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AS · Practice questions · Chromosome behaviour in mitosis

Name the stage.

Six original Cambridge-style questions on chromosome behaviour in mitosis, identifying stages, and root tip squash data.

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.
Keep these straight

Follow the chromosomes.

01
Recall
[4 marks]

Describe the behaviour of the chromosomes and the spindle during metaphase and anaphase.

  • Metaphase: spindle fibres attach to the centromere (kinetochore) of each chromosome. ✓
  • The chromosomes are lined up on the equator of the spindle. ✓
  • Anaphase: the centromeres divide and the spindle fibres shorten. ✓
  • Sister chromatids are pulled to opposite poles, centromere first. ✓
02
Explain
[3 marks]

Describe the changes to the nuclear envelope and nucleolus during mitosis, and explain why the nuclear envelope must break down.

  • The nucleolus disappears and the nuclear envelope breaks down at the end of prophase. ✓
  • Both re-form around each group of chromosomes in telophase. ✓
  • The envelope must break down so that spindle fibres can attach to the centromeres and move the chromosomes. ✓
03
Explain
[3 marks]

Compare cytokinesis and spindle formation in animal and plant cells.

  • Animal cells have centrosomes (centriole pairs) that move to the poles and organise the spindle; plant cells have no centrioles but still form a spindle. ✓
  • In animal cells the cell surface membrane is pulled in to form a cleavage furrow. ✓
  • In plant cells vesicles from the Golgi body fuse across the equator to form a cell plate, which becomes the new cell walls. ✓
04
Analysis
[3 marks]

Four drawings of dividing cells show: (A) chromosomes in a line across the middle; (B) visible thread-like chromosomes inside a nucleus; (C) two dense groups of chromosomes at opposite ends with a furrow between; (D) V-shaped chromosomes moving apart. Name each stage and put them in order.

  • A metaphase, B prophase, C telophase, D anaphase. ✓
  • Order: B, A, D, C. ✓
  • Reason: chromosomes condense in a nucleus, line up, separate, then form two new nuclei. ✓
05
Calculation
[3 marks]

A student counted cells in an onion root tip squash: interphase 380, prophase 64, metaphase 22, anaphase 14, telophase 20. Calculate the mitotic index, and the duration of prophase if the cell cycle takes 16 hours.

  • Total cells = 380 + 64 + 22 + 14 + 20 = 500; cells in mitosis = 120. ✓
  • Mitotic index = 120 ÷ 500 = 0.24. ✓
  • Prophase: 64 ÷ 500 = 0.128 of the cycle; 0.128 × 16 = 2.05 h (about 2.0 h). ✓
06
Analysis
[3 marks]

The mitotic index was 0.24 in the region 1 mm behind a root tip, and 0.02 in the region 8 mm behind the tip. Explain the difference, and give one way to make the estimates more reliable.

  • Near the tip is the meristem, where cells are dividing rapidly by mitosis to produce new cells for growth. ✓
  • Further back, cells have stopped dividing and are elongating and differentiating, so few are in mitosis. ✓
  • Reliability: count more cells, from several fields of view and several root tips, and calculate a mean. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.