when are homologous chromosomes for the same trait separated from each other during meiosis
meiosis I
2088518
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
Homologous chromosome
2088523
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
Homologous chromosome
251319
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
Meiosis
2088522
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
Homologous chromosome
2088518
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
Homologous chromosome
251318
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
Meiosis
251330
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,
Meiosis
251327
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
Meiosis
2088519
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.
Homologous chromosome
7327549
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
Cohesin
2678824
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
Nondisjunction
8249734
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
Dicentric chromosome
2088521
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
Homologous chromosome
15015776
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
Pseudolinkage
16674446
the genetic cause of 48, XXXY syndrome the oocyte would contain three X chromosome. This would be caused by two non-disjunction events during oogenesis. In meiosis I both sets of duplicated X chromosomes would have to be not separated. Then in meiosis II one set of X chromosomes would have to not separate and the other set would separate resulting in one oocyte with three X chromosomes. A normal sperm containing a Y chromosome would have to fertilize the XXX oocyte to make a XXXY zygote. The additional X chromosomes that are characteristic of this condition are associated with an
XXXY syndrome
2088526
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.
Homologous chromosome
262609
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
Mendelian inheritance
2088511
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)
Homologous chromosome
9354233
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
Complete linkage
7246777
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
Sister chromatids
251331
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
Meiosis
2088530
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
Homologous chromosome
6702969
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
Chromomere
2088524
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
Homologous chromosome
111336
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
Evolution
12967442
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
Sexual reproduction
312797
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
Ploidy
11013184
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
Chromosome segregation
6456199
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
RecLOH
1380522
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
Genetic recombination
6309953
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
Synapsis
2088517
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
Homologous chromosome
251329
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
Meiosis
725729
ploidal levels. Autopolyploids possess at least three homologous chromosome sets, which can lead to high rates of multivalent pairing during meiosis (particularly in recently formed autopolyploids, a.k.a. neopolyploids) and an associated decrease in fertility due to the production of aneuploid gametes. Natural or artificial selection for fertility can quickly stabilize meiosis in autopolyploids by restoring bivalent pairing during meiosis, but the high degree of homology among duplicated chromosomes causes autopolyploids to display polysomic inheritance. This trait is often used as a diagnostic criterion to distinguish autopolyploids from allopolyploids, which commonly display disomic inheritance after they progress past the neopolyploid stage.
Polyploidy
473710
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
Cell division
11013182
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
Chromosome segregation
2088514
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
Homologous chromosome
4505702
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
Sequence homology
11622120
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
REC8
17860857
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
Eleanor Carothers
8489983
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
Bivalent (genetics)
2088513
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
Homologous chromosome
475190
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
Rice
473543
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
Vertebrate
251334
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
Meiosis
6210823
-"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
Polysomy
5835662
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
Endoreduplication
3861437
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
Mongolian gerbil
9354231
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
Complete linkage
251336
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
Meiosis
2899127
DNA molecule. During mitosis the double chromosomes are split to produce 92 "single chromosomes", half of which go into each daughter cell. During meiosis, there are two chromosome separation steps which assure that each of the four daughter cells gets one copy of each of the 23 types of chromosome. Though cell reproduction that uses mitosis can reproduce eukaryotic cells, eukaryotes bother with the more complicated process of meiosis because sexual reproduction such as meiosis confers a selective advantage. Notice that when meiosis starts, the two copies of sister chromatids number 2 are adjacent to each other. During this time,
Cell growth
15068690
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
Origin and function of meiosis
18032692
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
Parthenogenesis in amphibians
251341
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
Meiosis
2088516
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
Homologous chromosome
11622121
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
REC8
2088528
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
Homologous chromosome
13596662
the target locus. Zygosity Zygosity () is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are
Zygosity
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Zygosity Zygosity () is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism
Zygosity
773828
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.
Dominance (genetics)
9346286
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
Balancer chromosome
2088535
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
Gametogenesis
11013191
ability depends on the microtubule motor dynein that regulates the movement of chromosomes to the poles of the meiotic spindle. Chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. During mitosis chromosome segregation occurs routinely as a step in cell division (see mitosis diagram). As indicated in the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome
Chromosome segregation
2076252
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
Aneuploidy
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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
Pathogen
2088529
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
Homologous chromosome
1380525
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
Genetic recombination
14022041
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
Meiotic recombination checkpoint
251339
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
Meiosis
5981375
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
Homologous recombination
251332
process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma). Prophase I has historically been divided into a series of substages which are named according to the appearance of chromosomes. The first stage of prophase I is the "leptotene" stage, also known as "leptonema", from Greek words meaning "thin threads".In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—become "individualized" to form visible strands within the nucleus. The two sister chromatids closely associate and are visually indistinguishable from one another. During leptotene, lateral elements
Meiosis
6639071
a result of the maternal and paternal chromosomes contributed from each gamete at fertilization of that offspring. During meiosis in gametes, homologous chromosomes experience genetic recombination and segregate randomly into haploid daughter cells, each with a unique combination of maternally and paternally coded genes. Dominant alleles will override the expression of recessive alleles. Test crosses are used to test an individual's genotype by crossing it with an individual of a known genotype. Individuals that show the recessive phenotype are known to have a homozygous recessive genotype. Individuals that show the dominant phenotype, however, may either be homozygous dominant or heterozygous.
Test cross
8489987
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
Bivalent (genetics)
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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
Chromatid
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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
Chromosome segregation
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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
Parthenogenesis
251333
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
Meiosis
81395
be thousands of copies per cell. Asexually reproducing species have one set of chromosomes that are the same in all body cells. However, asexual species can be either haploid or diploid. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes (23 pairs in humans with one set of 23 chromosomes from each parent), one set from the mother and one from the father. Gametes, reproductive cells, are haploid [n]: They have one set of chromosomes. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of
Chromosome
664535
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
Barbara McClintock
11330854
Dmc1's activity in purified systems and are also implicated as being required for Dmc1 function in cells. During meiosis, the two recombinases, Rad51 and Dmc1, interact with single-stranded DNA to form specialized filaments that are adapted for facilitating recombination between homologous chromosomes. Both Dmc1 and Rad51 have an intrinsic ability to self-aggregate. The presence of Rad51 filaments stabilizes adjacent Dmc1 filaments and conversely Dmc1 stabilizes adjacent Rad51 filaments. A model was proposed in which Dmc1 and Rad51 form separate filaments on the same single stranded DNA and cross-talk between the two recombinases affects their biochemical properties. During meiosis, even in
DMC1 (gene)
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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
Meiosis
737977
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",
Chromosomal crossover
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Sister chromatids A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis.
Sister chromatids
312803
virus, and HIV. "Homoploid" means "at the same ploidy level", i.e. having the same number of homologous chromosomes. For example, homoploid hybridization is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies, or happens soon after hybridization. Similarly, homoploid speciation contrasts with polyploid speciation. Zygoidy is the state where the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid. In the azygoid state the chromosomes are unpaired. It may be the natural state of
Ploidy
15068684
of eukaryotes during which meiosis and accompanying sexual capability did not yet exist. In addition, as noted by Wilkins and Holliday, there are four novel steps needed in meiosis that are not present in mitosis. These are: (1) pairing of homologous chromosomes, (2) extensive recombination between homologs; (3) suppression of sister chromatid separation in the first meiotic division; and (4) avoiding chromosome replication during the second meiotic division. Although the introduction of these steps seems to be complicated, Wilkins and Holliday argue that only one new step, homolog synapsis, that was particularly initiated in the evolution of meiosis from mitosis.
Origin and function of meiosis
737967
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
Chromosomal crossover
251320
some cases all four of the meiotic products form gametes such as sperm, spores, or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of
Meiosis
518587
the other). Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs
Hybrid (biology)
4862615
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,
Gene conversion
9637850
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
Parthenogenesis
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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
MSH5
11918306
Chiasma (genetics) In genetics, a chiasma (pl. chiasmata) is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis. In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy. The phenomenon of genetic chiasmata ("chiasmatypie") was discovered and described in 1909 by Frans Alfons Janssens, a Professor at the University of Leuven in Belgium. When each tetrad, which is composed of
Chiasma (genetics)
7971329
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
Coprinopsis cinerea
419178
messenger RNA. The inheritable unit that may influence a trait is called a gene. A gene is a portion of a chromosome, which is a very long and compacted string of DNA and proteins. An important reference point along a chromosome is the centromere; the distance from a gene to the centromere is referred to as the gene's locus or map location. The nucleus of a diploid cell contains two of each chromosome, with homologous (mostly identical) pairs of chromosomes having the same genes at the same loci. Different phenotypic traits are caused by different forms of genes, or alleles,
Phenotypic trait
9346299
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
Balancer chromosome
11013181
Chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. During mitosis chromosome segregation occurs routinely as a step in cell division (see mitosis diagram). As indicated in the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome forms two copies called chromatids. These chromatids separate to opposite poles, a process facilitated by a protein complex referred to
Chromosome segregation
11013185
DNA replication. Thus the two chromatids comprising each chromosome separate into different nuclei, so that each nucleus gets a single set of chromatids (now called chromosomes) and each nucleus becomes included in a haploid gamete (see stages following prophase II in the meiosis diagram). This segregation process is also facilitated by cohesin. Failure of proper segregation during prophase II can also lead to aneuploid gametes. Aneuploid gametes can undergo fertilization to form aneuploid zygotes and hence to serious adverse consequences for progeny. Meiotic chromosomal crossover (CO) recombination facilitates the proper segregation of homologous chromosomes. This is because, at the end
Chromosome segregation
160564
invade surrounding tissues. In certain lymphocytes in the human immune system, V(D)J recombination generates different genomic sequences such that each cell produces a unique antibody or T cell receptors. During meiosis, diploid cells divide twice to produce haploid germ cells. During this process, recombination results in a reshuffling of the genetic material from homologous chromosomes so each gamete has a unique genome. Genomes are more than the sum of an organism's genes and have traits that may be measured and studied without reference to the details of any particular genes and their products. Researchers compare traits such as karyotype (chromosome
Genome
251232
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)
Flowering plant
5981349
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
Homologous recombination
10590085
Immature ovum An immature ovum is a cell that goes through the process of oogenesis to become an ovum. It can be an oogonium, an oocyte, or an ootid. An oocyte, in turn, can be either primary or secondary, depending on how far it has come in its process of meiosis. Oogonia are the cells that turn into primary oocytes in oogenesis. They are diploid, i.e. containing both pairs of homologous chromosomes. There are 23 chromosome pairs. Thus, there are 46 chromosomes. Each chromosome, however, hasn't yet duplicated itself. As such, there is only one chromatid on each chromosome, making
Immature ovum