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,

Chromosome Behaviour in the Mitotic Cell Cycle (Cambridge IGCSE/A‑Level Biology 9700 – Topic 5 & 5.2)

1. What the cell must achieve

• Goal: One parent cell divides to give two genetically identical daughter cells.
• The process is divided into two parts:

  • Interphase (G₁, S, G₂) – cell growth, DNA replication, telomere maintenance and preparation for division.
  • Mitosis – division of the nucleus, followed immediately by cytokinesis (division of the cytoplasm).

2. Prerequisite knowledge (Topic 5.1)

2.1 Chromosome structure

• DNA is wrapped around histone proteins → chromatin.
• During S‑phase each DNA molecule is duplicated, producing two identical sister chromatids joined at a centromere.
• Both ends of a chromatid are capped by telomeres, which protect the DNA from degradation and from end‑to‑end fusion.

2.2 Telomere biology (syllabus requirement)

• Telomeres shorten each time a chromosome is replicated because DNA polymerase cannot fully copy the very ends.
• The enzyme telomerase adds repetitive DNA sequences to restore telomere length in cells that must divide repeatedly (e.g., stem cells, germ cells).
• Loss of telomere maintenance leads to chromosome instability, contributes to ageing and is a key factor in tumour formation.

2.3 Stem cells and tumour formation

• Stem cells retain the ability to divide indefinitely while remaining undifferentiated.
• If the control mechanisms that regulate stem‑cell mitosis fail, the resulting uncontrolled division can give rise to a tumour.

3. Overview of the mitotic cell cycle

1. Interphase

   • G₁ – cell grows, synthesises proteins and organelles.
   • S – DNA replication; each chromosome now consists of two sister chromatids.
   • G₂ – further growth, synthesis of mitotic proteins, telomere lengthening by telomerase.

2. Prophase
3. Prometaphase
4. Metaphase
5. Anaphase
6. Telophase
7. Cytokinesis (cleavage furrow in animal cells; cell plate in plant cells)

4. Behaviour of chromosomes, nuclear envelope, plasma membrane and spindle at each mitotic stage

4.1 Prophase

• Chromosomes: Chromatin coils into visible chromosomes; each chromosome = 2 sister chromatids.
• Nuclear envelope: Still intact but begins to fragment at discrete points.
• Spindle:

  • Animal cells – duplicated centrosomes migrate to opposite poles and nucleate microtubules, forming the spindle poles.
  • Plant cells – a single dispersed microtubule‑organising centre (MTOC) nucleates spindle microtubules; a pre‑prophase band of microtubules appears around the future division plane.

• Plasma membrane / cell wall:

  • Animal cell – membrane remains continuous and flexible.
  • Plant cell – rigid cell wall stays intact; plasma membrane stays attached to the wall.

4.2 Prometaphase

• Nuclear envelope: Disassembles completely, allowing spindle microtubules to contact chromosomes.
• Spindle fibres: Kinetochore microtubules attach to protein complexes (kinetochores) at the centromere of each chromatid.
• Chromosomes: Remain condensed; each sister chromatid is now linked to a spindle fibre from opposite poles.
• Plasma membrane / cell wall: No change; the cell wall of plant cells continues to constrain the cell shape.

4.3 Metaphase

• Chromosomes: Align on the metaphase plate, an imaginary plane equidistant from the two spindle poles.
• Spindle tension: Opposing kinetochore fibres generate tension that ensures each chromatid is attached to a different pole (the “bivalent” checkpoint).
• Nuclear envelope: Absent.
• Plasma membrane / cell wall: Remain unchanged.

4.4 Anaphase

• Centromeric cohesion is released; sister chromatids separate and are pulled toward opposite poles by shortening kinetochore microtubules.
• Polar (astral) microtubules lengthen, pushing the spindle poles farther apart.
• Animal cell: The plasma membrane begins to indent at the future cleavage site (future furrow).
• Plant cell: Golgi‑derived vesicles accumulate at the cell centre, marking the site where the cell plate will form.

4.5 Telophase

• Chromatids reach the poles and de‑condense into chromatin.
• New nuclear envelopes re‑form around each set of chromosomes; nucleoli re‑appear.
• Spindle fibres disassemble.
• Animal cell: The contractile ring of actin‑myosin deepens the cleavage furrow, completing cytokinesis.
• Plant cell: Vesicles fuse to produce a membranous cell plate, which matures into a new cell wall separating the daughter cells.

5. Plant vs. Animal cells – Summary comparison

6. Practical skills required by the syllabus

• Photomicrograph identification – be able to label the five mitotic stages (including prometaphase) in:

  • Onion root tip (plant tissue).
  • Animal tissue such as mouse intestinal epithelium or human cheek cells.

• Diagram drawing – produce a clear, labelled sketch of each stage showing:

  • Chromosomes (condensed or de‑condensed).
  • Spindle fibres (kinetochore, polar, astral).
  • Nuclear envelope (intact, fragmented, re‑forming).
  • Cell‑surface change (cleavage furrow or cell plate).

• Explanation of errors – in a short paragraph, describe how incorrect spindle‑kinetochore attachment can cause aneuploidy and why aneuploid cells are a hallmark of many tumours.

7. Key points to remember for revision

• The order of events – interphase → prophase → prometaphase → metaphase → anaphase → telophase → cytokinesis – is identical in plant and animal cells; only the supporting structures differ.
• Breakdown of the nuclear envelope in prometaphase is essential for spindle attachment to kinetochores.
• Centrosomes are the main MTOCs in animal cells; plants use a single MTOC plus a pre‑prophase band to define the division plane.
• Telomere maintenance by telomerase is crucial for cells that divide repeatedly; failure contributes to ageing and cancer.
• Accurate chromosome segregation depends on proper spindle‑kinetochore attachment; mis‑segregation → aneuploidy → tumour formation.

8. Suggested revision diagram