Question

How did Watson and Crick's suggestion that nitrogen containing bases in nucleotides pair together in DNA support Chargaff's rule; and allow the double helix to have a constant width (as indicated by Franklin's X-ray crystallography studies?

Answer 1

The nitrogenous bases of DNA are: purines and pyrimidines. Purines are adenine (A) and guanine (G). Pyrimidines are thymine (T) and cytosine (C).

The DNA structure states that adenine pairs with thymine, and cytosine pairs with guanine.

Chargaff's rule states that in DNA there is 1:1 ratio of purines to pyrimidines. It means A=T and G=C.

This supports Watson-Crick model of DNA. It states that two DNA strands are held together by hydrogen bonds between nitrogenous bases. It shows that Adenine always bonds with Thymine and Guanine always bonds with Cytosine.

Franklin discovered the X-ray diffraction pattern of DNA to elucidate double helical structure of DNA.

The diffraction pattern indicates that dark portions at bottom and top is repeated again and again. Watson and Crick found that this feature is bases that are hydrogen-bonded. Therefore, DNA double helix has a constant width whuch is repeated with the pairing of purine with pyrimidine on DNA ladder.