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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. Points of crossing over become visible as chiasma after the synaptonemal complex dissembles and the homologous chromosomes slightly apart from each other. 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 two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I of meiosis, but ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meiosis Crossover
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tetrad (chromosomal Formation)
Tetrad ('group of 4') or tetrade may refer to: * Tetrad (area), an area 2 km x 2 km square * Tetrad (astronomy), four total lunar eclipses within two years * Tetrad (chromosomal formation) * Tetrad (general relativity), or frame field ** Tetrad formalism, an approach to general relativity * Tetrad (geometry puzzle), a set of four simply connected disjoint planar regions in the plane * Tetrad (meiosis), the four cells produced by meiotic cell division * Tetrad (music), a set of four notes ** Tetrad (chord), a series of four notes * Tetrad (symbol), or tetractys, a triangular figure of ten points arranged in four rows, and mystical symbol * Medical tetrad, a group of four signs or symptoms which characterise a specific medical condition * Nibble, or tetrade, a 4-bit group * a tuple of length 4 * Tetrad Islands, in the Antarctic See also * * 4 * Triad (other) ('group of 3') * Pentad (other) ('group of 5') * Dark tetrad, group of four undesirable personality tr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Holocentric Chromosome
Holocentric chromosomes are chromosomes that possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes. They were first described in cytogenetic experiments in 1935. Since this first observation, the term holocentric chromosome has referred to chromosomes that: i) lack the primary constriction corresponding to the centromere observed in monocentric chromosomes; and ii) possess multiple kinetochores dispersed along the entire chromosomal axis, such that microtubules bind to the chromosome along its entire length and move broadside to the pole from the metaphase plate. Holocentric chromosomes are also termed ''holokinetic'', because, during cell division, the sister chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes. Holocentric chromosomes have evolved several times during both animal and plant evolution, and are currently reported in about eight hundred diverse speci ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bivalent (genetics)
A bivalent is one pair of chromosomes (sister chromatids) in a tetrad. A tetrad is the association of a pair of homologous chromosomes (4 sister chromatids) 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. In most organisms, each replicated chromosome (composed of two identical sisters chromatid) elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as 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 Contents * 1 Formation * 2 Structure * 3 Function * 4 References Formation The formation of a bivalent occurs during the first division of meiosis (in the Z ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Genetic Recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) ''interchromosomal'' recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes (random orientation of pairs of homologous chromosomes in meiosis I); & (2) ''intrachromosomal'' recombination, occurring through crossing over. During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a se ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chromosomal Crossover
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. 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 on the discovery of Frans Alfons Janssens who described the phenomenon in 1909 and had called it "chiasmatypie". The term '' chiasma'' is linked, if not identical, to chromosomal crossover. Morgan immediately saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of hi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Melanoplus Femurrubrum
''Melanoplus femurrubrum'', the red-legged grasshopper, is a species of grasshopper belonging to the genus '' Melanoplus''. It is one of the most common grasshoppers found in Mexico, the United States, and Canada. This grasshopper is frequently used as a model organism in scientific studies, due to their abundance throughout North America and behavioral response to changes in climate. Identification ''Melanoplus femurrubrum'' is a medium-sized grasshopper, in which males can range in length from - , whereas females can range from - long. This grasshopper has a reddish-brown back, a greenish-yellow belly, and red hind tibiae, hence its specific name ''femurrubrum'' (''femur'' = thigh, ''rubrum'' = red). Wings of ''M. femurrubrum'' typically extend beyond the tip of the abdomen. Males have an enlarged abdomen, with a U-shaped sub-genital plate. Habitat ''Melanoplus femurrubrum'' can be found in a variety of habitats found throughout most of North America, but prefer grasslan ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pachytene
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chromatids
A chromatid (Greek ''khrÅmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes. Chromatid pairs are normally genetically identical, and said to be homozygous. However, if mutations occur, they will present slight differences, in which case they are heterozygous. The pairing of chromatids should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome. Sister chromatids Chromatids may be sister or non-sister chromatids. A sister chromatid is either one of the two chromatids of the same chromosome joined together by a common centromere. A pair of sister chromatids is called a dyad. Once sister chromatids have separated (during the anaphase of m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
