In DNA, the two strands of the double helix are held together by complementary base pairing through hydrogen bonds. The pairing follows strict rules discovered by Erwin Chargaff and confirmed by Watson and Crick's model of DNA structure.
The two base-pairing rules are:
- Adenine (A) always pairs with thymine (T) via two hydrogen bonds.
- Guanine (G) always pairs with cytosine (C) via three hydrogen bonds.
These pairings occur because of the molecular shapes and hydrogen-bonding capacity of the bases. Adenine and thymine are complementary in size and bonding sites, as are guanine and cytosine. A purine (adenine or guanine, which are larger, double-ringed bases) always pairs with a pyrimidine (thymine or cytosine, which are smaller, single-ringed bases), maintaining a uniform width of the DNA double helix.
Adenine - thymine is therefore one of the two correct base pairs in a DNA molecule. The other options listed (cytosine - adenine, cytosine - thymine, guanine - adenine) violate Chargaff's rules and would result in mismatched hydrogen bonding and irregular helix geometry.
Exam tip: Remember the pairing rule with the mnemonic: A-T (Apple-Tree) and G-C (Good-Cat). In RNA, thymine is replaced by uracil, so adenine pairs with uracil instead.
