state that the bases adenine and guanine are purines with a double ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single ring structure (structural formulae for bases are not expected)
State that the bases adenine and guanine are purines with a double‑ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single‑ring structure.
Key Definitions
Purine: A nitrogenous base that contains two fused carbon‑nitrogen rings.
Pyrimidine: A nitrogenous base that contains a single carbon‑nitrogen ring.
Classification of DNA Bases
Base
Category
Ring Structure
Adenine (A)
Purine
Double ring
Guanine (G)
Purine
Double ring
Cytosine (C)
Pyrimidine
Single ring
Thymine (T)
Pyrimidine
Single ring
Uracil (U)
Pyrimidine
Single ring
Why the Distinction Matters
The double‑ring purines pair with single‑ring pyrimidines in the DNA double helix, maintaining a uniform width of the helix. Adenine pairs with thymine (or uracil in RNA) via two hydrogen bonds, while guanine pairs with cytosine via three hydrogen bonds.
DNA Replication Overview
Initiation at origins of replication.
Unwinding of the double helix by helicase.
Stabilisation of single strands by single‑strand binding proteins.
Primase synthesises short RNA primers.
DNA polymerase adds nucleotides complementary to the template strand (5'→3').
Removal of RNA primers and replacement with DNA.
Ligation of fragments to form a continuous strand.
Suggested diagram: A schematic of the DNA double helix showing purine‑pyrimidine pairing and the replication fork with key enzymes.