Explain the role of ribosomes in translation during protein synthesis.

IB DP Biology Standard Level (2023 syllabus) — D1.2 Protein synthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Ribosomes are ribonucleoprotein complexes composed of a small and a large subunit that assemble around the mRNA.
The ribosome moves along the mRNA in the 5′→3′ direction, reading successive codons.
It holds tRNA molecules in the A and P sites, allowing complementary base‑pairing between codon and anticodon.
The ribosome catalyses peptide bond formation between adjacent amino acids, thereby elongating the polypeptide chain.

Examiner tips

Use the exact terms – small/large subunit, A and P sites, 5′→3′ direction, peptide bond formation.
Show the sequence of events: assembly, movement, tRNA binding, catalysis.
Mention that the ribosome is the site of translation.
Keep the answer concise and to the point.

Common mistakes

Confusing the direction of ribosome movement (5′→3′) with the direction of mRNA synthesis.
Using “ribosomal RNA” instead of “ribosome” as the site of peptide bond formation.
Omitting the role of the A and P sites or the tRNA binding step.

Mark scheme (4 marks)

Ribosomes are composed of two subunits (large and small) that assemble around the mRNA.
The ribosome moves along the mRNA (5′ to 3′) reading successive codons.
The ribosome holds tRNA molecules in place (at A and P sites) so that complementary base pairing between codons and anticodons can occur.
The ribosome catalyses peptide bond formation between adjacent amino acids, elongating the polypeptide chain.

Key terms in this question

ribosome · translation