Abstract
In order to determine the form of replicating DNA, E. coli B3 and K12 Hfr were labelled for various periods with [3H]thymidine. Their DNA was then extracted gently and observed by autoradiography. The results and conclusions can be summarized as follows. (1) The chromosome of E. coli consists of a single piece of two-stranded DNA, 700 to 900 μ long. (2) This DNA duplicates by forming a fork. The new (daughter) limbs of the fork each contain one strand of new material and one strand of old material. (3) Each chromosome length of DNA is probably duplicated by one fork. Thus, when the bacterial generation time is 30 min, 20 to 30 μ of DNA is duplicated each minute. (4) Totally unexpected was the finding that the distal ends of the two daughter molecules appear to be joined during the period of replication. The reason for this is obscure. Conceivably the mechanism that, in vivo, winds the daughter molecules lies at the point of their union rather than, as commonly supposed, in the fork itself. (5) The chromosomes of both B3 (F−) and K12 (Hfr) appear to exist as a circle which usually breaks during extraction.
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