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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

Coverage 2023–2025 · Updated 16 Jul 2026

The Genetic Code Is Triplet, Degenerate and Nearly Universal

The genetic code has several linked properties:

  • Triplet: three mRNA bases form one codon.
  • Degenerate: most amino acids are specified by more than one codon.
  • Unambiguous: each codon specifies only one amino acid or stop signal.
  • Nearly universal: the same codons usually have the same meanings across organisms, with limited exceptions.
  • Punctuated: a start codon establishes the reading frame; stop codons end translation.

Degeneracy means some base substitutions are silent, but it does not mean one codon can represent several different amino acids.

Codon type Function
AUG usually codes for methionine and can establish the start of translation
UAA, UAG, UGA stop signals recognized by release factors; they do not add an amino acid

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.

Genetic code features

[ 5 ]

Describe the genetic code and its relationship to polypeptides and proteins.

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.