[The contact model of crossing over: definition of conditions required for the co-orientation of homologs].
Three phenomena have been examined: (1) independence of the segregation of structurally normal homologues from the position of crossover exchange relative to the centromere; (2) independence of chromosomal rearrangement ability to disturb disjunction from its position on chromosome; and (3) dependent, predominantly distal position of crossover exchanges in nondisjunction chromosomes. All three phenomena are explained in terms of the contact model of crossing over. According to the model, disjunction of the homologues is provided by a series of local contacts between them. The maximal number of contacts per arm is constant and is determined by the size of the absolute interference zone. The co-orientating influence of the contact decreases with the movement of its site away from the centromeric region responsible for the co-orientation. The role of the contact consists of bringing the centromeric regions of the homologues together. Co-orientation success does not depend on the presence or absence of a crossover exchange at the site of the contact. The nondisjunction chromosomes are selected with regard to the absence of proximal (or close to proximal) contacts. Hence, the exchanges formed on the basis of these contacts, are co-orientation-associated. In the disjunction chromosomes, the entire set of contacts is present. Any of these can be the first to develop and generate a crossover exchange. Because of this, exchanges in the disjunction chromosomes do not demonstrate association with the co-orientation. According to the model, the distortion of the previous contact disturbs (retards or prevents) the formation of the next contact. Hence, distortion of the contact formation in the site of rearrangement, irrespective to its location, would negatively affect the formation of the contact in the proximal region, and would promote nondisjunction.
Chadov
B F
BF
Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirsk, Russia.
D000818:Animals; D002503:Centromere; D020090:Chromosome Segregation; D003434:Crossing Over, Genetic; D004330:Drosophila / Q000235:genetics*; D015321:Gene Rearrangement; D017344:Genes, Insect; D008957:Models, Genetic; D009630:Nondisjunction, Genetic; D012689:Sequence Homology, Nucleic Acid