Mitosis — Phases and Significance

Mitosis is the process by which a eukaryotic cell divides its nucleus to produce two genetically identical daughter nuclei, each with the same number of chromosomes as the parent. It is followed by cytokinesis, which divides the cytoplasm. This lesson covers OCR A-Level Biology A specification point 2.1.6 (b) — the main stages of mitosis.

1. Introduction — Why Mitosis?

Mitosis is essential for:

Growth — a fertilised egg becomes a trillion-cell adult almost entirely through mitotic divisions.
Repair and replacement — the lining of the gut is replaced every few days; red blood cells are produced continuously in the bone marrow; wound healing is all mitosis.
Asexual reproduction — many plants reproduce vegetatively; Hydra buds; strawberries send out runners; all rely on mitosis.
Maintenance — tissues such as skin and the blood continuously replace cells lost through wear and tear.

A key feature: the daughter cells produced by mitosis are genetically identical to the parent cell (barring rare mutations) because DNA is replicated precisely in S phase and sister chromatids are separated accurately by the spindle.

Key Definition — Mitosis: A type of nuclear division that produces two daughter nuclei, each genetically identical to the parent and to each other, containing the same number of chromosomes as the parent cell.

2. Prerequisites — What Has Already Happened

Before a cell enters mitosis, it has already:

Replicated its DNA in S phase, so each chromosome now consists of two identical sister chromatids joined at a centromere.
Duplicated its centrosomes (in animal cells) — each will organise one end of the spindle.
Passed the G₂/M checkpoint, confirming the replication is complete and DNA damage is repaired.

A cell entering mitosis therefore has double the DNA content of a G₁ cell (usually written 2c → 4c to indicate the DNA quantity). The chromosome number, however, is unchanged — a human cell still has 46 chromosomes, but each is now two chromatids.

3. The Four Phases of Mitosis

Mitosis is traditionally divided into four phases: prophase, metaphase, anaphase and telophase. Some textbooks add prometaphase as a distinct phase between prophase and metaphase, but OCR groups it with prophase. Mitosis is a continuous process — the phases shade into each other.

graph LR
  A[Prophase<br/>chromosomes condense<br/>nuclear envelope breaks down] --> B[Metaphase<br/>chromosomes align at equator]
  B --> C[Anaphase<br/>sister chromatids separate]
  C --> D[Telophase<br/>nuclear envelopes reform]
  D --> E[Cytokinesis<br/>cytoplasm divides]

3.1 Prophase

During prophase:

Chromatin condenses by supercoiling around histones, becoming visible under a light microscope as discrete chromosomes. Each chromosome consists of two sister chromatids joined at the centromere.
The nucleolus disappears — rRNA synthesis halts.
The nuclear envelope breaks down (towards the end of prophase), releasing the chromosomes into the cytoplasm.
Centrosomes move to opposite poles of the cell (in animal cells). Plant cells have no centrioles but still form a functional spindle.
Spindle fibres (made of microtubules) begin to form, radiating from each pole.
Spindle microtubules start to attach to chromosomes at specialised protein structures called kinetochores, located at centromeres.

3.2 Metaphase

During metaphase:

Each chromosome is attached by its kinetochore to spindle microtubules from both poles.
Chromosomes are moved to the equator (metaphase plate) of the cell by the combined pulling and pushing of microtubules.
At this stage chromosomes are maximally condensed — this is the best phase in which to photograph a karyotype.
The spindle assembly checkpoint (Lesson 7) ensures every chromosome is correctly attached before anaphase begins.

3.3 Anaphase

During anaphase:

The centromeres divide, releasing each pair of sister chromatids.
The now-separated chromatids (each now a chromosome in its own right) are pulled towards opposite poles of the spindle by shortening of the kinetochore microtubules.
Chromatids appear V-shaped as the centromere leads the arms towards the pole.
Anaphase is the shortest phase of mitosis — it is all over in a few minutes.

3.4 Telophase

During telophase:

Chromatids reach the poles of the cell.
Nuclear envelopes reform around each group of chromatids, re-enclosing them into daughter nuclei.
Chromosomes decondense back into chromatin — they become invisible under the light microscope.
Nucleoli reappear — rRNA synthesis resumes.
The spindle fibres break down.

At the end of telophase, the cell has two distinct nuclei. It still needs to split its cytoplasm in two — that is cytokinesis.

4. Cytokinesis

Cytokinesis is the division of the cell body and is technically separate from mitosis. It proceeds differently in animal and plant cells.

4.1 Animal Cells

A ring of actin and myosin filaments forms just beneath the plasma membrane at the equator. This contractile ring contracts like a drawstring, forming a cleavage furrow that pinches the cell in two.

Lesson 8: Mitosis — Phases and Significance