title: [From the phenomenon of nondisjunction to the problem of chromosome co-orientation (75th anniversary of Bridges' article].
abstract: The concepts of the mechanism of chromosome nondisjunction in Drosophila are described in a historical retrospective. Evidences are given for the appropriateness of the term co-orientation in the traditional sense used by geneticists treating nondisjunction. There are 6 variants of co-operation in Drosophila meiosis depending upon the number and particular chromosomes involved in co-orientation. The classical chromosome nondisjunction is a variant of co-orientation in the bivalent composed of two homologous chromosomes. By comparing the different variants of pairing (pairing in bi- and multivalents) resulting in co-orientation, the elementary events preceding co-orientation may be identified. The author reviews his recent data concerning the similarities of the co-orientation of two homologs and the co-orientation of two nonhomologs in Drosophila meiosis. The concept of the role of pairing in the precentromeric heterochromatic region during chromosome co-orientation is considered, and the hypothesis of delayed pairing in this region during meiotic prophase is put forward. Based on the suggested hypothesis clarified are (i) the relationship of pairing, crossing over, and disjunction of homologous chromosomes (ii) the relationship of crossing over and co-orientation of nonhomologous chromosomes, and (iii) the time when the contact resulting in nonhomolog co-orientation takes place.
authors.forename: B F
authors.initials: BF
authors.lastname: Chadov
mesh_terms: D000818:Animals; D002875:Chromosomes; D004330:Drosophila / Q000235:genetics*; D008540:Meiosis / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid
vernacular_title: Ot fenomena neraskhozhdeniia k probleme koorientatsii khromosom (K 75-letiiu vykhoda v svet stat'i Bridzhesa).