SUMMARY: In Salmonella typhimurium PG154 the time taken for a round of DNA replication () at 37° is approximately 48 min. The time between the end of a round of DNA replication and the following cell division () is approximately 25 min. at 37°. In a temperature-sensitive mutant of defective in the initiation of DNA synthesis, cell division at the restrictive temperature occurs at an approximately normal rate for 75 min., suggesting that = 75 min.; the cells then elongate and form filaments. After about a further 1 h. in minimal medium at 38° cell division recommences at the ends of the filaments to form small cells. Filaments can divide at each end to produce more than one small cell. Once formed, small cells do not grow further. They do not synthesize DNA, RNA or protein and contain little DNA. Complete inhibition of DNA synthesis at the time of the shift stops cell division within about min. and almost completely stops the production of small cells. A period of growth at 38° before the inhibition of DNA synthesis allows more division and the production of an almost normal number of small cells. Both the early divisions and the production of small cells are considered to be the result of the termination of rounds of DNA replication in the absence of new initiations.


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