Why do purines pair with pyrimidines?
Purines pair with pyrimidines because their size and shape make them a perfect fit for hydrogen bonding
Purines and pyrimidines are base pairs.
The two most common base pairs are A-T and C-G.
These nucleotides are complementary —their shape allows them to bond together with hydrogen bonds.
In the A-T pair, the purine (adenine) has two binding sites, and so does the pyrimidine (thymine)
In the C-G pair, the purine (guanine) has three binding sites, and so does the pyrimidine (cytosine).
The hydrogen bonding between complementary bases is what holds the two strands of DNA together.