Explain how the sequence of bases in a gene determines the sequence of amino acids in a polypeptide.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The DNA base sequence is copied by RNA polymerase into a complementary mRNA strand. Each set of three bases (a codon) on the mRNA specifies a particular amino acid. During translation, tRNA molecules with anticodons complementary to the codons bring the corresponding amino acids to the ribosome. The ribosome links the amino acids by peptide bonds in the order dictated by the mRNA codons, producing a polypeptide whose primary structure matches the gene’s base sequence.
Examiner tips
- Use the word ‘transcribed’ and ‘translated’ to show process steps; mention RNA polymerase and tRNA anticodons; link codons to amino acids and peptide bonds; keep answer concise to fit 4 marks.
Common mistakes
- Confusing DNA with mRNA sequence; forgetting the role of tRNA anticodons; omitting the peptide bond formation step; using vague terms like ‘copy’ instead of ‘transcribe’.
Mark scheme (4 marks)
- The base sequence of DNA is transcribed into a complementary mRNA sequence (by RNA polymerase), producing a pre-mRNA / mRNA strand.
- Each group of three consecutive bases on mRNA forms a codon, and each codon specifies a particular amino acid.
- During translation, tRNA molecules with complementary anticodons carry specific amino acids to the ribosome and base-pair with the codons on mRNA.
- Amino acids are joined by peptide bonds in the order dictated by the mRNA sequence, so the linear sequence of codons determines the primary structure (sequence of amino acids) of the polypeptide.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →