Figure 26.4: Hydrogen Bonding Between Base pairs in DNA.

WHY THERE IS RESTRICTION OF BASE PAIRING, ADENINE WITH THYMINE OR GUANINE TO CYTOSINE ?
As discussed earlier, adenine or guanine is purine and has 9 membered ring wheras thymine or cytosine is pyrimidine of 6 membered. As explained in Figure 26.5, presence of both purines (bulky side chain) or pyrimidines (small side chain) make it difficult to accommodate or too short to form hydrogen bonding within the DNA strands. In addition, purine and pyrimidine has perfect match of hydrogen accepter and donor sites. As a strict requirement of base pairing, two chains are complementary to each other.