describe the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of mitosis are expected: prophase,

Published by Patrick Mutisya · 8 days ago

Chromosome Behaviour in Mitosis – Cambridge A‑Level Biology 9700

Chromosome Behaviour in Mitosis

The mitotic cell cycle ensures that a single parent cell divides to give two genetically identical daughter cells. The behaviour of chromosomes, the nuclear envelope, the cell surface membrane and the spindle apparatus is coordinated and follows a predictable sequence of stages.

Key Stages of Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

Behaviour of Structures at Each Stage

Prophase

  • Chromatin condenses into visible chromosomes; each consists of two sister chromatids joined at the centromere.

  • The nucleolus disappears.

  • The nuclear envelope begins to fragment.

  • Centrosomes (animal cells) or microtubule‑organising centres (plant cells) migrate to opposite poles and begin to nucleate the mitotic spindle.

  • In plant cells, a pre‑prophase band of microtubules forms around the future division plane.

Metaphase

  • Chromosomes align at the metaphase plate, an imaginary plane equidistant from the spindle poles.

  • Each sister chromatid is attached to spindle fibres (kinetochore microtubules) emanating from opposite poles.

  • The nuclear envelope is completely broken down, allowing spindle fibres to contact chromosomes.

  • In animal cells, the cell surface membrane remains intact; in plant cells, the cell wall is still present, but the plasma membrane is flexible enough to accommodate spindle movements.

Anaphase

  • Sister chromatids separate at the centromere and are pulled toward opposite poles by shortening kinetochore microtubules.

  • Polar microtubules lengthen, pushing the spindle poles further apart.

  • The cell surface membrane may begin to indent (animal cells) or a new cell plate starts to form (plant cells).

Telophase

  • Chromatids reach opposite poles and de‑condense back into chromatin.

  • Nuclear envelopes re‑form around each set of chromosomes, re‑establishing separate nuclei.

  • The nucleolus re‑appears.

  • Spindle fibres disassemble.

  • In animal cells, a cleavage furrow forms, deepening until the cell splits (cytokinesis).

  • In plant cells, vesicles coalesce at the centre of the cell to form a new cell plate, which becomes the new cell wall separating the daughter cells.

Comparison of Plant and Animal Cells During Mitosis

FeatureAnimal CellPlant Cell
CentrosomesPresent; act as main microtubule‑organising centres.Absent; microtubules nucleate from dispersed MTOCs.
Pre‑prophase bandNot present.Forms around future division plane during prophase.
Cell surface changesCleavage furrow forms during telophase → cytokinesis.Cell plate forms from vesicle fusion during telophase → cytokinesis.
Spindle attachmentKinetochore fibres attach directly to chromosomes.Similar attachment; however, the rigid cell wall restricts spindle movement.
Presence of cell wallAbsent.Present; remains intact throughout mitosis.

Suggested diagram: A side‑by‑side schematic of animal and plant cells showing chromosome alignment, spindle orientation, and the distinct cytokinetic mechanisms (cleavage furrow vs. cell plate) at each mitotic stage.

Key Points to Remember

  • All stages of mitosis are identical in terms of chromosome behaviour; differences lie in the structures that support division (centrosomes, cell wall, cytokinetic apparatus).

  • The nuclear envelope disassembles in prophase–metaphase and re‑assembles in telophase, allowing spindle access to chromosomes.

  • Spindle fibres are essential for accurate chromosome segregation; errors can lead to aneuploidy.

  • Plant cells cannot form a cleavage furrow because of the rigid cell wall; they instead construct a new cell wall from the inside out.