A student says: 'Every cell in a human body contains the same DNA, so every cell must make the same proteins.' Explain why this statement is incorrect.

AQA A-Level Biology (7402) — 3.4.2 DNA and Protein Synthesis · Explain · 5 marks · View as Markdown

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

Human body cells are specialised for different functions. For example, a pancreatic cell produces the hormone insulin, whereas a muscle cell does not.

Model answer (5 marks)

DNA contains genes that code for proteins. All human cells have the same DNA sequence, but only certain genes are switched on (expressed) in a particular cell type. The genes that are expressed are transcribed into mRNA, which is then translated into protein. Thus, although the DNA is identical, different cells produce different proteins – for example, pancreatic cells express the insulin gene while muscle cells do not.

Examiner tips

  • Use the term ‘genes’ and ‘expression’ to show understanding of regulation.
  • Show the sequence: DNA → mRNA → protein, emphasising that only expressed genes are transcribed.
  • Give a clear example (insulin vs muscle) to demonstrate the consequence of differential expression.
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Mark scheme (5 marks)

  1. DNA contains genes / sections of DNA that code for proteins
  2. All body cells contain the same genes / the same DNA sequence
  3. Only certain genes are switched on / expressed in a particular cell type
  4. Only the mRNA produced from active/expressed genes is used to make protein (in that cell)
  5. Therefore different cell types produce different proteins / reference to example (e.g. pancreatic cells produce insulin but muscle cells do not)

Key terms in this question

DNA · protein

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