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AS · Practice questions · Chromosomes and the cell cycle

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Six original Cambridge-style questions on chromosomes, the mitotic cell cycle, telomeres, stem cells and uncontrolled cell division.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
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Replication in S, division in M.

01
Recall
[4 marks]

Describe the structure of a chromosome at the start of mitosis. Refer to DNA, histones, chromatids, the centromere and telomeres.

  • Each chromatid is one DNA molecule wound round histone proteins (chromatin), tightly coiled. ✓
  • There are two genetically identical sister chromatids, formed by replication in S phase. ✓
  • They are held together at the centromere. ✓
  • Each end of each chromatid has a telomere of repeated, non-coding base sequences. ✓
02
Explain
[3 marks]

Outline what happens in each of the three phases of interphase.

  • G1: the cell grows and makes proteins (including enzymes for replication) and organelles. ✓
  • S: every DNA molecule is replicated, so each chromosome becomes two sister chromatids. ✓
  • G2: further growth, organelles such as mitochondria divide, and the DNA is checked before mitosis. ✓
03
Data
[3 marks]

The mass of DNA in a cell was measured through one cell cycle. It was 6.6 pg for the first 9 hours, rose steadily to 13.2 pg over the next 7 hours, stayed at 13.2 pg for 5 hours, then fell to 6.6 pg. Identify the phases and explain the fall.

  • First 9 h at 6.6 pg: G1. ✓
  • Steady rise to 13.2 pg over 7 h: S phase, DNA replication; the 5 h at 13.2 pg is G2 (and mitosis). ✓
  • The fall to 6.6 pg happens at cytokinesis: the replicated DNA is shared equally between two cells, each with the original mass. ✓
04
Explain
[3 marks]

Explain the role of telomeres, and suggest why most body cells can only divide a limited number of times.

  • DNA polymerase cannot replicate the very end of a linear DNA molecule, so the ends get shorter at each replication. ✓
  • Telomeres are non-coding repeats, so it is telomere that is lost rather than genes. ✓
  • Most body cells lack active telomerase, so once the telomeres are critically short the cell stops dividing, protecting the genes. ✓
05
Explain
[3 marks]

After a deep cut, the skin heals and the new tissue has the same genotype as the surrounding skin. Explain how this happens.

  • Stem cells near the wound divide repeatedly by mitosis. ✓
  • Mitosis produces daughter cells that are genetically identical to the parent cell. ✓
  • Some daughter cells differentiate into specialised skin cells, replacing the cells lost and repairing the tissue. ✓
06
Explain
[4 marks]

Explain how exposure to a carcinogen can lead to the formation of a malignant tumour.

  • The carcinogen causes mutations in genes that control the cell cycle, for example a proto-oncogene becomes an oncogene or a tumour suppressor gene is inactivated. ✓
  • The cell no longer responds to the signals that stop division, and damaged cells are not destroyed. ✓
  • It divides repeatedly by mitosis without control, forming a mass of genetically identical cells: a tumour. ✓
  • If tumour cells invade surrounding tissue and spread through the blood or lymph to form secondary tumours, the tumour is malignant. ✓

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