When cultures of undergoing balanced growth are shifted from one medium to another, a definite pattern of rate changes is observed. Shifts from a low to a high growth rate result in a strict succession of events: RNA synthesis is immediately affected and its rate rapidly increases to that characteristic of the new medium; the increase in optical density shows a lag of a few minutes before the new rate is attained; DNA synthesis and cell division, on the other hand, continue at the old rate for appreciable periods of time and then abruptly shift to the new rates. The times at which these shifts take place are, at 37°, invariably 20 and 70 min., regardless of the actual growth rates before and after the shift. This rate maintenance effect on DNA synthesis and cell division is discussed in terms of specific rate-controlling mechanisms.


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