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D1.2.10—Elongation of polypeptide

Polypeptide elongation adds amino acids sequentially as ribosomes translocate along mRNA, while polysomes translate one transcript simultaneously in active cells.

Syllabus
First assessment 2025
Objective
D1.2.10
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks2

Common command terms

  • Explain

Scoring notes

Common mistake
Describing several ribosomes on one mRNA as several genes rather than one transcript being translated many times.

Recent exam appearances

November 2025Paper2 ["SL"] · TZ12(c)[ 2 ]D1.2.10—Elongation of polypeptide
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Elongation Repeats a Three-Step Ribosome Cycle

During elongation, a matching tRNA enters, a peptide bond forms and the ribosome moves to the next codon.

Repeating this cycle extends the chain in a fixed direction while mRNA advances through the ribosome. Energy from charged tRNAs and factors supports the process.

Track each cycle: codon recognition; chain transfer; translocation; empty tRNA exit.

After one cycle the chain has one additional amino acid and the next codon is exposed.

Elongation is not random amino-acid addition; codon recognition controls the order. The cycle stops at a termination signal rather than continuing until the physical end of the mRNA.

Elongation of polypeptide

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain.

Command terms

Explain

What earns marks

Build the answer around this relationship: Elongation lengthens a polypeptide one amino acid at a time.

Watch for

Describing several ribosomes on one mRNA as several genes rather than one transcript being translated many times.

Representative question

Question 1

[Maximum number: 2]

Six polypeptides are shown in the diagram. Explain the different lengths of these polypeptides.

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

  • Elongation lengthens a polypeptide one amino acid at a time.
  • Ribosomes translocate along mRNA as translation proceeds.
  • A polysome consists of many ribosomes on one mRNA molecule.
  • Farther-moving ribosomes usually carry longer growing polypeptides.