Relation between cell growth and cell division: IV. The synthesis of DNA,RNA, and protein from division to division in Tetrahymena

The incorporation of ¹⁴C methionine into protein, ¹⁴C adenine into RNA, and tritium-thymidine into DNA has been followed autoradiographically over the cell cycle in small groups of Tetrahymena in synchrony. ¹⁴C methionine incorporation occurs at a constant rate through the entire interphase. The rate of ¹⁴C adenine incorporation into RNA is low during the first half of interphase and high during the last half. The synthesis of DNA, as indicated by tritium-thymidine incorporation, is restricted to the first half of interphase.     

Phased cell division in a natural population of Dinophysis sacculus and the in situ measurement of potential growth rate

A method for the in situ estimation of the potential growthrate of the dinoflagellate Dinophysis sacculus based on thepattern of cell division and the duration of the cell cyclephases according to the model of McDuff and Chisholm (Limnol.Oceanogr., 27, 783–788, 1982) is proposed. In order toobserve the cell cycle and measure the in situ growth rate ofD.sacculus, water samples were taken from Alfacs Bay in theEbro River Delta, Spain, in May and June 1994, and kept in thelaboratory under a light-dark cycle similar to that in the field.The pattern of cell division in the D.sacculus population wasestablished by intensive sampling over 24 h periods. The maximumfrequency of cells undergoing cytokinesis was observed at dawnand that of recently divided cells 2 h later. Based on the patternof cell division and the duration of the various cell cyclephases, a generation time between 2 and 5 days was estimated.Subsequently, in situ estimates of the growth rate for D.sacculuswere carried out in St Carles de la Ràpita harbour duringOctober 1995. A mean generation time of 7 days was estimated.     

A new gene controlling the frequency of cell division per round of DNA replication in Escherichia coli

Summary A novel mutant of Escherichia coli, named cfcA1, was isolated from a temperature-sensitive dnaB42 strain, and found to have the following characteristics. Division arrest and lethality induced by inhibition of DNA replication was reduced and delayed in the cfcA1 dnaB42 strain, as compared with the parental dnaB42 strain. Two types of inhibition of division induced by the addition of nalidixic acid or hydroxyurea were suppressed by the cfcA1 mutation. Under permissive conditions for DNA replication, the colony forming ability of cfcA1 cells was significantly reduced as compared with that of cfc ⁺ cells; conversely the division rate of cfcA1 cells was higher than that of cfc ⁺ cells. The cfcA1 mutation partially restored division arrest induced in the thermosensitive ftsZ84 mutant at the restrictive temperature and suppresed the UV sensitivity of the lon mutation. The mutation was mapped at 79.2 min on the E. coli chromosome. Taking these properties into account, it is hypothesized that the cfcA gene is involved in determining the frequency of cell division per round of DNA replication by interacting with the FtsZ protein which is essential for cell division.     

DNA replication initiation, doubling of rate of phospholipid synthesis, and cell division in Escherichia coli.

In synchronized culture of Escherichia coli, the specific arrest of phospholipid synthesis (brought about by glycerol starvation in an appropriate mutant) did not affect the rate of ongoing DNA synthesis but prevented the initiation of new rounds. The initiation block did not depend on cell age at the time of glycerol removal, which could be before, during, or after the doubling in the rate of phospholipid synthesis (DROPS) and as little as 10 min before the expected initiation. We conclude that the initiation of DNA replication is not triggered by the preceding DROPS but requires active phospholipid synthesis. Conversely, when DNA replication initiation was specifically blocked in a synchronized culture of a dnaC(Ts) mutant, two additional DROPS were observed, after which phospholipid synthesis continued at a constant rate for at least 60 min. Similarly, when DNA elongation was blocked by thymine starvation of a synchronized culture, one additional DROPS was observed, followed by linear phospholipid accumulation. Control experiments showed that specific inhibition of cell division by ampicillin, heat shock, or induction of the SOS response did not affect phospholipid synthesis, suggesting that the arrest of DROPS observed was due to the DNA replication block. The data are compatible with models in which the DROPS is triggered by an event associated with replication termination or chromosome segregation.     

Requirements for DNA replication preceding cell division in Tetrahymena pyriformis

Populations of Tetrahymena pyriformis were synchronized by 30 min heat shocks at 34 °C separated by 160 min intervals at the normal growth temperature. The cells initiate DNA synthesis immediately after the cellular division, and the S period of the population lasts about 80 min. It was found that DNA replication is a prerequisite for the following synchronous division. Inhibition of the DNA synthesis in early S by starvation of the cells for thymidine prevents the forthcoming division. However, inhibition in the latter half of S does not prevent the subsequent division. Thus the cells have synthesized enough DNA to permit cell division before the end of a normal S period. These results are discussed in relation to the organization of the genome replication in the highly polyploid macronucleus.     

Linking cell division to cell growth in a spatiotemporal model of the cell cycle

Cell division must be tightly coupled to cell growth in order to maintain cell size, yet the mechanisms linking these two processes are unclear. It is known that almost all proteins involved in cell division shuttle between cytoplasm and nucleus during the cell cycle; however, the implications of this process for cell cycle dynamics and its coupling to cell growth remains to be elucidated. We developed mathematical models of the cell cycle which incorporate protein translocation between cytoplasm and nucleus. We show that protein translocation between cytoplasm and nucleus not only modulates temporal cell cycle dynamics, but also provides a natural mechanism coupling cell division to cell growth. This coupling is mediated by the effect of cytoplasmic-to-nuclear size ratio on the activation threshold of critical cell cycle proteins, leading to the size-sensing checkpoint (sizer) and the size-independent clock (timer) observed in many cell cycle experiments.     

The bacterial cell cycle: DNA replication, nucleoid segregation, and cell division

Data on the bacterial cell cycle published in the last 10-15 years are considered, with a special stress on studies of nucleoid segregation between dividing cells. The degree of similarity between the eukaryotic mitotic apparatus and the apparatus performing nucleoid separation is discussed.