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A1.2.8—Complementary base pairing

Complementary base pairing links specific bases, allowing DNA replication, transcription, and translation to use predictable molecular matching in cells in IB Biology exam contexts.

Syllabus
First assessment 2025
Objective
A1.2.8
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Deduce

Scoring notes

Common mistake
Mixing up covalent sugar-phosphate bonds with hydrogen bonds between complementary bases.

Recent exam appearances

November 2025Paper1B HL · TZ11(a)(i)[ 1 ]A1.2.8—Complementary base pairing
May 2025Paper2 HL · TZ33(c)[ 1 ]A1.2.8—Complementary base pairing
May 2018Paper1 HL · TZ19[ 1 ]A1.2.8—Complementary base pairing
Practice this objective

Coverage 2018–2025 · Updated 14 Jul 2026

Complementary Pairing Makes Copying Predictable

Complementary base pairing means each base has a specific partner, allowing a strand to determine the sequence of its partner.

Hydrogen bonds give the pairs selectivity: adenine pairs with thymine in DNA (uracil in RNA), while cytosine pairs with guanine. This matching is the mechanism behind faithful copying and template reading.

Apply the pairing rule in order:

  • A ↔ T (or U in RNA)
  • C ↔ G
  • preserve the strand positions while switching to the partner base

The DNA sequence A–C–G–T has the complementary sequence T–G–C–A.

Complementary does not mean identical: the partner strand has different letters in the opposite direction.

Complementary base pairing

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Deduce.

Command terms

Identify / Deduce

What earns marks

Build the answer around this relationship: Complementary base pairing should be described using precise molecular vocabulary.

Watch for

Mixing up covalent sugar-phosphate bonds with hydrogen bonds between complementary bases.

Representative question

Question 1

[Maximum number: 1]

Deduce the base indicated by X on the diagram.

Retrieve The Core Rules

The core chain is: nucleotides have three parts; condensation builds the sugar-phosphate backbone; base order stores information; complementary pairing lets DNA copy and express that information.

  • Nucleotide parts: phosphate, pentose sugar, nitrogenous base.
  • Polymer rule: condensation forms a sugar-phosphate backbone.
  • Code rule: base order stores information and triplet codons specify amino acids.
  • Pairing rule: A-T or A-U, and C-G, enables replication and gene expression.
  • DNA/RNA contrast: DNA uses deoxyribose/T and is usually double-stranded; RNA uses ribose/U and is usually single-stranded.

Concept essentials

  • Complementary base pairing 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