The cell cycle, mitosis and mitotic index

What you’ll learn

• Explain how DNA replication and mitosis maintain genetic information.
• Recognise the main events of mitosis and distinguish chromosomes from chromatids.
• Calculate and interpret a mitotic index.

Understand the idea

The cell cycle includes interphase, nuclear division and usually cytokinesis. During interphase the cell grows, carries out normal functions and replicates its DNA before division. A replicated chromosome contains two sister chromatids joined at the centromere region. DNA replication doubles the DNA content but does not immediately double the chromosome count when chromosomes are counted by their centromeres.

In prophase chromosomes condense, the spindle forms and the nuclear envelope breaks down. During metaphase chromosomes align at the equator, attached to spindle fibres. In anaphase sister chromatids separate and move to opposite poles; in telophase nuclear envelopes reform. Cytokinesis divides the cytoplasm. Mitosis generally produces nuclei with the same chromosome complement and genetic information as the original nucleus, apart from new mutations. It supports growth, repair and asexual reproduction.

Core practical 6 uses stained root-tip cells to observe mitosis. The meristem near the tip contains many dividing cells; a thin preparation reduces overlap, and staining makes chromosomes visible. Mitotic index is the fraction of observed cells in mitosis, sometimes expressed as a percentage. A high value can suggest active cell division, but a treatment that traps cells in mitosis can also increase it. Estimating stage duration from a snapshot requires an approximately steady, asynchronous population and representative sampling.

Key formulae

$$\text{mitotic index}=\frac{\text{number of cells in mitosis}}{\text{total number of cells observed}}$$

$$\%\text{ of cells in mitosis}=100\times\text{mitotic index}$$

Worked example

A root-tip sample contains 180 interphase cells and 45 cells in mitosis. Calculate the mitotic index as a fraction and percentage.

1. Count all observed cells: 180 + 45 = 225.
2. The fraction in mitosis is 45/225 = 0.20.
3. Multiply by 100 to express this as 20%.

Answer: Mitotic index 0.20, or 20%.