Extracts of pneumococcal cells which had incorporated [32P]DNA were fractionated in order to determine the fate of the incorporated radioactivity. Immediately after entry, about half of the DNA was found to be converted to a single-stranded form; the other half was degraded to dialysable oligonucleotides and inorganic phosphate. Very little native [32P]DNA was present. On incubation of the cells for various lengths of time after introduction of the DNA, radioactivity was rapidly incorporated into the native DNA fraction. The sources of this radioactivity were (1) the fragments initially present which apparently were incorporated by means of normal synthetic processes, and (2) the single-stranded DNA some of which may have been transferred intact. This latter transfer appears to be the route by which genetic information is transferred. The increase of radioactivity in native DNA and the depletion of the single-stranded DNA corresponded in time with recovery of a genetic factor introduced by the donor DNA. The extent to which this genetic factor was recovered, determined both by direct measurement of its transforming activity and by measurement of the frequency of transformed cells, corresponded to a physical integration of about one-quarter of the donor DNA taken up by the cells. © 1962, Academic Press Inc. (London) Ltd.. All rights reserved.
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