THE CELL CYCLES OF CHLAMYDOMONAS AND CHLORELLA

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Abstract
Summary: In Chlorella and Chlamydomonas two periods of relatively constant duration determine total cell cycle length and cell size is stabilised by control of whether 2, 4, 8 or 16 daughter cells are formed. A timer runs through G1 phase and leads to the attainment just before S phase of a commitment to divide without further requirement for growth. The commitment timer requires growth for its continuation, is stabilized against a raising or lowering of temperature between 20 and 30°C but it is only partly stabilized against slow growth due to low energy supply. In Chlamydomonas 137c, at 25°C when growth is limited by light energy, the period between the beginning of growth in fully formed daughter cells and the attainment of first commitment to divide approximates to 5 + 0.75 × doubling time (h), at protein doubling times between 5 and 25 h. Under the same conditions division processes occurring in the period after first commitment also confer time properties since they are completed in 6 h. Postcommitment events are extended in the first cycle following a temperature reduction but develop temperature compensation in the second cell cycle. The mean generation time of Chlamydomonas 137c approximates to 11 + 0.75 × mass doubling time (h).In both algae cell size is stabilized by an influence of mother cell mass on the number of successive further commitments that can occur following first commitment to divide. Each commitment is to perform a single further round of DNA replication and achieve a further doubling of cell number without further requirement for growth. Comparison of cells of different size when growing at the same rate shows that time, not size, controls the start of division events, and that size and not growth rate determines division number. In Chlamydomonas, division numbers provide a constant mean apportionment of 20 pg Lowry positive material to committed daughter cells at the time of final commitment. It is proposed that commitments continue until the mass available for further apportionment falls below a critical minimum of 27 pg, therefore, the minimum apportionment will be 13.5 pg and the maximum 27 pg per committed daughter ceDs. This predicted twofold range of cell sizes is confirmed by cell size frequency measurements made by microscopy and by fluorescence‐activated cell sorting. Size frequency measurements made after growth intermediate between 2, 4 or 8 in a cell cycle are consistent with increments of commitment under size control. Multiple fission cell cycles are compared with others.