5.2 Chromosome Behaviour in Mitosis

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. 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 — including the four main stages: prophase, metaphase, anaphase, and telophase.
  2. Interpret photomicrographs, diagrams, and microscope slides of cells at different stages of mitosis, and identify each main stage.

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What is Mitosis?

Mitosis is a type of nuclear division (division of the nucleus) that produces two genetically identical daughter nuclei — meaning the new nuclei are exact copies of each other and of the original parent nucleus. Each daughter nucleus contains the same number of chromosomes as the parent.

Mitosis is one continuous process, but scientists divide it into four named stages to make it easier to study:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

A helpful memory trick: P-M-A-T (think: "People Meet And Talk").

After mitosis is complete, the cytoplasm (the liquid filling the cell) divides in a process called cytokinesis, producing two separate daughter cells.

Key Structures Involved in Mitosis

Before looking at the stages, it helps to understand the key structures that play a role in mitosis:

  • Chromosomes — Thread-like structures in the nucleus that carry genetic information (DNA). They are only visible under a microscope when they condense (shorten and thicken) during cell division.
  • Chromatids — When a chromosome is copied (replicated) before mitosis, it consists of two identical strands called sister chromatids, joined together at a point called the centromere.
  • Centromere — The region where the two sister chromatids are held together.
  • Nuclear envelope — The double membrane (thin layer) that surrounds and protects the nucleus.
  • Nucleolus — A small, dense structure found inside the nucleus; it disappears during mitosis and reforms afterward.
  • Centrosomes — Small organelles that organise the spindle. In animal cells, each centrosome contains a pair of structures called centrioles. Plant cells do not have centrioles but still form a spindle.
  • Spindle fibres — Thin protein threads (made of microtubules) that grow out from the centrosomes and move chromosomes during mitosis. Together, all these fibres form the spindle.
  • Cell surface membrane — The thin outer membrane surrounding the whole cell.

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