Distinct controls of dna replication and of nuclear division in the cell cycles of the chlorococcal alga Scenedesmus quadricauda

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Abstract
In the course of the cell cycle of Scenedesmus quadricauda, the syntheses of RNA and total protein occur in steps. Each step represents an approximate doubling of the preceding amount of RNA or protein per cell. The increase in protein content per cell runs parallel to, but with a constant delay behind, the corresponding RNA steps. When protein synthesis is suppressed (e.g. by maintaining the cells in the dark) after an RNA synthesis step has already occurred the cells double their DNA content, but no corresponding nuclear division occurs and uninuclear daughter cells with double the amount of DNA may be formed. Under conditions of phosphorus or nitrogen starvation RNA synthesis is stopped while protein synthesis continues. In this case, the number of DNA replication rounds corresponds to the reduced RNA content while the number of nuclear divisions tends to follow the number of protein synthesis steps until one genome per nucleus is attained. These results indicate that with each doubling of RNA content the cells become committed to DNA replication, while doubling of protein content is required for the commitment to the corresponding nuclear divisions.