Question

explain how chargaff's rule of base pairing helped watson and crick model dna

Answer 1

Edwin Chargaff's rule is actually more an observation. Before Watson and Crick solved the structure of double helical DNA, Chargaff observed that adenine and thymine are present in DNA in equal amounts, as are cytosine and guanine. He never actually determined that adenine pairs with thymine and cytosine pairs with guanine - that was the jump Watson made. Because adenine and thymine are present in equal amounts, he figured out that they must pair up with each other. Similar reasoning for C and G.

I encourage you to look up Edwin Chargaff, as he is one of the more interesting biochemists of this century and he tends to get overshadowed (as do most other people involved in solving the DNA structure) by Watson, Crick and to some extent Rosalind Franklin.