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D1.2.7—Complementary base pairing

Complementary base pairing between mRNA codons and tRNA anticodons ensures that each codon recruits the correct amino acid carrier during translation.

Syllabus
First assessment 2025
Objective
D1.2.7
Level
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2022
Most common paperPaper1
Typical marks1–3

Common command terms

  • Outline
  • Explain

Scoring notes

Common mistake
Using DNA base-pairing letters instead of RNA uracil when writing anticodons.

Recent exam appearances

November 2022Paper1 ["SL"] · TZ010[ 1 ]D1.2.7—Complementary base pairing
May 2019Paper1 ["SL"] · TZ29[ 1 ]D1.2.7—Complementary base pairing
May 2017Paper2 ["SL"] · TZ15(a)[ 3 ]D1.2.7—Complementary base pairing
May 2012Paper1 ["SL"] · TZ27[ 1 ]D1.2.7—Complementary base pairing
Practice this objective

Coverage 2012–2022 · Updated 16 Jul 2026

Complementary Pairing Transfers Information

A three-base tRNA anticodon forms complementary base pairs with a three-base mRNA codon during translation.

Correct codon–anticodon pairing positions the tRNA carrying the amino acid specified by that codon, transferring mRNA sequence information into polypeptide sequence.

Read the mRNA codon 5′→3′; write the antiparallel complementary anticodon; pair A–U and C–G; identify the amino acid from the mRNA codon, not the anticodon.

mRNA codon 5′-AUG-3′ pairs with tRNA anticodon 3′-UAC-5′, positioning the tRNA carrying methionine.

Codon and anticodon are complementary and antiparallel, not identical. DNA–RNA pairing during transcription is a different Objective.

Complementary base pairing

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline / Explain.

Command terms

Outline / Explain

What earns marks

Build the answer around this relationship: mRNA codons pair with complementary tRNA anticodons.

Watch for

Using DNA base-pairing letters instead of RNA uracil when writing anticodons.

Representative question

Question 1

[Maximum number: 3]

Outline how translation depends on complementary base pairing.

Core Protein Synthesis

  • Transcription: RNA polymerase builds complementary mRNA from the DNA template; A pairs with U and C with G.
  • Translation: ribosomes read mRNA codons 5′ to 3′ while tRNA anticodons deliver specific amino acids for peptide-bond formation.
  • Genetic code: codons are triplets; the code is degenerate and almost universal, with start and stop signals. Use mRNA—not DNA—when reading a code table.
  • Information flow: codon order determines amino-acid sequence, which determines protein folding and function.
  • Expression and variation: cells regulate which genes are transcribed. A mutation may change a codon, primary structure and phenotype, as in sickle-cell haemoglobin.

Concept essentials

  • mRNA codons pair with complementary tRNA anticodons.
  • RNA base pairing uses A with U and C with G.
  • Correct amino acid addition depends on both anticodon pairing and tRNA charging.
  • The anticodon identifies the codon read at the ribosome, not the original DNA triplet directly.