AP Syllabus focus:
‘Nucleic acids show conserved base pairing: purine bases pair with pyrimidines, so A pairs with T or U and G pairs with C.’
Nucleic acid structure depends on how nucleotides chemically fit together. In AP Biology, understanding purines vs pyrimidines and complementary base pairing explains DNA’s consistent width and RNA’s use of uracil.
Nucleotides and nitrogenous base categories
A nucleic acid strand is a chain of repeating nucleotide subunits. The key variability is the nitrogenous base, which determines pairing and information encoding.
Nucleotide: A monomer of nucleic acids made of a pentose sugar, a phosphate group, and a nitrogenous base.
Purines (two-ring bases)
Purines have a larger, double-ring structure:
This figure summarizes the chemical structures of the five nitrogenous bases and classifies them as purines (adenine, guanine; two fused rings) or pyrimidines (cytosine, thymine, uracil; single ring). It also shows how a base attaches to a pentose sugar and phosphate to form a nucleotide, and highlights the key structural difference between ribose (RNA) and deoxyribose (DNA). Source
Adenine (A)
Guanine (G)
Pyrimidines (one-ring bases)
Pyrimidines have a smaller, single-ring structure:
Cytosine (C)
Thymine (T) (primarily in DNA)
Uracil (U) (primarily in RNA)
Purine / Pyrimidine: Purines are double-ring nitrogenous bases (A, G); pyrimidines are single-ring nitrogenous bases (C, T, U).
Conserved complementary base pairing
The syllabus emphasis is that nucleic acids show conserved base pairing: a purine pairs with a pyrimidine. This conserved pairing maintains a uniform distance between the sugar-phosphate backbones in double-stranded nucleic acids.
DNA base pairing rules
In DNA, complementary base pairing is:
This figure presents the chemical structures of the two canonical DNA base pairs (A–T and G–C) and marks the hydrogen bonds that stabilize each pairing. It reinforces the idea that correct pairing depends on matching hydrogen-bond donors/acceptors and purine–pyrimidine geometry, which keeps the double helix a consistent width. Source
A pairs with T
G pairs with C
These pairings are favoured because the shapes and hydrogen-bonding patterns align correctly, producing stable base pairs and consistent helix geometry.
This diagram shows Watson–Crick complementary base pairing in double-stranded DNA, with adenine pairing to thymine via two hydrogen bonds and guanine pairing to cytosine via three hydrogen bonds. The figure also emphasizes the antiparallel orientation of the two sugar-phosphate backbones, connecting base pairing rules to double-helix structure. Source
RNA base pairing rules
In RNA, thymine is typically replaced by uracil, so:
A pairs with U
G pairs with C
RNA is often single-stranded, but the same pairing rules apply within folded regions (for example, hairpins), where segments of the same molecule base-pair with each other.
Why “purine with pyrimidine” matters
Pairing a large base with a small base is structurally efficient:
Purine–pyrimidine pairing keeps the nucleic acid double-strand width relatively constant.
Purine–purine would be too wide; pyrimidine–pyrimidine would be too narrow, disrupting stable stacking and backbone positioning.
Key takeaways for AP Biology accuracy
Students should be able to state and apply:
The categories:
Purines: A, G
Pyrimidines: C, T, U
The conserved pairing logic:
Purine pairs with pyrimidine
The specific base-pair outcomes:
DNA: A–T, G–C
RNA: A–U, G–C
FAQ
Uracil is less chemically complex to produce than thymine.
Thymine can help mark DNA damage (e.g., distinguishing true bases from certain altered cytosines).
Yes, “wobble” and other atypical pairings can occur under specific conditions.
They are generally less stable than standard A–T/U and G–C pairing.
G–C pairs typically form more hydrogen bonds than A–T/U.
This can increase local stability where G–C content is higher.
Base stacking is the stabilising interaction between adjacent bases along a strand.
Correct pairing supports optimal stacking geometry, improving overall stability.
No. Their derivatives occur in other biomolecules (e.g., ATP, NAD).
The same base categories apply even when the nucleotide is not in a nucleic acid polymer.
Practice Questions
State the complementary base pairs found in DNA and identify which of the paired bases are purines. (2 marks)
Correct DNA base pairs stated: A–T and G–C (1)
Identifies purines as A and G (1)
Explain why nucleic acids show conserved base pairing such that purines pair with pyrimidines, and relate this to the specific pairing of A with T (or U) and G with C. (5 marks)
States that purines pair with pyrimidines (1)
Identifies purines (A, G) and pyrimidines (C, T, U) (1)
Links purine–pyrimidine pairing to maintaining a consistent strand/helix width (1)
States A pairs with T in DNA and with U in RNA (1)
States G pairs with C (1)
