Implications

That DNA was the molecule that carried hereditary information was established through a number of studies, in particular those of Oswald Avery, Colin McLead and Maclyn McCarty.

One key piece of information that directed Watson and Crick in their model building was "Chargaff's rules".

There are four "bases" in DNA - two purines and two pyrimidines.

Edwin Chargaff found that the DNA of different organisms differed in the amounts of adenine (A), thymine (T), cytosine (C) and guanine (G) they contained.

However the amount of adenine always equaled the amout of thymine and the amount of cytosine always equaled the amount of guanine.

In the Watson & Crick model, "only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine)."

A prediction of their model was that "The sequences of bases on a single chain does not appear to be restricted in any way."

Therefore, "if the sequences of bases on one chain is given, then the sequence on the other chain is automatically determined."

• The authors remark that "It has not escaped our notice that the specific pairing we have postulated immediately suggest a possible copying mechanism for genetic material."  What is that mechanism?  What evidence do the present to support this idea?