, in fact only a decade after the chromosome theory of Bovery and Sutton had been established ]. This first meiotic, reductional, division leads to the chromosome number of the two daughter nuclei being halved, with the second meiotic division giving rise to haploid gametes, as illustrated in Figure 1b.Ĭrossover interference was first described nearly a century ago by Sturtevant and Muller in Drosophila melanogaster The positional control by interference seemingly creates bivalents of optimal mechanical stability, promoting regular segregation at the subsequent Meiosis I Anaphase. physical connections that hold parental homologs (bivalents) together. ranked length and centromere index of the chromosomes involved, (2) the specific bivalent/multivalent length and flexibility, dependent on the way this structure is positioned within the nucleus and the size of the respective meiocyte nuclei, (3) the frequency characteristics of the oscillatory movements at respectively the telomeres and the kinetochores.Ĭompletion of reciprocal recombination/crossing-over between parental half chromosomes (chromatids) together with chromatid cohesion, leads to the formation of chiasmata, i.e. The parameters to vary in this type of experiment will include: (1) the mitotic karyotype, i.e. Further studies are underway to analyze mechanical/mathematical aspects of this model for the origin of crossover/chiasma interference, using string replicas of the homologous chromosomes at the Prophase stage of Meiosis I. The crossover/chiasma frequency distribution in humans and mice with normal karyotypes as well as in carriers of structural chromosome rearrangements are those expected on the COM model. This model adequately explains the salient features of crossover/chiasma interference, where (1) there is normally at least one crossover/chiasma per bivalent, (2) the number is correlated to bivalent length, (3) the positions are dependent on the number per bivalent, (4) interference distances are on average longer over the centromere than along chromosome arms, and (5) there are significant changes in carriers of structural chromosome rearrangements. The C hromosome O scillatory M ovement (COM) model for crossover/chiasma interference implies that, during Prophase of Meiosis I, oscillatory movements of the telomeres (attached to the nuclear membrane) and the kinetochores (within the centromeres) create waves along the length of chromosome pairs (bivalents) so that crossing-over and chiasma formation is facilitated by the proximity of parental homologs induced at the nodal regions of the waves thus created. In spite of much research addressing this question, the underlying mechanism(s) for the phenomenon called crossover/chiasma interference is/are still unknown and this constitutes an outstanding biological enigma. In fact, the number and distribution of crossovers are strictly regulated with crossovers/chiasmata formed in optimal positions along the length of individual chromosomes, facilitating regular chromosome segregation at the first meiotic division. It is now nearly a century since it was first discovered that crossovers between homologous parental chromosomes, originating at the Prophase stage of Meiosis I, are not randomly placed.
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