Onset of DNA synthesis during the cell cycle in chemostat cultures.

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Abstract
The onset of DNA synthesis during the growth and division cycle was measured for continuous glucose-limited cultures of Escherichia coli THU and for cultures in exponential growth in the same medium. The H3-thymidine incorporation was measured in cells partially separated by size in a sucrose gradient. Cell volumes were related to their mean value at birth by a comparative scale that assumes that mean cell volumes increase linearly from birth to division. For freely growing cultures, only the unusually small cells at birth fail to synthesize DNA. As growth rates are decreased in chemostat cultures, the initiation of DNA synthesis is increasingly delayed requiring the passage of about 2/3 of the interdivision cycle when generation times become longer than 2 hrs. Synthesis of DNA ceases before the cycle is terminated by fission.