Question

As we examine DNA, we see that the four component molecules create the rung-like crossbars of the ladder-like structure. (Use your text for helpful diagrams.) Why do the four components pair up only as AT (or TA) and CG (or GC)? Why wouldn’t other combinations such as TC, CT, AG, and GA also allow a ladder-like structure to be made?

Answer 1

The nitrogenous bases of DNA are categorized into two types depending on their structure: Pyrimidines which contains a six-membered ring-like cytosine, thymine and uracil and purines which contains six members rings attached to a five-membered ring like in guanine and adenine.

The base pairing in DNA double-helical structure always takes place between purine and pyrimidines as:

1. The pairing between the purines and pyrimidines results in uniform diameter between two strands which provides stability to the ladder-like the structure of DNA.

2. The chemical side groups attached to the nitrogenous bases allow the formation of hydrogen bonds between purine and pyrimidines.

Therefore, A always binds T through two hydrogen bonds and G always bind C through three hydrogen bonds.