The yeast Candida utilis was continuously synchronized by the phased method of cultivation with the nitrogen source as the growth-limiting nutrient. The doubling time (phasing period) of celLs was 6 h. Both cell number and deoxyribonucleic acid synthesis showed a characteristic stepwise increase during the phased growth. The time of bud emergence coincided with the time of initiation of deoxyribonucleic acid synthesis. Size distribution studies combined with microscopic analysis showed that the cells expanded only during the unbudded phase of growth. Usually the cells stopped increasing in size about 30 min before bud emergence, and the arrest of the increase in cell volume coincided with the exhaustion of nitrogen from the medium. There was no net change in the volume of cells during the bud expansion phase of growth, suggesting that as the bud expanded, the volume of the mother portion of the cell decreased. After division the cells expanded slightly. The postdivision expansion of cells, unlike the growth before bud initiation, occurred in the absence of the growth-limiting nutrient. The newly formed daughter cells were smaller than the mother cells and expanded at a faster rate, so that both types of cells reached maximum size at the same time. Possible reasons for the different rates of expansion of mother and daughter cells are discussed. Mitchison (18) suggested that the cell cycle can be considered as two loosely coupled and temporally overlapping sequences ofevents. One is referred to as the DNA division cycle, and the other is referred to as the growth cycle. Cycle markers that can be identified with the DNA division cycle are the start and the finish of the S period, stages of mitosis, and cell division. Most of the macromolecular syntheses, including the production of step and peak enzymes and cell envelope synthesis, are considered to be part of the growth cycle. Under normal conditions of growth these cycles are coordinated. This is borne out by the fact that cells of a particular type display a characteristic and rather narrow range of sizes (7). Cell division without precisely controlled coordination between growth and DNA division cycles would result in the production of excessively large or infinitesimally small cells. However, the mechanism of this coordination is poorly understood. One suggestion is that attainment of a critical cell size might be a prerequisite for the continuation of the cellular events that lead to cell division (8). The existence of such "size control" over cell division has been verified in several microorganisms (4,7,8,10,32). t National Research Council publication no. 17960.The progression through the cell cycle of two newborn cells of equal size under the same environmental conditions would be expected to be at the same rate, so that the cells might attain the critical size at the same time. Most likely these cells would have the same generation time as well. However, if the newborn cells were not the same size, it is quite likely that the smaller cell would take a lo...

Under the phased method of cultivation the yeast Candida utilis grew and divided synchronously. The newly formed cells were relatively small, and a new cell cycle was not initiated until the cells could attain a certain minimum size (critical size). Although the cells expanded to some extent after division, the critical size was not reached until a fresh supply of medium was provided. With the arrival of the fresh supply of growth medium at the beginning of the phasing period, the cells expanded rapidly, and new cell cycles were initiated. The cells continued to expand until the growth-limiting nutrient (nitrogen source) was exhausted or until 90 min, which ever occurred first. Usually, buds emerged at a constant time after the start of the phasing period. The time of bud emergence was independent of the size attained by the cells during the expansion phase of growth. The results indicated that it was initiation of the cell cycle that was under size control, and not bud emergence. Bud emergence seemed to be under the control of a timer. The start of this timer seemed to be at or immediately after the beginning of the phasing period. Protein synthesis was essential for the initiation and expansion of buds. However, inhibition of protein synthesis by cycloheximide did not prevent unbudded cells or the parent portion of budded cells from expanding. Cycloheximide seemed to abolish the control mechanism(s) which prevented the cells from expanding after they had reached the maximum size.