A student states: 'Every cell in a human body contains the same DNA, yet cells look and behave very differently from one another.' Explain how cells with identical DNA can produce different proteins, and why the order of bases in DNA is important for protein production.

AQA A-Level Biology (7402) — 3.1.5 Nucleic Acids · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Different genes are expressed in different cell types, so only a subset of the genome is transcribed into mRNA in each cell.
A gene is a section of DNA that codes for a specific protein.
The order of bases in a gene determines the sequence of amino acids in the protein.
Each amino acid is specified by a triplet of bases (a codon).
The amino‑acid sequence determines the protein’s shape and therefore its function, so different proteins give cells different properties.

Examiner tips

  • Use the word "expressed" to link genes to proteins. Mention transcription and translation. Show the codon‑amino acid relationship. Explain that protein shape gives function.

Common mistakes

  • Saying all genes are expressed in every cell. Forgetting that codons are triplets. Confusing DNA sequence with protein sequence.

Mark scheme (5 marks)

  1. Different genes are expressed (switched on) in different cell types
  2. A gene is a section of DNA that codes for a specific protein
  3. The order/sequence of bases in a gene determines the order of amino acids in a protein
  4. Each amino acid is coded for by a sequence of three bases (a triplet / codon)
  5. The order of amino acids determines the shape/structure of the protein, so different proteins have different functions

Key terms in this question

DNA

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