigma). The female gamete is produced inside the embryo sac of the ovule. Meiosis is a central feature of gametogenesis, but the adaptive function of meiosis is currently a matter of debate. A key event during meiosis is the pairing of homologous chromosomes and recombination (exchange of genetic information) between homologous chromosomes. This process promotes the production of increased genetic diversity among progeny and the recombinational repair of damage in the DNA to be passed on to progeny. To explain the adaptive function of meiosis (as well as

pos: Diploidization status. This is usually achieved by elimination of duplicated genes. The main goals of diploidization are: (1) To ensure proper gene dosage; and (2) to maintain stable cellular division processes. It should be noted that this process does not need to occur rapidly for all chromosomes in one or few steps. In recent polyploid events, segments of the genome may still remain in a tetraploid status. In other words, diploidization is a long ongoing process that is shaped by both intrinsic and evolutionary drives. Normally, homologous chromosomes pair up in bivalents during meiosis and separate into different daughter cells. However,

pos: Chromosome segregation as cohesin. Upon proper segregation, a complete set of chromatids ends up in each of two nuclei, and when cell division is completed, each DNA copy previously referred to as a chromatid is now called a chromosome. Chromosome segregation occurs at two separate stages during meiosis called prophase I and prophase II (see meiosis diagram). In a diploid cell there are two sets of homologous chromosomes of different parental origin (e.g. a paternal and a maternal set). During the phase of meiosis labeled “interphase s” in the meiosis diagram there is a round of DNA replication, so that each of

pos: 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

pos: Chromosome segregation metaphase I and anaphase I in the meiosis diagram. Different pairs of chromosomes segregate independently of each other, a process termed “independent assortment of non-homologous chromosomes”. This process results in each gamete usually containing a mixture of chromosomes from both original parents. Improper chromosome segregation can result in aneuploid gametes having either too few or too many chromosomes. The second stage at which segregation occurs during meiosis is prophase II (see meiosis diagram). During this stage, segregation occurs by a process similar to that during mitosis, except that in this case prophase II is not preceded by a round of

pos: Meiosis of preformed mRNA), regulating 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 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,

pos: Coprinopsis cinerea mushroom cap. Meiosis is a specialized cell division process, occurring in diploid cells, in which a single round of DNA replication occurs, and is followed by two divisions to produce four haploid daughter nuclei. During meiosis homologous chromosomes pair with each other and undergo a DNA repair process in which DNA damage is removed and genetic information is recombined. Burns et al. studied the expression of genes involved in the 15-hour meiotic process encompassing time points prior to the haploid nuclear fusion that forms the diploid zygote to the final formation of the four haploid products. They compared expression of

pos: Chromomere chromatid that are aligned and produce alternating dark and light bands when stained. The dark bands are the chromomere. It is unknown when chromomeres first appear on the chromosome. Chromomeres can be observed best when chromosomes are highly condensed. The chromomeres are present during leptotene phase of prophase I during meiosis. During zygotene phase of prophase I, the chromomeres of homologs align with each other to form homologous rough pairing (homology searching). These chromomeres helps provide a unique identity for each homologous pairs. There are more than 2000 chromomeres on 20 chromosomes of maize. Chromomeres are organized in a discontinuous

pos: 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

pos: Barbara McClintock maize chromosomes triggered scientific interest in maize cytogenetics, and attributed to her 10 of the 17 significant advances in the field that were made by Cornell scientists between 1929 and 1935. In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during meiosis. The following year, McClintock and Creighton proved the link between chromosomal crossover during meiosis and the recombination of genetic traits. They observed how the recombination of chromosomes seen under a microscope correlated with new traits. Until this point, it had only been hypothesized that genetic recombination could occur during meiosis, although it

pos: Aneuploidy 1 to 22, from largest to smallest. The 23rd pair of chromosomes are the sex chromosomes. Normal females have two X chromosomes, while normal males have one X chromosome and one Y chromosome. The characteristics of the chromosomes in a cell as they are seen under a light microscope are called the karyotype. During meiosis, when germ cells divide to create sperm and egg (gametes), each half should have the same number of chromosomes. But sometimes, the whole pair of chromosomes will end up in one gamete, and the other gamete will not get that chromosome at all. Most embryos

pos: Cohesin segregates to opposite poles. Without cohesin, the cell would be unable to control sister chromatid segregation since there would be no way of ensuring whether the spindle fiber attached on each sister chromatid is from a different pole. 2. It facilitates spindle attachment onto chromosomes. 3. It facilitates DNA repair by recombination. 4. Recently many novel functions of cohesin have been discovered in many different cellular processes. Cohesin has been shown to be responsible for transcription regulation, DNA double strand break repair, chromosome condensation, pairing of homologous chromosomes during meiosis I, mono-orientation of sister kinetochores during meiosis I, non-homologous centromere

pos: Parthenogenesis in squamata is widespread among squamates. Parthenogenesis can result from either full cloning of the mother's genome, or through the combination of haploid genomes to create a "half-clone". Both mechanisms of parthenogenesis are seen in reptiles. Females can produce full clones of themselves through a modification of the normal meiosis process used to produce haploid egg cells for sexual reproduction. The female's germ cells undergo a process of premeiotic genome doubling, or endoreduplication, so that two consecutive division cycles in the process of meoisis result in a diploid, rather than haploid, genome. Whereas homologous chromosomes pair and separate during meiosis I in

pos: Pseudolinkage been rearranged during crossover, both haploid sets of chromosomes in the individual have the same rearrangement. As a result, all chromosomes will find a single partner with which to pair at meiosis, and there will be no deleterious consequences for the progeny. In translocation heterozygote, however, certain patterns of chromosome segregation during meiosis produce genetically unbalanced gametes that at fertilization become deleterious to the zygote. In a translocation heterozygote, the two haploid sets of chromosomes do not carry the same arrangement of genetic information. As a result, during prophase of the first meiotic division, the translocated chromosomes and their normal

pos: 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

pos: 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

pos: Evolution of sexual reproduction outweighs the costs. Sexual reproduction derives from recombination, where parent genotypes are reorganized and shared with the offspring. This stands in contrast to single-parent asexual replication, where the offspring is identical to the parents. Recombination supplies two fault-tolerance mechanisms at the molecular level: "recombinational DNA repair" (promoted during meiosis because homologous chromosomes pair at that time) and "complementation" (also known as heterosis, hybrid vigor or masking of mutations). The issue features in the writings of Aristotle, and modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin (1731-1802) in the 18th century. August Weismann picked up the thread

pos: Meiotic recombination checkpoint double-strand breaks that result in cell cycle arrest and apoptosis. The "Drosophila" CHEK2 ortholog mnk and the p53 ortholog dp53 are required for much of the cell death observed in early oogenesis when oocyte selection and meiotic recombination occur. Meiotic recombination checkpoint The meiotic recombination checkpoint monitors the meiotic recombination during meiosis, and blocks the entry into metaphase I if the recombination is not efficiently processed. Meiotic recombination contributes to the cells in three different ways. First, to achieve proper segregation, each pair of homologous chromosomes must be linked to each other to maintain a certain level of tension between

pos: Synapsis appear to be initiation sites for meiotic silencing. In female "Drosophila melanogaster" fruit flies, meiotic chromosome synapsis occurs in the absence of recombination. Thus synapsis in "Drosophila" is independent of meiotic recombination, consistent with the view that synapsis is a precondition required for the initiation of meiotic recombination. Meiotic recombination is also unnecessary for homologous chromosome synapsis in the nematode "Caenorhabditis elegans". 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

pos: Gene conversion due to base mismatch repair during homologous recombination: if one of the four chromatids during meiosis pairs up with another chromatid, as can occur because of sequence homology, DNA strand transfer can occur followed by mismatch repair. This can alter the sequence of one of the chromosomes, so that it is identical to the other. Meiotic recombination is initiated through formation of a double-strand break (DSB). The 5’ ends of the break are then degraded, leaving long 3’ overhangs of several hundred nucleotides. One of these 3’ single stranded DNA segments then invades a homologous sequence on the homologous chromosome,

pos: Eukaryote can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells. These act as sex cells (gametes). Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis. The domain Eukaryota appears to be monophyletic, and makes up one of the domains of life in the three-domain system. The two other domains, Bacteria and Archaea,

pos: 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

pos: Genetic 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 recombination can occur with small probability at any location along 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

pos: Meiotic recombination checkpoint Meiotic recombination checkpoint The meiotic recombination checkpoint monitors the meiotic recombination during meiosis, and blocks the entry into metaphase I if the recombination is not efficiently processed. Meiotic recombination contributes to the cells in three different ways. First, to achieve proper segregation, each pair of homologous chromosomes must be linked to each other to maintain a certain level of tension between them. Such tension is supposed to help the assembly of spindles and generally depends on meiotic recombination. Secondly, meiotic recombination increases the genetic diversity of gametes, making them readily adapt to new environment. Thirdly, meiotic recombination is a DNA

pos: 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

pos: MSH5 important role in meiotic recombination. In the worm "Caenorhabditis elegans", the MSH5 protein is required during meiosis both for normal spontaneous and for gamma-irradiation induced crossover recombination and chiasma formation. Meiotic recombination is often initiated by double strand breaks. MSH5 mutants retain the competence to repair DNA double-strand breaks that are present during meiosis, but they accomplish this repair in a way that does not lead to crossovers between homologous chromosomes. The known mechanism of non-crossover recombinational repair is called synthesis dependent strand annealing (see homologous recombination; see also Bernstein et al.). MSH5 thus appears to be employed in directing

pos: MSH4 of pairing and synapsis of homologous chromosomes. In an "MSH4" mutant of rice, chiasma frequency was dramatically decreased to about 10% of the wild-type frequency, although the synaptonemal complex was normally installed. It is likely that MSH4 interacts with MSH5 to promote the majority of crossovers during rice meiosis. In general it appears that MSH4 acts during meiosis to direct the recombinational repair of some DNA double-strand breaks towards the crossover option rather than the non-cross over option (see Homologous recombination). MSH4 has been shown to interact with MLH1, MSH5 and MLH3. MSH4 MutS protein homolog 4 is a protein

pos: Eleanor Carothers expeditions, held in 1915 and 1919. During her time at the University of Iowa, she completed her most important work, in the field of genetics and cytology, using grasshopper embryos to study the independent assortment of heteromorphic homologous chromosomes. This was the first physical evidence that homologous chromosomes separated independently during meiosis, which is one source of genetic variation in sexually-reproducing organisms. Carothers retired from the University of Iowa in 1941 and moved from Iowa to Kingman, Kansas, where she continued to conduct research for the Woods Hole Marine Biological Lab. In 1954, she moved to Murdock, Kansas, where she

pos: REC8 a member of the kleisin family of SMC (structural maintenance of chromosome) protein partners. The protein localizes to the axial elements of chromosomes during meiosis in both oocytes and spermatocytes. REC8 protein appears to participate with other cohesins STAG3, SMC1ß and SMC3 in sister chromatid cohesion throughout the whole meiotic process in human oocytes. In the mouse, the homologous protein is a key component of the meiotic cohesion complex, which regulates sister chromatid cohesion and recombination between homologous chromosomes. Multiple alternatively spliced variants, encoding the same protein, have been found for this gene. Rec8 remains in complex with SMC proteins

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: Homologous chromosome separated in meiosis I, the sister chromatids from each pair are separated. The two haploid(because the chromosome no. has reduced to half. Earlier two sets of chromosomes were present, but now each set exists in two different daughter cells that have arisen from the single diploid parent cell by meiosis I) daughter cells resulting from meiosis I undergo another cell division in meiosis II but without another round of chromosomal replication. The sister chromatids in the two daughter cells are pulled apart during anaphase II by nuclear spindle fibers, resulting in four haploid daughter cells. Homologous chromosomes do not function

neg: Meiosis 1c). Prophase I is typically the longest phase of meiosis. During prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). This often results in chromosomal crossover. This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The

neg: Homologous chromosome emanating from opposite spindle poles attach to each of the homologs (each pair of sister chromatids) at the kinetochore. In anaphase I of meiosis I the homologous chromosomes are pulled apart from each other. The homologs are cleaved by the enzyme separase to release the cohesin that held the homologous chromosome arms together. This allows the chiasmata to release and the homologs to move to opposite poles of the cell. The homologous chromosomes are now randomly segregated into two daughter cells that will undergo meiosis II to produce four haploid daughter germ cells. After the tetrads of homologous chromosomes are

neg: Origin and function of meiosis repair, referred to as double-strand damage. One common example of double-strand damage is the double-strand break. In this case, genetic information (nucleotide sequence) is lost from both strands in the damaged region, and proper information can only be obtained from another intact chromosome homologous to the damaged chromosome. The process that the cell uses to accurately accomplish this type of repair is called recombinational repair. Meiosis is distinct from mitosis in that a central feature of meiosis is the alignment of homologous chromosomes followed by recombination between them. The two chromosomes which pair are referred to as non-sister chromosomes, since

neg: Genetic recombination 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 is a common mechanism used in DNA repair. 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

neg: Homologous chromosome helps make a population more stable by providing a wider range of genetic traits for natural selection to act on. In prophase I of meiosis I, each chromosome is aligned with its homologous partner and pairs completely. In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere. During the zygotene stage of prophase I, the homologous chromosomes pair up with each other. This pairing occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the homologous chromosomes along their lengths.

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: Pseudolinkage homologs assume a crosslike configuration in which four chromosomes, rather than the normal two, pair to achieve a maximum of synapsis between similar regions. We denote the chromosomes carrying translocated material with a T and the chromosomes with a normal order of genes with an N. Chromosomes N1 and T1 have homologous centromeres found in wild type on chromosome 1; N2 and T2 have centromeres found in wild type on chromosome 2. During anaphase of meiosis I, the mechanisms that attach the spindle to the chromosomes in this crosslike configuration still usually ensure the disjunction of homologous centromeres, bringing homologous

neg: Complete linkage absence of recombination, only parental phenotypes are expected. Genetic Linkage is the tendency of alleles, which are located closely together on a chromosome, to be inherited together during the process of meiosis in sexually reproducing organisms. During the process of meiosis, homologous chromosomes pair up, and can exchange corresponding sections of DNA. As a result, genes that were originally on the same chromosome can finish up on different chromosomes. This process is known as genetic recombination. The rate of recombination of two discrete loci corresponds to their physical proximity. Alleles that are closer together have lower rates of recombination than

neg: Meiosis pair in addition to cohesin between sister chromatids. Kinetochore microtubules shorten, pulling homologous chromosomes (which consist of 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. 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

neg: Meiosis 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 a prolonged G2-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 is

neg: Bivalent (genetics) Bivalent (genetics) A bivalent, sometimes referred to as a tetrad, is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. The formation of a bivalent occurs during the first division of meiosis (in the pachynema stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as

neg: 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: Homologous chromosome their chromosomal arms. There are two main properties of homologous chromosomes: the length of chromosomal arms and the placement of the centromere The actual length of the arm, in accordance with the gene locations, is critically important for proper alignment. Centromere placement can be characterized by four main arrangements, consisting of being either metacentric, submetacentric, telocentric, or acrocentric. Both of these properties are the main factors for creating structural homology between chromosomes. Therefore, when two chromosomes of the exact structure exist, they are able to pair together to form homologous chromosomes. Since homologous chromosomes are not identical and do not

neg: Immature ovum specifically in prophase I of meiosis. The halt is called dictyate. Most degenerate by atresia, but a few go through ovulation, and that's the trigger to the next step. Thus, an immature ovum can spend up to ~55 years as a primary oocyte (the last ovulation before menopause). The secondary oocyte is the cell that is formed by meiosis I in oogenesis. Thus, it has only one of each pair of homologous chromosomes. In other words, it is haploid. However, each chromosome still has two chromatids, making a total of 46 chromatids (1N but 2C). The secondary oocyte continues the

neg: Bivalent (genetics) highlighting the robustness of the chiasma and the cohesion that hold the bivalents together. Bivalent (genetics) A bivalent, sometimes referred to as a tetrad, is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. The formation of a bivalent occurs during the first division of meiosis (in the pachynema stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-strand breaks during the leptotene

neg: Homologous chromosome originate from the same organism, they are different from sister chromatids. Sister chromatids result after DNA replication has occurred, and thus are identical, side-by-side duplicates of each other. Humans have a total of 46 chromosomes, but there are only 22 pairs of homologous autosomal chromosomes. The additional 23rd pair is the sex chromosomes, X and Y. If this pair is made up of an X and Y chromosome, then the pair of chromosomes is not homologous because their size and gene content differ greatly. The 22 pairs of homologous chromosomes contain the same genes but code for different traits in

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: Flowering plant is the pairing of homologous chromosomes and homologous recombination (the exchange of genetic information) between homologous chromosomes. This process promotes the production of increased genetic diversity among progeny and the recombinational repair of damages in the DNA to be passed on to progeny. To explain the adaptive function of meiosis in flowering plants, some authors emphasize diversity and others emphasize DNA repair. Apomixis (reproduction via asexually formed seeds) is found naturally in about 2.2% of angiosperm genera. One type of apomixis, gametophytic apomixis found in a dandelion species involves formation of an unreduced embryo sac due to incomplete meiosis (apomeiosis)

neg: Homologous chromosome their allelic forms since one was inherited from the mother and one from the father. So humans have two homologous chromosome sets in each cell, meaning humans are diploid organisms. Homologous chromosomes are important in the processes of meiosis and mitosis. They allow for the recombination and random segregation of genetic material from the mother and father into new cells. Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell. It reduces the chromosome number in a germ cell by half by first

neg: Sexual reproduction material from both gametes. In a process called genetic recombination, genetic material (DNA) joins up so that homologous chromosome sequences are aligned with each other, and this is followed by exchange of genetic information. Two rounds of cell division then produce four daughter cells with half the number of chromosomes from each original parent cell, and the same number of chromosomes as both parents. For instance, in human reproduction each human cell contains 46 chromosomes in 23 pairs. Meiosis in the parents' gonads produce gamete cells which only contain 23 chromosomes each. When the gametes are combined via sexual intercourse

neg: Vertebrate thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs only rarely, if at all. Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms. However, meiosis may have been maintained during evolution by the efficient recombinational repair of DNA damages that meiosis provides, an advantage that could be realized at each generation. The mangrove killifish ("Kryptolebias marmoratus") produces both eggs and sperm by meiosis and

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: Prophase the spindle apparatus at locations called foci. The mitotic spindle is of great importance in the process of mitosis and will eventually segregate the sister chromatids in metaphase. The nucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production. This indicates a redirection of cellular energy from general cellular metabolism to cellular division. The nuclear envelope stays intact during this process. Meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II. Prophase I the most complex phase in all of meiosis because homologous chromosomes must pair and

neg: Diploidization when multiple copies of similar chromosomes are present in the nucleus, homeologous chromosomes can also pair with homologous chromosomes resulting in the formation of trivalents or multivalents. The formation of multivalents results in unequal division of the chromosomes and lead to daughter cells lacking one or few chromosomes. When homeologous chromosomes pair up though bivalents or multivalents, illegitimate genetic crossovers may occur. Since the chromosomes may differ in genetic structure and content, segments of the chromosome may be shuffled around resulting in massive gene loss. Additionally, illegitimate recombinations may also result in dicentric chromosomes lead to chromosome breakage during anaphase.

neg: Prophase exchange genetic information. Prophase II is very similar to mitotic prophase. Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in mitotic prophase, several crucial events occur within these phases such as pairing of homologous chromosomes and the reciprocal exchange of genetic material between these homologous chromosomes. Prophase I occurs at different speeds dependent on species and sex. Many species arrest meiosis in diplotene of prophase I until ovulation. In humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to

neg: Taraxacum officinale Sexual reproduction involves a reduction of the somatic chromosome number by meiosis followed by a restoration of the somatic chromosome number by fertilization. Diploid dandelions have eight pairs of chromosomes, and meiosis is regular with normal pairing of homologous chromosomes at the metaphase I stage of meiosis. The second group consists of polyploid (mostly triploid) apomicts, meaning that both a viable embryo as well as a functional endosperm is formed without prior fertilization. In contrast to the sexual diploids, the pairing of chromosomes at metaphase I in triploid apomicts is strongly reduced. However pairing is still sufficient to allow some

neg: Endoreduplication of the adult female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs rarely, if at all. Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the maintenance of meiosis for millions of years. Perhaps the efficient recombinational repair of DNA damages at each generation provided by meiosis has been a sufficient advantage to maintain meiosis. Endoreduplication Endoreduplication (also

neg: Bivalent (genetics) a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I. Resolution of the DNA recombination intermediate into a crossover exchanges DNA segments between the two homologous chromosomes at a site called a chiasma (plural: chiasmata). This physical strand exchange and the cohesion between the sister chromatids along each chromosome ensure robust pairing of the homologs in diplotene phase. The structure, visible by microscopy, is called a bivalent. A bivalent is the association of two

neg: Meiotic recombination checkpoint spindle pole bodies can separate. The event is triggered by the activation of M-Cdk in late prophase I. Then the spindle assembly checkpoint examines the attachment of microtubules at kinetochores and APC can initiate the metaphase I. The special chromosome separation in meiosis, homologous chromosomes separation in meiosis I and chromatids separation in meiosis II, requires special tension between homologous chromatids and non-homologous chromatids for distinguishing microtubule attachment and it relies on the programmed DNA double strand break (DSB) and repair in prophase I. Therefore meiotic recombination checkpoint can be a kind of DNA damage response at specific time spot.

neg: Homologous recombination in E. coli and other bacteria, and "rad51" and "dmc1" in yeast and other eukaryotes, including humans). Multiplicity reactivation has also been demonstrated in numerous pathogenic viruses. Without proper homologous recombination, chromosomes often incorrectly align for the first phase of cell division in meiosis. This causes chromosomes to fail to properly segregate in a process called nondisjunction. In turn, nondisjunction can cause sperm and ova to have too few or too many chromosomes. Down's syndrome, which is caused by an extra copy of chromosome 21, is one of many abnormalities that result from such a failure of homologous recombination in

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: Pathogen process involving meiosis and syngamy. Meiosis involves the intimate pairing of homologous chromosomes and recombination between them. Examples of eukaryotic pathogens capable of sex include the protozoan parasites "Plasmodium falciparum", "Toxoplasma gondii", "Trypanosoma brucei", "Giardia intestinalis", and the fungi "Aspergillus fumigatus", "Candida albicans" and "Cryptococcus neoformans". Viruses may also undergo sexual interaction when two or more viral genomes enter the same host cell. This process involves pairing of homologous genomes and recombination between them by a process referred to as multiplicity reactivation. Examples of viruses that undergo this process are herpes simplex virus, human immunodeficiency virus, and vaccinia virus. The

neg: Meiosis of the synaptonemal complex assemble. Leptotene is of very short duration and progressive condensation and coiling of chromosome fibers takes place. The "zygotene" stage, also known as "zygonema", from Greek words meaning "paired threads", occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. In some organisms, this is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex. Pairing is brought about in a zipper-like fashion

neg: Meiomitosis other chromosome pair), the homologous chromosomes are segregated together. In these cells chiasma occurs and sister chromatids are partly held together by cohesions; when cell division occurs, the sister chromatids are sheared and mis-segregated which results in an aneuploid state. Melanoma may be prone to meiomitosis due to the high frequency of testis antigen that is expressed in tumor cells, two of which are known meiosis proteins. Melanoma tumor cells that replicate via meiomitosis may become immortal due to the expression of telomerase, but this comes at the cost of meiotic proteins and genomic instability. Instability is different from tumor

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: Wilfred Eade Agar He found out that apart from rare mutations, the genetic constitution of these crustacea remained unchanged, generation after generation. This work provided the first direct evidence from animals that the segregation of genes is according to the segregation of homologous chromosomes at meiosis. In 1919, he accepted the chair of zoology at the University of Melbourne; his notable projects concerned marsupial chromosomes and inheritance in cattle. Doing it, he was afraid of giving up most of his ambitions; his anxiety seemed justified when he realized that he was to give all the lectures and conduct a big amount of the

neg: REC8 REC8 Meiotic recombination protein REC8 homolog is a protein that in humans is encoded by the "REC8" gene. Rec8 is a meiosis-specific component of the cohesin complex that binds sister chromatids in preparation for the two divisions of meiosis. Rec8 is sequentially removed from sister chromatids. It is removed from the arms of chromosomes in the first division - separating homologous chromosomes from each other. However, Rec8 is maintained at centromeres so that sister chromatids are kept joined until anaphase of meiosis II, at which point removal of remaining cohesin leads to the separation of sister chromatids. This gene encodes

neg: Homologous chromosome adhering homologous chromosomes vary among organisms, proper functioning of those mechanisms is imperative in order for the final genetic material to be sorted correctly. Proper homologous chromosome separation in meiosis I is crucial for sister chromatid separation in meiosis II. A failure to separate properly is known as nondisjunction. There are two main types of nondisjunction that occur: trisomy and monosomy. Trisomy is caused by the presence of one additional chromosome in the zygote as compared to the normal number, and monosomy is characterized by the presence of one fewer chromosome in the zygote as compared to the normal number.

neg: Chromatid during mitosis or meiosis. SCEs appear to primarily reflect DNA recombinational repair processes responding to DNA damage (see articles Sister chromatids and Sister chromatid exchange). "Non-sister chromatids", on the other hand, refers to either of the two chromatids of paired homologous chromosomes, that is, the pairing of a paternal chromosome and a maternal chromosome. In chromosomal crossovers, non-sister (homologous) chromatids form chiasmata to exchange genetic material during the prophase I of meiosis (See Homologous recombination). Chromatid A chromatid (Greek "khrōmat-" 'color' + "-id") is one copy of a newly copied chromosome which is still joined to the original chromosome by

neg: Parasexual cycle homologous chromosomes break and rejoin, leading to chromosomes that are hybrids of the parental types. It can also occur during mitosis but at a much lower frequency because the chromosomes do not pair in a regular arrangement. Nevertheless, the result will be the same when it does occur—the recombination of genes. Occasionally, nondisjunction of chromosomes occurs during division of a diploid nucleus, so that one of the daughter nuclei has one chromosome too many (2n+1) and the other has one chromosome too few (2n–1). Such nuclei with incomplete multiples of the haploid number are termed "aneuploid", as they do not

neg: Dicentric chromosome syndrome. The formation of dicentric chromosomes and their implications on centromere function are studied in certain clinical cytogenetics laboratories. Most dicentric chromosomes are known to form through chromosomal inversions, which are rotations in regions of a chromosome due to chromosomal breakages or intra-chromosomal recombinations. Inversions that exclude the centromere are known as paracentric inversions, which result in unbalanced gametes after meiosis. During prophase of meiosis I, homologous chromosomes form an inversion loop and crossover occurs. If a paracentric inversion has occurred, one of the products will be acentric, while the other product will be dicentric. The dicentric chromatid is pulled

neg: Meiosis the kinetochores are known as "kinetochore microtubules". Other microtubules will interact with microtubules from the opposite centrosome: these are called "nonkinetochore microtubules" or "polar microtubules". A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton. Homologous pairs move together along the metaphase plate: As "kinetochore microtubules" from both centrosomes attach to their respective kinetochores, 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

neg: 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

neg: Klinefelter syndrome infants; they may also struggle in school, both academically and with sports. The extra chromosome is retained because of a nondisjunction event during paternal or maternal meiosis I (gametogenesis). Nondisjunction occurs when homologous chromosomes, in this case the X and Y or two X sex chromosomes, fail to separate, producing a sperm with an X and a Y chromosome or an egg with two X chromosomes. Fertilizing a normal (X) egg with this sperm produces an XXY offspring (Klinefelter). Fertilizing a double X egg with a normal sperm also produces an XXY offspring (Klinefelter). Another mechanism for retaining the extra

neg: Homologous recombination plants, animals, fungi and protists. In cells that divide through mitosis, homologous recombination repairs double-strand breaks in DNA caused by ionizing radiation or DNA-damaging chemicals. Left unrepaired, these double-strand breaks can cause large-scale rearrangement of chromosomes in somatic cells, which can in turn lead to cancer. In addition to repairing DNA, homologous recombination also helps produce genetic diversity when cells divide in meiosis to become specialized gamete cells—sperm or egg cells in animals, pollen or ovules in plants, and spores in fungi. It does so by facilitating chromosomal crossover, in which regions of similar but not identical DNA are exchanged

neg: Cell division Cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell (mitosis), and a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes (meiosis). Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated

neg: Dominance (genetics) as haploid, and created through meiosis. These gametes then fuse during fertilization during sexual reproduction, into a new single cell zygote, which divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each (non-gamete) cell as its parents. Each chromosome of a matching (homologous) pair is structurally similar to the other, and has a very similar DNA sequence (loci, singular locus). The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene also has a corresponding homologue, which may exist in different versions called alleles.

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: Homologous chromosome and future research on the subject of homologous chromosome is heavily focused on the roles of various proteins during recombination or during DNA repair. In a recently published article by Pezza et al. the protein known as HOP2 is responsible for both homologous chromosome synapsis as well as double-strand break repair via homologous recombination. The deletion of HOP2 in mice has large repercussions in meiosis. Other current studies focus on specific proteins involved in homologous recombination as well. There is ongoing research concerning the ability of homologous chromosomes to repair double-strand DNA breaks. Researchers are investigating the possibility of exploiting

neg: Aurora B kinase B enzyme inhibitor (Aurora B localization is not affected), the condensin complex localizes normally. Aurora B localizes to the paired arms of homologous chromosomes in metaphase I of C. elegans meiosis, and perturbs microtubule dynamics in mitosis. Release of this cohesion, which is dependent on Aurora B, is required for progression to anaphase I and segregation of homologous chromosomes. In mitotic vertebrate B lymphocytes, the proper centromeric localization of a number of Aurora B binding partners requires cohesin. The Aurora B complex is necessary for cytokinesis in vertebrates, C. elegans, Drosophila, and fission yeast. In various cell types, overexpression of

neg: Chromosomal crossover crossover numbers, and a large number of meioses with non exchange chromosomes. Nevertheless, this mutant gave rise to spore 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 grasshopper "Melanoplus femur-rubrum" was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured. Irradiation during the leptotene-zygotene stages of meiosis (that is, prior to the pachytene period in which crossover recombination occurs) was found to increase subsequent chiasma frequency. Similarly, in the grasshopper "Chorthippus brunneus",