Chiasma (genetics)
Encyclopedia
A chiasma in genetics
, is thought to be the point where two homologous non-sister chromatid
s exchange genetic material during chromosomal crossover
during meiosis
(sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of meiosis
, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene stage. When each bivalent (tetrad), which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.
where recombination frequency is:
The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens
, a Jesuit professor at the University of Leuven in Belgium
. A bivalent refers to the two homologous chromosomes (4 chromatids). The chiasmata refers to the actual break of the phosphodiester bond during crossing over. The larger the number of map units between the genes, the more crossing over occurs.
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, is thought to be the point where two homologous non-sister chromatid
Chromatid
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...
s exchange genetic material during chromosomal crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
(sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene stage. When each bivalent (tetrad), which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.
- chiasma frequency = 2 x recombination frequency
where recombination frequency is:
- recombination frequency = (no. of recombinants) / (total no. of progeny)
The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens
Frans Alfons Janssens
Frans Alfons Janssens . Ordained as a priest in 1886. He obtained a PhD in Natural Science with the highest honors and a scholarchip to attend the most prestigious foreign laboratories. He worked with Professor Kjeldahl at the Hansen Institute Carlsberg Brewery in Copenhagen. He was also a teacher...
, a Jesuit professor at the University of Leuven in Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
. A bivalent refers to the two homologous chromosomes (4 chromatids). The chiasmata refers to the actual break of the phosphodiester bond during crossing over. The larger the number of map units between the genes, the more crossing over occurs.