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Treatment of Allium cepa meristematic cells in metaphase with the topoisomerase II inhibitor ICRF-193, results in bridging of the sister chromatids at anaphase. Separation of the sisters in experimentally generated acentric chromosomal fragments was also inhibited by ICRF-193, indicating that some non-centromeric catenations also persist in metaphase chromosomes. Thus, catenations must be resolved by DNA topoisomerase II at the metaphase-to-anaphase transition to allow segregation of sisters. A passive mechanism could maintain catenations holding sisters until the onset of anaphase. At this point the opposite tension exerted on sister chromatids could render the decatenation reaction physically more favorable than catenation. But this possibility was dismissed as acentric chromosome fragments were able to separate their sister chromatids at anaphase. A timing mechanism (a common trigger for two processes taking different times to be completed) could passively couple the resolution of the last remaining catenations to the moment of anaphase onset. This possibility was also discarded as cells arrested in metaphase with microtubule-destabilising drugs still displayed anaphase bridges when released in the presence of ICRF-193. It is possible that a checkpoint mechanism prevents the release of the last catenations linking sisters until the onset of anaphase. To test whether cells are competent to fully resolve catenations before anaphase onset, we generated multinucleate plant cells. In this system, the nuclei within a single multinucleate cell displayed differences in chromosome condensation at metaphase, but initiated anaphase synchronously. When multinucleates were treated with ICRF-193 at the metaphase-toanaphase transition, tangled and untangled anaphases were observed within the same cell. This can only occur if cells are competent to disentangle sister chromatids before the onset of anaphase, but are prevented from doing so by a checkpoint mechanism.

Condensed sister chromatids possess a protein scaffold or axial core to which loops of chromatin are attached. The sister cores are believed to be dynamic frameworks that function in the organization and condensation of chromatids. Chromosome structural proteins are implicated in the establishment of sister chromatid cohesion and in the maintenance of epigenetic phenomena. Both processes of templating are tightly linked to DNA replication itself. It is a question whether the structural basis of sister chromatid cores is templated during S phase. As cells proceed through the cell cycle, chromatid cores undergo changes in their protein composition. Cytologically, cores are first visualized at the start of prometaphase. Still, core assembly can be induced in G1 and G2 when interphase cells are fused with mitotic cells. In this study, we asked if chromatid cores are similarly able to assemble in S-phase cells. We find that the ability to assemble cores is transiently lost during local replication, then regained in chromosome regions shortly after they have been replicated. We propose that core templating occurs coincident with DNA replication and that the competence for the assembly of the sister chromatid cores is acquired shortly after passage of replication forks.

After using a silver staining procedure two deeply stained structures appear in spermatogonial, first and second meiotic metaphase chromosomes of grasshoppers. First of all, rounded structures of about 0.4–0.6 um in diameter, which represent kinetochores. Moreover a longitudinal axes, which runs the whole length of each chromatid and which seems to represent a core or scaffold-like structures. This axis is always in close relation with kinetochores. The homologous axes of chromosomes in first meiotic metaphase are very close together at the point were chiasmata take place.

In microspores of Allium cepa, the nucleolus generally takes the form of two hemispherical masses showing the same degree of development, consisting of the fibrillar and the granular components intermingled. The nucleolar organizing region (NOR) can be observed between the two nucleolar masses and is often in close connection with the masses of chromatin. It consists of a series of zones similar in density to the chromatin itself, arranged in a relatively orderly pattern in a region of low density. These dense zones consist of an accumulation of filaments measuring 100 Å, while the less dense area consists of similar but less concentrated filaments connecting the dense zones with one another. Apparently it is this latter area which is in connection with the nucleolar masses. It may be supposed that in order to carry out its function the NOR assumes a pufflike structure.

By means of different treatments which inhibit cytokinesis in root-tip cells (Phalaris canariensis) it has been possible to analyse the cell plate formation by blocking, with a certain selectivity, each one of its morphological phases. Cytokinesis appears to be a continuous process; however, it proved possible to divide it into four morphological phases: production of the small vesicles by Golgi apparatus, accumulation of these vesicles in the equatorial region, arrangement at the equatorial plane and coalescence to form the new cell wall. The heat shock (38–40°C) inhibited cytokinesis blocking the production of the small vesicles. c-Mitotic drugs (colchicine and cyclohexane) inhibited the accumulation phase, and the high hydrostatic pressure (400 atm) the arrangement phase. The last phase of cytokinesis—fusion of the small vesicles—could not be inhibited without affecting the previous phases. In short, the small vesicles produced by Golgi bodies are an essential factor for cytokinesis, and their behaviour suggests the intervention of cytoplasmic currents which could determine the course of the cell plate formation.

Caffeine inhibits cytokinesis in plant cells by blocking cell plate formation. This drug disturbs neither the production nor the accumulation of the Golgi vesicles, but it prevents their arrangement. In consequence, the coalescence of these vesicles, which contain the cell plate precursors, is greatly disturbed and no cell plate is formed.

A study of the effect of ethidium bromide was carried out on root meristems of Allium cepa L. This inhibitor of nucleic acid synthesis possesses a powerfully inhibitive influence on RNA synthesis. A parallel study was also carried out on the structural effects of the drug on the nucleolus, under the light microscope with silver impregnation, and by electron microscopy. Nucleolar segregation is observable quite clearly during the first few hours of treatment, and in subsequent hours (six) a morphological change can be observed in the area known as the pars granulosa, culminating in complete dispersion after 12 hours, leaving a wide electron-clear zone around the pars fibrosa. Thus, the effect of ethidium bromide is found to be appreciably different from that of other inhibitors of RNA synthesis producing degranulation of the nucleolus in a centrifugal direction, and the failure to produce granular RNP from the fibrillar RNP points to a functional effect of the drug on the latter.