Ontogenes and the Paradox of Homologous Pairing

The mutations in ontogenes have been shown to drastically increase the nondisjunction of X chromosomes in the D. melanogaster meiosis. This means that ontogenes are involved in the process that brings the homologs together although both the genes and ontogenes are finally paired. The phenomenon named the paradox of homologous pairing is described. Chromosomal rearrangements (inversions and translocations) lead to formation of specific topological figures (loops and crosses) during pairing. The mutual arrangement of the nucleotide sequences of homologous ontogenes before and after formation of such figures is different. Their arrangement coincides after a figure is formed and the pairing looks homologous. However, before the figure is formed, their arrangement does not match and the pairing is actually nonhomologous. The available data on ontogenes allows this paradox to be resolved. It is assumed that the sequence of each ontogene possesses a factor that 1) is a product of this nucleotide sequence;2) is co-located with this sequence;and 3) generates approaching independently of nucleotide sequence position in space. The sole candidate to the role of this factor is the DNA conformation of ontogene. The conformation in the form of a solenoid of DNA is able to generate an electromagnetic field independent of the orientation of the DNA itself. The proposed resolution of the paradox is considered in terms of the problem of genetic homology.

Recombination-Independent Mechanisms and Pairing of Homologous Chromosomes during Meiosis in Plants

Meiosis is the specialized eukaryotic cell division that permits the halving of ploidy necessary for game- togenesis in sexually reproducing organisms, This involves a single round of DNA replication followed by two successive divisions. To ensure balanced segregation, homologous chromosome pairs must migrate to opposite poles at the first meiotic division and this means that they must recognize and pair with each other beforehand. Although understanding of the mechanisms by which meiotic chromosomes find and pair with their homologs has greatly advanced, it remains far from being fully understood. With some notable exceptions such as male Drosophila, the recognition and physical link- age of homologs at the first meiotic division involves homologous recombination. However, in addition to this, it is clear that many organisms, including plants, have also evolved a series of recombination-independent mechanisms to facili- tate homolog recognition and pairing. These implicate chromosome structure and dynamics, telomeres, centromeres, and, most recently, small RNAs. With a particular focus on plants, we present here an overview of understanding of these early, recombination-independent events that act in the pairing of homologous chromosomes during the first meiotic division,

Molecular Mechanisms of Homologous Chromosome Pairing and Segregation in Plants

In most eukaryotic species, three basic steps of pairing, recombination and synapsis occur during prophase of meiosis I. Homologous chromosomal pairing and recombination are essential for accurate segregation of chromosomes. In contrast to the well-studied processes such as recombination and synapsis, many aspects of chromosome pairing are still obscure. Recent progress in several species indicates that the telomere bouquet formation can facilitate homologous chromosome pairing by bringing chromosome ends into close proximity, but the sole presence of telomere clustering is not sufficient for recognizing homologous pairs. On the other hand, accurate segregation of the genetic material from parent to offspring during meiosis is dependent on the segregation of homologs in the reductional meiotic division （MI） with sister kinetochores exhibiting mono-orientation from the same pole, and the segregation of sister chromatids during the equational meiotic division （MII） with kinetochores showing bi-orientation from the two poles. The underlying mechanism of orientation and segregation is still unclear. Here we focus on recent studies in plants and other species that provide insight into how chromosomes find their partners and mechanisms mediating chromosomal segregation.