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
- SL
Exam analysis
Chance of appearing6%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1
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 ["SL"] · TZ16[ 1 ]D1.2.8—Genetic code features
May 2024Paper1 ["SL"] · TZ210[ 1 ]D1.2.8—Genetic code features
May 2022Paper1 ["SL"] · TZ110[ 1 ]D1.2.8—Genetic code features
November 2019Paper2 ["SL"] · TZ02(c)[ 1 ]D1.2.8—Genetic code features
November 2019Paper2 ["SL"] · TZ02(a)[ 1 ]D1.2.8—Genetic code features
Practice this objective
Coverage 2015–2025 · Updated 16 Jul 2026
The Genetic Code Maps Codons to Amino Acids
The genetic code is a triplet, degenerate and nearly universal mapping from mRNA codons to amino acids or stop signals.
Four bases taken two at a time give only 16 combinations, fewer than the 20 amino acids; triplets give 4³ = 64 codons, enough for all amino acids plus stop signals.
Degenerate means more than one codon can specify the same amino acid. Universal means the same codon usually specifies the same amino acid across organisms, with limited exceptions.
UUU and UUC both code for phenylalanine, showing degeneracy; UUU has one defined meaning, so degeneracy is not ambiguity.
Read codons in a fixed triplet frame. Universality is a broad biological pattern, not a claim that no exceptions exist.
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.