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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

Coverage 2012–2022 · Updated 16 Jul 2026

Two Recognition Events Protect Amino-Acid Identity

At the ribosome, a tRNA anticodon pairs antiparallel with a complementary mRNA codon. This selects which tRNA can occupy the decoding site.

Before translation, a specific aminoacyl-tRNA synthetase attaches the correct amino acid to its matching tRNA. This charging step links anticodon identity to amino-acid identity.

Recognition Example What it secures
codon–anticodon pairing mRNA 5′–AUG–3′ pairs with tRNA 3′–UAC–5′ the correct tRNA is selected
enzyme–tRNA/amino-acid recognition methionine is attached to the initiator tRNA the selected tRNA carries the correct amino acid

Complementary base pairing alone cannot check which amino acid is attached. Translation fidelity depends on both correct pairing and correct tRNA charging.

Trace Information—and the Places Where It Can Change

DNA template → complementary mRNA → codons read 5′→3′ → charged tRNAs selected → peptide bonds join amino acids → polypeptide primary structure

Checkpoint Question to ask
transcription Is the mRNA complementary to the template and written 5′→3′?
code table Are mRNA codons grouped from the correct start?
tRNA selection Does the anticodon pair and is the tRNA correctly charged?
elongation Has the ribosome advanced one codon while the chain remains on the peptidyl-tRNA?
mutation Did the codon, amino acid or reading frame actually change?

A sequence change matters biologically only through a causal path: altered primary structure → altered interactions or folding → altered protein activity → possible cellular or organism effect.

Complementary base pairing

[ 3 ]

Outline how translation depends on complementary base pairing.

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.