A "Point of No Return" in the Cell Cycle of Chlorella

In a Chlorella culture growing synchronously at pH 6.3 undera 12 hr light/12 hr dark regime, DNA replication occurs betweenthe 8th and the 12th hour of the cycle, the main period of proteinand chlorophyll synthesis occurring between the 4th and 12thhour of the cycle. When the culture is transferred to alkalinepH at any time up to the 8 hr of the cycle, autospore releaseis prevented, and the pattern of synthesis of DNA, protein andchlorophyll is altered. However, when the culture is transferredto alkaline conditions after the 8th hour of the cycle, thepattern follows that of a culture growing at pH 6.3 with respectto cell number and volume, as well as protein, chlorophyll andDNA contents. Thus, a transition point seems to occur afterthe 8 hr of the cycle. The existence of such a point was alsodemonstrated by reciprocal experiments in which Chlorella wascultured at an alkaline pH and transferred to pH 6.3 at varioustimes in the cell cycle. ¹ Present address: Applied Research Institute, Ben-Gurion Universityof the Negev, P.O. Box 1025, Beer-Sheva 84110, Israel. (Received October 2, 1981; Accepted January 20, 1982)