Explain how the sequence of nucleotides in a gene is used to determine the sequence of amino acids in a polypeptide, referring to both transcription and translation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During transcription, RNA polymerase reads the template strand of DNA and synthesises a complementary mRNA strand using base‑pairing (A–U, T–A, G–C). The mRNA is a copy of the gene’s coding sequence.
The mRNA is read in non‑overlapping sets of three nucleotides called codons. Each codon specifies a particular amino acid (or a start/stop signal) according to the genetic code.
During translation, tRNA molecules with an anticodon complementary to the codon bind to the ribosome. Each tRNA carries the amino acid that matches its anticodon.
As the ribosome moves along the mRNA, peptide bonds are formed between successive amino acids, producing a polypeptide whose primary sequence directly reflects the original gene sequence.
The mRNA is read in non‑overlapping sets of three nucleotides called codons. Each codon specifies a particular amino acid (or a start/stop signal) according to the genetic code.
During translation, tRNA molecules with an anticodon complementary to the codon bind to the ribosome. Each tRNA carries the amino acid that matches its anticodon.
As the ribosome moves along the mRNA, peptide bonds are formed between successive amino acids, producing a polypeptide whose primary sequence directly reflects the original gene sequence.
Examiner tips
- Use the exact terms ‘transcription’, ‘translation’, ‘codon’, ‘anticodon’, ‘peptide bond’.
- Show the base‑pairing rules for DNA→RNA and codon→anticodon.
- Explain the directionality (5’→3’) and the role of the ribosome.
Mark scheme (4 marks)
- During transcription, RNA polymerase reads the template strand of DNA and synthesises a complementary mRNA strand using base pairing (A–U, T–A, G–C).
- Each non-overlapping triplet of three consecutive nucleotides on the mRNA is a codon that corresponds to a specific amino acid (or start/stop signal) according to the genetic code.
- During translation, tRNA molecules with a specific anticodon carry a specific amino acid and undergo complementary base pairing with the matching codon on the mRNA at the ribosome.
- Peptide bonds form between successive amino acids as the ribosome moves along the mRNA, producing a polypeptide whose primary sequence is dictated by the original gene sequence.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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