Question

Watson and Crick came to reject several potential models for DNA structure. If they considered it, they would have rejected a model where the bases A and G are paired, and where the bases C and T are paired. Why would W and C know that these alternative pairings did not fit what was then known about DNA in the early 1950’s? There are 2 main reasons for why they would know these were wrong. Can you explain both of them?

Answer 1

Erwin Chargaff proposed a rule known as Chargaff’s rule on the basis of which Watson and Crick correctly solve the correct model for B-DNA.

The first reason is the specific hydrogen bonding. They know that Adenine can pair with thymine via formation of two hydrogen bonds while guanine pairs with cytosine via formation of three hydrogen bonds.

The second reason is Chargaff’s rule. Chargaff’s rule says that in a double stranded DNA molecule the number of guanine units is equal to the number of cytosine units and the number of adenine units is equal to the number of thymine units.

Now, the number of units an only become correctly equal and chargaff's rule is proved when the pairing occurs between A and T and G and C not between the bases A and G and bases C and T.