The Timing of Division in Chlamydomonas

The duration of the cell cycle of Chlamydomonas is determined by two timed periods which each follow an initiating stimulus. A commitment timer, initiated by the beginning of growth in autonomous daughter cells, is dependent upon concurrent growth and controls the time of first commitment to divide. Division processes, which are then initiated, occupy a second timer period of 6 h duration at 25⚬C. Absence of division control by an endogenous oscillator was indicated by an unfluctuating ability to resume cycles of consistent duration when daughter cells were held in darkness or when growing cells in light were subjected to darkness. A consistent time from the beginning of daughter growth to commitment was also observed when daughter formation was delayed by an earlier temperature reduction. Ethanol, which commonly affects circadian oscillators, did not disturb synchrony The division cycle is controlled by growth and not by direct response to light, since commitments ceased when growth was prevented by lack of CO2in the light. Growth in daughter cells initiated the timer leading to commitment since commitment was reached at similar times after resupply of CO2or reillumination. Division synchrony in diurnal cycles of periodic illumination is not caused by entrainment of an oscillator but by the growth requirement of the hourglass timer leading to commitment. A model of division control is presented.