<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>
Yes, because each of the four nucleotide bases present in DNA have a corresponding base: adenine bases always pair with thymine bases, and cytosine bases always pair with guanine bases. For example, if one strand of a DNA molecule was CTAGGT, the sequence of the second strand would be, from left to right, GATCCA.
The chronic long term use of alcohol
