In their April 1953 DNA structure paper, Watson and Crick noted: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
A month later, in their second paper, they published their hypothesis for the replication of DNA:
"…If the actual order of the bases on one of the pairs of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.
Previous discussions of self-duplication have usually involved the concept of a template, or mould. Either the template was supposed the copy itself directly or it was to produce a 'negative', which in its turn was to act as a template and produce the original 'positive' once again. In no case has it been explained in detail how it would do this in terms of atoms and molecules.
Now our model for deoxyribonucleic acid is, in effect, a pair of templates, each of which is complementary to the other. We imagine that prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains, where we only had one before. Moreover, the sequence of the pairs of bases will have been duplicated exactly."
This mechanism - termed semiconservative replication - was confirmed experimentally in 1958 by Matthew Meselson and Franklin Stahl.
"Why, then, did we change our minds and, within only a few weeks, write the more speculative paper of May 30? The main reason was that when we sent the first draft of our initial paper to King's College we had not yet seen the papers by the researchers there.
Consequently we had little idea of how strongly their X-ray evidence supported our structure. Jim had seen the famous 'helical' X-ray picture of the B form reproduced by Franklin and Gosling in their paper, but he certainly had not remembered enough details to construct the arguments about Bessel functions and distances that the experimentalists gave. I myself at that time had not seen the picture at all. Consequently we were mildly surprised to discover that they had got so far and delighted to see how well their evidence supported our idea. Thus emboldened, Jim was easily persuaded that we should write a second paper."
Crick, 'What Mad Pursuit' (1988), 66-7.
Watson J D, Crick F H (1953) Genetic implications of the structure of deoxyribonucleic acid. Nature 171: 964-967.