The Growth of the Nucleus in Dividing and Non-dividing Cells of the Pea Root

The growth of the nucleus and the cell in the pea root was followedthrough the mitotic cycle and subsequently in post-mitotic developmentby comparing cells and nuclei from the meristem, at differentstages of interphase, and cells and nuclei from two regionsof the enlarging zone of the root. Measurements of cell andnuclear volumes were made in sections of fixed roots. Measurementsof nuclear volume, DNA content, and dry mass were made on isolatednuclei. Growth in the mitotic cycle was characterized by a doublingof DNA and nuclear dry mass and a five-fold increase of nuclearvolume. Since cell volume doubled, a differential hydrationof cytoplasm and nucleus is inferred. Post-mitotic growth wascharacterized by a four-fold or greater increase in cell volume,with vacuolation and a continued increase of cytoplasmic constituents,but a cessation of nuclear growth except by uptake of water;the only increase in nuclear dry matter appeared to be in cellsbecoming endopolyploid. The concentration of dry matter in thenucleus fell as the nuclei enlarged in the mitotic cycle andin post-mitotic growth. The relationships between the measuredparameters are examined to see whether they might be indicativeof causal relationships.     

THE NUCLEAR CELL CYCLE IN CHLAMYDOMONAS (CHLOROPHYCEAE)1

The timing of replication and division of the Chlamydomonas Ehrenberg nucleus in the vegetative cell cycle and at gametogenesis was examined, using fluorescence microspectrophotometry with two fluorochromes, mithramycin and 4′,6-diamidino-2-phenylindole (DAPI). Under appropriate conditions, these bind specifically to DNA, and the fluorescence of the DNA fluorochrome complex is a quantitative measure of the DNA content. The alga is a haplont, which produces 2ⁿ daughter cells at the time of vegetative reproduction; cytokinesis and daughter cell release lag behind karyokinesis. No nucleus was found to contain more than the 2c quantity of DNA. Hence daughter cell production proceeds by doubling of the nuclear DNA followed by karyokinesis, in a repetitive sequence. As reported previously for C. reinhardtii Dangeard, the gametes of C. moewusii Gerloff contain the 1c amount of nuclear DNA. Several conflicting interpretations of the cell cycle sequence proposed in the literature were resolved.     

Nuclear reorganization of mammalian DNA synthesis prior to cell cycle exit

In primary mammalian cells, DNA replication initiates in a small number of perinucleolar, lamin A/C-associated foci. During S-phase progression in proliferating cells, replication foci distribute to hundreds of sites throughout the nucleus. In contrast, we find that the limited perinucleolar replication sites persist throughout S phase as cells prepare to exit the cell cycle in response to contact inhibition, serum starvation, or replicative senescence. Proteins known to be involved in DNA synthesis, such as PCNA and DNA polymerase delta, are concentrated in perinucleolar foci throughout S phase under these conditions. Moreover, chromosomal loci are redirected toward the nucleolus and overlap with the perinucleolar replication foci in cells poised to undergo cell cycle exit. These same loci remain in the periphery of the nucleus during replication under highly proliferative conditions. These results suggest that mammalian cells undergo a large-scale reorganization of chromatin during the rounds of DNA replication that precede cell cycle exit.     

Cell reproduction cycle of mycoplasma

The cell reproduction cycle of parasitic wall-free bacteria, mycoplasma, is reviewed. DNA replication of Mycoplasma capricolum starts at a fixed site neighboring the dnaA gene and proceeds to both directions after a short arrest in one direction. The initiation frequency fits to the slow speed of replication fork and DNA content is set constant. The replicated chromosomes migrate to one and three quarters of cell length before cell division to ensure delivery of the replicated DNA to daughter cells. The cell reproduction is based on binary fission but a branch is formed when DNA replication is inhibited. Mycoplasma pneumoniae has a terminal structure, designated as an attachment organelle, responsible for both host cell adhesion and gliding motility. Behavior of the organelle in a cell implies coupling of organelle formation to the cell reproduction cycle. Several proteins coded in three operons are delivered sequentially to a position neighboring the previous organelle and a nascent one is formed. One of the duplicated attachment organelles migrates to the opposite pole of the cell before cell division. It is becoming clear that mycoplasmas have specialized cell reproduction cycles adapted to the limited genome information and parasitic life.