2.3.5 Recognising the Stages of Mitosis

Key Stages of Mitosis:

1. Prophase:

• Chromosomes condense and become visible as sister chromatids joined by a centromere.

• The nuclear envelope begins to break down.

• Spindle fibres start forming from the centrioles, which move to opposite poles of the cell. Diagram Features:

• Chromosomes are distinct and thickened.

• No nuclear membrane visible.

2. Metaphase:

• Chromosomes align along the equator of the cell (metaphase plate).

• Spindle fibres attach to the centromeres of each chromosome. Diagram Features:

• Chromosomes are neatly lined up in the centre of the cell.

• Spindle fibres are clearly visible.

3. Anaphase:

• Spindle fibres contract, pulling the sister chromatids apart at the centromere.

• The chromatids (now individual chromosomes) are pulled to opposite poles of the cell. Diagram Features:

• Chromatids are visibly separated and moving toward opposite poles.

• The cell may begin elongating.

4. Telophase:

• Chromosomes reach the poles and begin to decondense, becoming less visible.

• A nuclear envelope reforms around each set of chromosomes.

• Spindle fibres disintegrate. Diagram Features:

• Two nuclei are forming, with chromosomes enclosed within.

• The cell appears ready to divide.

Tips for Recognising Stages:

• Look for chromosome arrangement:
  • Condensed in prophase, aligned in metaphase, separating in anaphase, and decondensing in telophase.
• Identify key structural changes like the breakdown or reformation of the nuclear envelope and the presence of spindle fibres.