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9700 · 5.2

Chromosome behaviour in mitosis — practice questions

Practice and worked examples for 9700 Chromosome behaviour in mitosis. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A cell from a plant's root tip is observed undergoing mitosis. Describe the key events that occur during anaphase in this cell and explain the importance of precise chromosome segregation during this stage.

Show solution outline
  1. Centromere division: The centromeres, which hold the sister chromatids together, divide.
  2. Sister chromatid separation: This separation transforms each chromatid into an individual, un-replicated chromosome.
  3. Chromosome migration: Shortening spindle fibres (specifically kinetochore microtubules) pull these newly formed chromosomes towards opposite poles of the cell. The poles are defined by the organizing centres from which the spindle fibres originate.
  4. Significance: Precise chromosome segregation in anaphase is critical because it ensures that each new daughter cell receives an identical and complete set of genetic information. Failure to do so would result in daughter cells with an incorrect number of chromosomes (aneuploidy), which can lead to cell dysfunction, developmental abnormalities, or even cell death. This genetic identicality is essential for proper growth, tissue repair, and maintaining the organism's genetic stability.

Worked example 2

A student observes a micrograph of an onion root tip and counts the number of cells in each stage of the cell cycle. They count a total of 300 cells. Of these, 54 cells are observed to be in a stage of mitosis (prophase, metaphase, anaphase, or telophase). Calculate the mitotic index for this tissue sample.

Show solution outline
  1. State the formula: The mitotic index is the ratio of cells undergoing mitosis to the total number of cells, usually expressed as a percentage. Mitotic Index=Number of cells in mitosisTotal number of cells×100%\text{Mitotic Index} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells}} \times 100\%
  2. Identify the given values:
    • Number of cells in mitosis = 54
    • Total number of cells = 300
  3. Substitute the values into the formula: Mitotic Index=54300×100%\text{Mitotic Index} = \frac{54}{300} \times 100\%
  4. Calculate the result: 54300=0.18\frac{54}{300} = 0.18 0.18×100%=18%0.18 \times 100\% = 18\%
  5. Final Answer: The mitotic index for the onion root tip tissue is 18%. This indicates that at any given moment, 18% of the cells in this region are actively dividing.
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