Summary: Computer reconstructions of 659 and 1325 whole mounted, shadowed cells, randomly chosen from cultures of undergoing balanced growth and doubling in mass every 83 min and 30 min, respectively, were used to analyse the cell cycle. The size limits and duration of phases of the cell cycle were estimated by applying a method previously described by the authors, details of which are given here to allow others to use the method. Deeply constricted cells whose primary septal radius, , was less than or equal to 0·18 μm were considered as belonging to an -phase ending the cell cycle. The statistical parameters of these -phase cells were used to calculate the mean and coefficient of variation of dividing cells. These latter values, in turn, predicted the moments of the total population well enough so that the method's assumptions were judged adequately satisfied. Therefore, the method was considered applicable to other phases and sub-phases of the cell cycle of these two cultures. The -phase cells were further classified as having either 0, 1 or 2 secondary growth zones, allowing us to calculate the percentage of newborn cells without growth zones. In the slow-growing cells. 69% of the cells arose with no growth zone. On the other hand, in more rapidly growing cells 16% of the cells or less arose with no growth zone. Our calculations showed that they could exist without a growth zone for only 2 and 0·1 min, respectively. We also classified cells as possessing a ‘birth site’ if the volume between the two daughter bands was greater than 0, but less than 0·06 μm. From the statistical properties of such cells with new growth zones, the mean pole time, , was estimated. We also estimated from the size of cells in -phase. The major conclusion is that the pole time is only slightly greater than the mass doubling time at both growth rates. Since DNA synthesis in takes longer ( = 50 to 52 min) than the mass doubling time in rich medium (30 min), a new round of chromosome replication must be initiated before the old round of synthesis is completed (dichotomous replication). Consequently, wall band splitting and initiation of chromosome replication do not occur simultaneously. It was also concluded that the cell initiates wall band splitting, resulting in pole formation and cell division, when the growth zones cannot function rapidly enough to allow the increase of surface area required to accommodate continuing production of cytoplasm.


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