||Bacteria Escherichia coli
||Helmstetter CE. DNA synthesis during the division cycle of rapidly growing Escherichia coli B/r. J Mol Biol. 1968 Feb 14 31(3):507-18. P.518 top paragraphPubMed ID4866336
||P.508 top paragraph: "Cultures growing exponentially in various media were pulse-labeled with [14C]thymidine
and the amount of label incorporated into cells of different ages was determined. The sorting-out of the labeled cells according to age was accomplished by binding the cells to a membrane filter at the end of the labeling period and collecting the
new-born cells which were eluted continuously from the membrane. Measurement of the radioactivity in the effluent cells as a function of elution time yielded the rate of thymidine incorporation into their ancestors as a function of age."
||P.518 top paragraph: "(d) Conclusions: [Researchers'] measurements of the rate of thymidine incorporation during the division cycle of E. coli B/r growing at rates between 22 and 40 minutes per doubling indicate the following: (1) The cell age at the start of rounds of DNA replication varies with the growth rate. With increasing growth rate, rounds start at increasing fractions of a generation before a given division.
(2) The time for a complete round of replication is quite constant and equals about 41 minutes. (3) In rapidly growing E. coli B/r, a new round of replication begins before the previous round has ended-that is, individual chromosomes contain multiple forks." For replication of chromosome of 42-67 min in E. coli B/r with doubling time between 24 -100 min see BNID 100090. The seeming paradox of chromosome replication time being longer than doubling time is resolved by the observation that E. coli under ideal conditions employs nested replication forks that begin replicating the granddaughter cells while the daughter cells are still engaged in replication.