Mitosis — Phases and Significance

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.6 — Cell division, diversity and cellular organisation (refer to the official OCR H420 specification document for exact wording). It develops the main stages of mitosis (prophase, metaphase, anaphase, telophase), cytokinesis in animal and plant cells, and the significance of mitosis for growth, repair and asexual reproduction.

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 develops the OCR H420 Module 2.1.6 content on 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.

4.2 Plant Cells

Plant cells cannot form a cleavage furrow because the rigid cell wall prevents membrane inward movement. Instead, vesicles from the Golgi apparatus assemble at the equator into a cell plate, which grows outwards until it fuses with the existing plasma membrane and cell wall, dividing the cell in two. New cell wall material (cellulose) is deposited on both sides of the cell plate.

graph TD
  A[End of telophase] --> B{Cytokinesis}
  B --> C["Animal cell<br/>Cleavage furrow<br/>actin-myosin ring"]
  B --> D["Plant cell<br/>Cell plate forms<br/>vesicles fuse at equator"]

5. Observing Mitosis — Root Tip Squash

The classic OCR practical is to observe mitosis in cells from an onion or garlic root tip:

Snip the last 5 mm of a root tip — this is the meristematic region where cells are actively dividing.
Place in 1 mol dm⁻³ HCl at 60 °C for a few minutes to loosen the tissue and break down the middle lamella between cells.
Rinse in water and transfer to a microscope slide.
Add a drop of toluidine blue or acetic orcein stain, which binds DNA and stains chromosomes purple.
Cover with a coverslip and press firmly (the "squash") to spread cells into a single layer.
Observe under low then high magnification; identify cells in different phases of mitosis.

This practical produces beautiful images of cells in every phase and is a common exam context. Expect questions about identifying phases, calculating the mitotic index (proportion of cells actively dividing), and measuring cell sizes.

5.1 Mitotic Index

Mitotic index = (number of cells in mitosis) / (total number of cells observed). It is a measure of the proportion of a tissue that is actively dividing and is used clinically to assess tumours — a high mitotic index indicates fast-growing tissue.

6. Mitosis in Cancer and in Life

Growth of a human embryo — millions of mitoses per day in early development.
Gut epithelium — renewed every 3–5 days, with continuous mitosis in the crypts of Lieberkühn.
Bone marrow — produces billions of red blood cells per day via mitosis of stem cells.
Wound healing — cells around a wound divide to replace lost tissue.
Cancer — uncontrolled mitosis produces tumours. Chemotherapy drugs such as vinca alkaloids and taxanes work by disrupting the mitotic spindle; topoisomerase inhibitors block DNA supercoiling; nucleotide analogues block S-phase DNA synthesis.

7. Comparison with Meiosis

Meiosis (Lesson 9) is a different kind of division that produces four genetically varied haploid gametes. Mitosis is compared here only briefly:

Feature	Mitosis	Meiosis
Daughter cells	2	4
Chromosome number	Same as parent (diploid → diploid)	Halved (diploid → haploid)
Genetic variation	None (identical)	Yes (crossing over, independent assortment)
Homologues pair up?	No	Yes (in prophase I)
Role	Growth, repair, asexual reproduction	Production of gametes

8. Common Exam Mistakes

Confusing chromatids and chromosomes. A chromosome can consist of one or two chromatids, depending on whether S phase has happened.
Writing that DNA replicates "in prophase". It replicates in S phase, before mitosis begins.
Saying that chromosome number doubles in S phase. It does not — the number stays the same; each chromosome simply has two chromatids now.
Forgetting that centromeres divide in anaphase, separating sister chromatids.
Calling the metaphase plate a "physical plate". It is the equatorial plane along which chromosomes align — not a solid structure.
Confusing prophase and telophase — they look superficially similar, but the nuclear envelope is breaking down in prophase and reforming in telophase.
Saying mitosis "produces gametes". It produces somatic cells; gametes are produced by meiosis.
Confusing the cleavage furrow (animal cells) with the cell plate (plant cells).

9. Exam-Style Questions

Describe the main events that occur during metaphase and anaphase of mitosis. (5)
Explain why cytokinesis differs between plant and animal cells. (3)
Describe how you would prepare a stained slide of a root tip to observe mitosis. (5)
A section of dividing tissue contains 120 cells. 18 are in mitosis. Calculate the mitotic index and suggest what a high mitotic index might indicate. (3)

Model answer for (1): "During metaphase, each chromosome (consisting of two sister chromatids joined at a centromere) is attached by its kinetochore to spindle microtubules from both poles of the cell. Chromosomes are moved to the equator of the cell, forming the metaphase plate. During anaphase, the centromeres divide, allowing sister chromatids to separate. The now-independent chromatids are pulled towards opposite poles by shortening of the kinetochore microtubules, appearing V-shaped as the centromere leads the arms."

Summary

Mitosis produces two genetically identical daughter nuclei with the same chromosome number as the parent.
The four phases are prophase, metaphase, anaphase, telophase, preceded by interphase (particularly S phase, when DNA replicates).
Prophase: chromosomes condense, nuclear envelope breaks down, spindle forms.
Metaphase: chromosomes align at the equator, attached by kinetochores.
Anaphase: centromeres divide, sister chromatids move to opposite poles.
Telophase: nuclear envelopes reform, chromosomes decondense.
Cytokinesis follows: a cleavage furrow in animal cells, a cell plate in plant cells.
Mitosis is essential for growth, repair and asexual reproduction.
Uncontrolled mitosis underlies cancer; the mitotic index measures how much of a tissue is dividing.

Lesson 8: Mitosis — Phases and Significance