A scientist grows yeast cells in a nutrient solution. After several hours, the number of yeast cells in the solution has increased greatly. Explain how cell replication allows the number of yeast cells to increase, and why the new cells produced are able to function normally.

AQA A-Level Biology (7402) — 3.2.2 Cell Replication · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Yeast cells replicate by mitosis. Before division the DNA is copied, so each chromosome is duplicated. The cell then divides, giving each daughter cell a complete, identical set of chromosomes. One parent cell therefore produces two daughter cells, and the total number of cells doubles with each division. Because the new cells are genetically identical to the parent, they contain the same genes and can carry out the same functions.

Examiner tips

  • Use the phrase "mitosis" and "DNA duplication" to hit the key points. Show the sequence: DNA copied → cell divides → two identical cells. Mention that the number of cells doubles each cycle. Highlight genetic identity to justify normal function.

Common mistakes

  • Confusing mitosis with meiosis; not stating DNA duplication; forgetting that each daughter cell receives a full set of chromosomes; not linking genetic identity to normal function.

Mark scheme (5 marks)

  1. Yeast cells replicate by mitosis / cell division by mitosis occurs
  2. Before division, the DNA / chromosomes are copied / duplicated
  3. The cell then divides so that each daughter / new cell receives a full / identical copy of the DNA / chromosomes
  4. One parent cell produces two daughter cells / the number of cells increases with each division
  5. The new cells are genetically identical to the parent cell / contain the same genes, so they can carry out the same functions

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