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A1.2.12 (HL)—Purine-to-pyrimidine bonding and helix stability

Purine-to-pyrimidine pairing keeps the DNA helix a consistent width while hydrogen bonds stabilize complementary base pairs in the molecule in IB Biology exam contexts.

Syllabus
First assessment 2025
Objective
A1.2.12
Level
HL

Purines Pair with Pyrimidines to Fit the Helix

HL only

DNA pairs a two-ring purine with a one-ring pyrimidine, keeping the double helix at a nearly constant width.

A purine–purine pair would be too wide and a pyrimidine–pyrimidine pair too narrow. The A–T and C–G pairings also provide the hydrogen-bond pattern needed for stable, selective matching.

Classify before pairing:

  • purines: adenine and guanine
  • pyrimidines: cytosine and thymine
  • valid DNA pairs: A–T and C–G

Replacing an A–T pair with a G–T pair would put two pyrimidines together and disrupt the normal geometry and pairing pattern.

Base-pair stability is not just the number of hydrogen bonds; fitting the helix and stacking interactions also matter.

Retrieve The HL Extensions

HL only

HL adds directionality, helix geometry, DNA packaging, and classic evidence. These ideas stay separate: 5′/3′ explains strand direction; purine-pyrimidine pairing explains width; nucleosomes explain packaging; Hershey-Chase and Chargaff explain evidence for DNA and base pairing.

  • Directionality: phosphodiester bonds create 5′ and 3′ ends, and DNA strands are antiparallel.
  • Helix stability: purine-pyrimidine pairing keeps width constant and hydrogen bonds stabilize base pairs.
  • Packaging: DNA wraps around histone octamers and H1 binds linker DNA.
  • Evidence: Hershey-Chase supports DNA as genetic material; Chargaff supports complementary pairing.
  • Chargaff found purines equal pyrimidines across DNA samples.

Concept essentials

  • Purine-to-pyrimidine bonding and helix stability should be described using precise molecular vocabulary.
  • Structure and information must be kept separate: backbones provide continuity while bases carry sequence meaning.
  • Exam answers should match the command term and avoid adding unsupported extra claims.
ConceptIB Biology HL