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The recent discovery of a new class of RNA in a centrosome may be an indirect evidence of the centrosomal hypothesis of cellular ageing and differentiation. The presence of a reverse transcriptase domain in this class of RNA, together with its specificity to this organelle, makes the centrosome a place of information memorizing and reproduction. This feature of centrosome is one of the main points of centrosomal hypothesis of cell aging and differentiation.

Two original mechanisms of nuclear restitution related to different processes of meiotic division of pollen mother cells (PMCs) have been found in male meiosis of the lines of maize haploids no. 2903 and no. 2904. The first mechanism, which is characteristic of haploid no. 2903, consists in spindle deformation (bend) in the conventional metaphase-anaphase I. This leads to asymmetric incomplete cytokinesis with daughter cell membranes in the form of incisions on the mother cell membrane. As a result, the chromosomes of the daughter nuclei are combined into a common spindle during the second meiotic division, and a dyad of haploid microspores is formed at the tetrad stage. The frequency of this abnormality is about 50%. The second restitution mechanism, which has been observed in PMCs of haploid no. 2904, results from disturbance of the fusion of membrane vesicles (plastosomes) at the moment of formation of daughter cell membranes and completion of cytokinesis in the first meiotic division. This type of cell division yields a binuclear monad. In the second meiotic division, the chromosomes of the daughter nuclei form a common spindle, and meiosis results in a dyad of haploid microspores. The frequency of this abnormality is as high as 15%. As a result, haploid lines no. 2903 and no. 2904 partly restore fertility.

Tubulin-folding cofactor D is necessary for the assembly of tubulin heterodimers and, possibly, plays additional roles in the cell. The effects of cofactor D, microtubules, and/or tubulin dimers on the mitosis initiation were studied in Schizosaccharomyces pombe. It was found for the first time that S. pombe cells with the alp1-1315 and cdc25-22 mutations remained highly viable at 36 degrees C for 8 h, in contrast to cells with the alp1-1315 mutation alone. The progression of cdc25-22 alp1-1315 cells through mitosis after a cell division arrest at 36 degrees C was described. When transferred to 25 degrees C, cdc25-22 alp1-1315 cells displayed a lag of approximately 30 min in Plo1-GFP appearance in the spindle pole body (SPB), 1 h in chromosome condensation, and 75 min in spindle formation. Thus, the initiation of mitosis in cdc25-22 alp1-1315 cells was delayed as compared with cdc25-22 cells. Since treatment of cdc25-22 cells with a microtubule-destabilizing drug during an arrest is known to cause a premitotic arrest with low activity of the mitosis-promoting factor (MPF), it was assumed that an impaired integrity of microtubules and/or lack of tubulin dimers in the nucleus were responsible for the delayed mitosis initiation in cdc25-22 alp1-1315 cells and in cdc25-22 cells treated with a microtubule-destabilizing drug. The progression through mitosis after a cdc25-22 arrest was extremely slow in cdc25-22 alp1-1315 cells, which was attributed to the de novo formation of tubulin dimers.

The paper describes the phenomenon of disorganization of completely formed subcellular structures: division spindle, phragmoplast and chromosome telophase groups. These structures disintegrate into their elements (cytoskeletal fibers, chromosomes) that transform into chaotic system. Chaotization of cytoskeleton structures such as prophase spindle in mitosis or perinuclear ring in meiosis is a normal step of wild type plant cell division. Disintegration of division spindle and phragmoplast presumably indicate the abnormality of temporal regulation of cytoskeleton cycle during meiosis. Disintegration of telophase chromosome groups and the migration of the chromosomes backward to the equatorial area might mean the abnormal start of some prometaphase mechanisms, in particular, chromokinesins activation.

Data are presented in favor of universal significance of physical connections between pericentromeric regions of homologs in their orientation to the opposite poles of the first meiotic division in Drosophila melanogaster. Disturbances in the formation of such connections caused by structural or locus mutations are compensated for by the presence of pericentromeric chiasmata between homologs or (in the case of their absence) by chromocentral connections between nonhomologs being preserved up to the prometaphase. In the latter case, an interchromosome effect on chromosome disjunction and nonhomologous pairing is registered by genetic methods. Inhibition of the formation of the division spindle fibers during prometaphase of meiosis 1 by the long-term action of colcemide promotes the retention of connections between paired nonexchanged homologs and between nonhomologous chromosomes with abnormal homologous pairing because of heterozygosity for numerous inversions and transpositions (X and autosome 2). These connections are registered cytologically. Cytologically registered are also connections between normal X chromosomes and metacentric compounds by the arms of autosome 2 (C(2L)RM, C(2R)RM), which is the known case of the interchromosome effect on chromosome nondisjunction. It is supposed that cytologically detected associations between compounds are realized through a normal mechanism, as a result of interaction and formation of orienting connections between the homologous pericentromeric regions of these compounds. Cytological evidence is presented for colocation of compounds in the chromocentrally organized nucleus of somatic and germline cells.

Leukemia is a clonal proliferative disorder of the multipotent hematopoietic stem cells that leads to abnormal cell growth and (or) differentiation. The hallmark of the disease is the presence of oncogene expression in bone marrow or peripheral blood as a result of some chromosome translocations. The development of leukemia with transfer to disease progression presents a multistage process implicating series of molecular changes leading to chromosomal instability and aneuploidy. The most possible of these changes include the followings: appearance of additional chromosome translocations, activation of other not previously expressed oncogenes, loss of tumor suppressor genes, abnormal centrosome duplication, and dysfunction of the genes which coordinate the accurate chromosome alignment and chromosome segregation during mitosis. The two latter molecular changes which are controlled by mitotic spindle checkpoints play the role in leukemogenesis and are probably involved in apoptosis.

The progeny of SK-UT-1B cells that survived gamma-irradiation with 4 Gy up to the 80th passage was examined. Descendants of irradiated cells lost p53 transactivation properties. Simultaneously, in the presence of nocodazole coordination between M and S phases was disrupted. Meanwhile, descendants of irradiated cells maintained the accurate spindle assembly checkpoint. These data suggest that p53 transactivation function may be required for coordination of M and S phases, rather than for spindle assembly checkpoint. Since it is known that p53 regulates both these processes on the basis of data obtained, we suggest that functions of p53 required for coordination of M and S-phases and for spindle assembly checkpoint are separated. Besides, the data obtained indicate that radiation-induced chromosomal rearrangements are associated with activation of DNA recombination process.

The mechanism of restitution nuclei formation in male meoiosis of Triticum-Agropyrum F1 hybrid and monosomic wheat line Milturum 553 has been studied. Visualization of division spindle with Navashin's routine fixationle distribution method accompanied with acetocarmine-coloured squashed preparations revealed that a block in the kinetochore spindle thread formation, i.e. the absence of chromosome association with the division spindle, is the basic reason of restitution in these genotypes. On this background, the prometaphase spindle in abnormal meiosis of these lines demonstrates a number of vivid and inordinate aberrations revealing some regularities of spindle structure in the normal plant cell.

Chromosomal behaviour and spindle morphology were studied in microsporogenesis of two kinds of diploid potato clones: with normal meiosis, and with "fused spindles" (fs) occurring during the second meiotic division from prometaphase II (proMII) to telophase II (TII). For the first time, morphological effect of fs was found at the late proMII stage to be expressed as two interrelated processes: 1) abnormal chromosome movement, which resulted in joining two groups of chromosomes in the central zone of meiocytes, and 2) abnormal formation of two spindles in the direction to two division poles instead of four poles that actually led to the formation of a united bipolar spindle. Thus, it is not the fusion of two parallel spindles but the formation of united bipolar spindle that constitutes fs abnormality, while the parallel co-orientation of two spatially separated meiotic spindles is a norm in diploid potato. These primary abnormalities detected at proMII resulted in abnormalities at its subsequent meiotic stages: formation of fused spindle and united metaphase plate at MII, bipolar chromosome segration at anaphase II, formation of two telophase nuclei at TII and dyads at the tetrad stage. The results obtained evidence the polar division disturbance in diploid potato clones with fs abnormality.