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
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper1
Typical marks1
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 2023Paper2 ["HL"] · TZ22(b)[ 2 ]D1.2.7—Complementary base pairing
May 2022Paper1 ["HL"] · TZ128[ 1 ]D1.2.7—Complementary base pairing
May 2021Paper1 ["HL"] · TZ228[ 1 ]D1.2.7—Complementary base pairing
May 2019Paper1 ["HL"] · TZ28[ 1 ]D1.2.7—Complementary base pairing
May 2012Paper1 ["HL"] · TZ29[ 1 ]D1.2.7—Complementary base pairing
Practice this objective
Coverage 2012–2023 · Updated 16 Jul 2026
Complementary Pairing Transfers Information
Complementary base pairing allows DNA to specify an mRNA sequence and codons to be recognized by tRNA anticodons.
Hydrogen-bonding patterns and base size make particular pairs fit. The same principle connects template reading during transcription with codon decoding during translation.
Check strand direction and pair type before predicting the next nucleotide or amino acid. Check pairing; strand direction; codon reading frame; and the matching anticodon.
A DNA template produces complementary mRNA, whose codon pairs with a matching tRNA anticodon. A single template base determines its partner in mRNA, which then helps select the tRNA carrying the next amino acid.
Pairing is complementary, not identical: the message is related to the template rather than a literal copy.
Complementary base pairing
Assessment in practice
1 marksHow 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
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.