Synchronous cultures of Chlorella, that were obtained with minimum metabolic perturbation by centrifugal selection, reveal that progress through the cell cycle requires no change in the poly(A)+ mRNA population, although changes do occur during nutritional adaptation. Of the abundant soluble proteins, 93% are synthesized continuously through the cell cycle and those that are discontinuous show similar patterns in control cells. The synthesis of proteins is compared with parallel studies of accumulation of enzyme activity and it is shown that there is no discrepancy in their pattern of accumulation when both are studied under the same culture conditions. The eukaryote cell cycle can allow stable relative rates of synthesis of most proteins and balanced rates of accumulation of most enzyme activities. Macromolecule classes differ in their rates of accumulation throughout the cell cycle: total RNA increases linearly, poly(A)+ RNA accumulation is restricted to G1 phase, but total protein accumulation accelerates smoothly through G1, S and mitosis phases, pausing at cytokinesis. There is no evidence that the cell cycle requires an extensive programme of differential enzyme synthesis. The cycle can therefore proceed with minimum disturbance of metabolism required for growth.

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