Explain how the sequence of bases in a gene determines the sequence of amino acids in a protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Proteins are made up of long chains of amino acids. The order of amino acids determines the shape and function of each protein. The instructions for making proteins are stored in DNA inside the nucleus of a cell.
Model answer (5 marks)
The gene is a stretch of DNA that contains a specific sequence of bases, which acts as a code. During transcription, this DNA sequence is copied into a complementary strand of mRNA. The mRNA exits the nucleus and binds to a ribosome. At the ribosome, the mRNA is read in sets of three bases, called codons, and each codon specifies a particular amino acid. The ribosome links the amino acids together in the order dictated by the codons, forming a polypeptide chain that folds into a functional protein.
Examiner tips
- Use the exact terms: gene, transcription, mRNA, ribosome, codon, translation, polypeptide chain.
- Show the flow: DNA → mRNA → ribosome → codons → amino acids → protein.
- Keep the answer concise – one sentence per point to hit all 5 marks.
Common mistakes
- Confusing transcription with translation, or vice versa.
- Failing to mention that codons are triplets or that the ribosome is in the cytoplasm.
- Using vague language such as ‘sequence of bases’ without linking to mRNA and codons.
Mark scheme (5 marks)
- The gene (section of DNA) contains a specific sequence of bases / the base sequence acts as a code
- The sequence of bases on DNA is used to make a complementary strand of mRNA (transcription)
- The mRNA moves out of the nucleus to a ribosome
- Each sequence of three bases (triplet / codon) on the mRNA codes for a specific amino acid
- Amino acids are joined together in a specific order (translation) to form the protein / polypeptide chain
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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