D1.2.8—Genetic code features
The genetic code uses mRNA triplets to specify amino acids, start and stop signals, while degeneracy reduces some mutation effects.
- Syllabus
- First assessment 2025
- Objective
- D1.2.8
- Level
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1–4
Common command terms
- Identify
- Outline
- Suggest
- Describe
Scoring notes
- Common mistake
- Counting amino acids as if one base rather than three bases codes for each residue.
Recent exam appearances
November 2025Paper1A ["HL"] · TZ39[ 1 ]D1.2.8—Genetic code features
November 2025Paper1A ["HL"] · TZ110[ 1 ]D1.2.8—Genetic code features
May 2025Paper2 ["HL"] · TZ210(a)[ 4 ]D1.2.8—Genetic code features
May 2023Paper3 ["HL"] · TZ113(b)[ 1 ]D1.2.8—Genetic code features
Practice this objective
Coverage 2023–2025 · Updated 16 Jul 2026
The Genetic Code Maps Codons to Amino Acids
The genetic code assigns each three-base mRNA codon to an amino acid or stop signal.
Triplets provide enough combinations for all amino acids. The code is degenerate because several codons can specify one amino acid, but each codon has a defined meaning.
Read codons in threes, using the correct reading frame and checking start and stop signals. Check triplet grouping; reading frame; codon meaning; and stop signals.
Changing UUU to UUC may still specify phenylalanine because the code is degenerate. A codon table can show that two different triplets encode the same amino acid, reducing the effect of some substitutions.
Degenerate does not mean ambiguous for one codon; it means multiple codons can share an amino acid.
Genetic code features
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Outline / Suggest.
Command terms
Identify / Outline / Suggest / Describe
What earns marks
Build the answer around this relationship: A codon is a triplet of bases on mRNA.
Watch for
Counting amino acids as if one base rather than three bases codes for each residue.
Representative question
Question 1
Describe the genetic code and its relationship to polypeptides and proteins.
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
- A codon is a triplet of bases on mRNA.
- Most codons specify one amino acid in a polypeptide.
- Start and stop codons define where translation begins and ends.
- Degeneracy means multiple codons can code for the same amino acid.