<span>Two of the nitrogenous bases present in DNA (Guanine and Adenine) contain two rings and thus are much larger than the other two (Cytosine and Thymine), which have only one.
Nitrogenous bases with two rings (such as G and A) are known as PURINES, whereas nitrogenous bases with a single ring (such as C and T) are known as PYRIMIDINES.
A stable DNA structure is formed ONLY when the two strands are a constant distance
apart which ONLY happens when a PURINE (G or A) on one strand is paired with a PYRIMIDINE (C or T) on the other strand. The strands would have to move much further apart to permit a purine to pair with another purine and would have to move much closer together to permit a pyrimidine to pair with a pyrimidine.
(Source: https://www.amherst.edu/system/files/media/1536/ModGen_1C_TP.pdf )</span>
