Abstract

By following the nuclear division, chloroplast division, cytokinesis and zoospore liberation of Chlamydomonas reinhardi cells exposed to a high concentration of 6-methyl purine (6-MP), corroborative evidence was obtained for our previous conclusion that 6-MP-exposed cells are brought to the starting point of a new round of the cell cycle with abortion of the cycle in process (“return-to-start” effect). This effect did not occur after cells had passed a critical stage (transition point) which seemed to be situated shortly prior to the onset of nuclear division under the conditions used. When 6-MP was applied to cells after the transition point, it caused an advancing effect on their zoospore liberation. A cycloheximide (CHI)-inhibition step existed shortly alter the transition point for 6-MP. A model was proposed for the effects of 6-MP and CHI.

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