A dog has 78 chromosomes in each of its body cells. Explain how genetic information is stored and organised within these chromosomes, and explain why the dog's liver cells and muscle cells contain the same genetic information despite having different functions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Chromosomes are thread‑like structures that contain DNA.
DNA is a long double‑stranded molecule that forms a double helix.
A gene is a small section of DNA that codes for a protein, i.e. a sequence of amino acids.
All body cells are produced by mitosis from the original fertilised egg, so every cell contains the same set of chromosomes.
In different cell types only certain genes are switched on or expressed, but the DNA sequence itself is identical in liver and muscle cells.
DNA is a long double‑stranded molecule that forms a double helix.
A gene is a small section of DNA that codes for a protein, i.e. a sequence of amino acids.
All body cells are produced by mitosis from the original fertilised egg, so every cell contains the same set of chromosomes.
In different cell types only certain genes are switched on or expressed, but the DNA sequence itself is identical in liver and muscle cells.
Examiner tips
- Use the exact terms: chromosome, DNA, double helix, gene, protein, mitosis, expressed
- Show the logical sequence: chromosome → DNA → gene → protein; then explain mitosis and gene expression
Common mistakes
- Confusing DNA with RNA or proteins; writing ‘gene is a protein’
- Forgetting to mention that the DNA sequence is the same in all cells
Mark scheme (5 marks)
- Chromosomes are made of / contain DNA
- DNA is a long molecule made up of (two) strands / is a double helix
- A gene is a small section of DNA / a section of DNA that codes for a protein / codes for a sequence of amino acids
- Every body cell is produced by mitosis (from the original fertilised egg) / all body cells are derived from the same original cell
- Different genes are switched on / expressed in different cell types, but the DNA sequence itself is the same in both cell types
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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