Mitosis

AQA spec ref: 3.2.7 - Cell division: mitosis

Mitosis is the form of nuclear division that produces two genetically identical daughter cells from a single parent cell. It is the basis of growth, tissue repair, and asexual reproduction. The key point is genetic identity - because the chromosomes are copied before division and one complete set is distributed to each daughter cell, every new cell carries exactly the same genetic information as the original. Mitosis occurs in all somatic (body) cells of eukaryotes, and the entire process - DNA replication plus division - is called the cell cycle.

The Cell Cycle

The cell cycle has two main phases: interphase (the period between divisions, when the cell grows and replicates its DNA) and mitosis (division itself). Interphase is not a resting phase - it is the most metabolically active part of the cycle.

Interphase is divided into three sub-phases:

• G₁ (first gap phase) - the cell grows, synthesises proteins, and increases its organelle content. The cell checks that conditions are favourable before committing to DNA replication.
• S phase (synthesis) - DNA replication occurs. Every chromosome is copied, using the semi-conservative mechanism (see DNA replication and Protein synthesis). By the end of S phase, the cell contains twice the normal amount of DNA.
• G₂ (second gap phase) - the cell continues to grow and synthesises the proteins needed for division (e.g. tubulin for spindle fibres). The cell also proofreads the newly replicated DNA to correct errors before dividing.

After G₂, the cell enters mitosis. Cells that are no longer dividing (e.g. terminally differentiated nerve cells) exit the cycle into a quiescent state called G₀.

The Stages of Mitosis

Mitosis is continuous, but for exam purposes it is divided into four stages: prophase, metaphase, anaphase, and telophase (PMAT).

Prophase

The chromosomes condense and become visible under a light microscope - they shorten and thicken by supercoiling. Each chromosome consists of two identical sister chromatids joined at the centromere (this was produced during S phase replication). The nuclear envelope breaks down and disperses. The spindle begins to form from protein fibres (made of tubulin) that extend from the centrioles (in animal cells) or from microtubule organising centres at the poles.

Chromosomes condense so that they can be moved without tangling - long, uncondensed chromosomes would be impossible to segregate without knotting.

Metaphase

The spindle is fully formed and chromosomes align along the equator (metaphase plate) of the cell. Spindle fibres from opposite poles attach to the kinetochores - protein structures at the centromere of each chromatid. One kinetochore faces each pole. This attachment is highly regulated: the cell does not proceed past metaphase until every chromosome has spindle fibres correctly attached at both poles (this is the spindle assembly checkpoint).

Anaphase

The centromere splits. Spindle fibres shorten by depolymerisation, pulling the sister chromatids apart to opposite poles. Each chromatid is now considered a chromosome in its own right. This is the critical step - it ensures that each daughter cell receives one complete, identical copy of every chromosome.

The cell also elongates during anaphase, which helps move the poles further apart.

Telophase

The chromosomes arrive at the poles and begin to decondense. A new nuclear envelope forms around each set of chromosomes, and a nucleolus re-forms in each nucleus. The spindle breaks down. At the end of telophase, the cell has two genetically identical nuclei - the nuclear division is complete.

Cytokinesis

Cytokinesis (division of the cytoplasm) usually occurs alongside or just after telophase, and is technically separate from mitosis. In animal cells, a cleavage furrow forms as a ring of actin microfilaments constricts the cell, eventually pinching it into two. In plant cells, a cell plate forms at the equator from Golgi-derived vesicles containing cell wall materials; this expands outward until it fuses with the plasma membrane, creating a new cell wall between the two daughter cells.

Why Two Genetically Identical Daughter Cells?

This outcome depends on two sequential events: (1) during S phase, every chromosome is faithfully replicated to produce two identical sister chromatids; (2) during anaphase, sister chromatids are separated so one goes to each daughter cell. If either step fails, cells receive incorrect chromosome numbers or abnormal DNA - this underlies conditions like cancer, where cell cycle checkpoints are compromised.

Cell Cycle Checkpoints

The cell cycle has several checkpoints that prevent errors:

• G₁/S checkpoint - checks for DNA damage, adequate cell size, and favourable growth signals before committing to replication
• G₂/M checkpoint - checks that DNA replication is complete and accurate before entering mitosis
• Spindle assembly checkpoint (metaphase) - ensures all chromosomes are correctly attached to spindle fibres before anaphase proceeds

Mutations in genes encoding checkpoint proteins (e.g. p53, Rb) can lead to uncontrolled division - cancer. Cells with damaged DNA are normally destroyed by apoptosis (programmed cell death); if apoptosis also fails, mutant cells accumulate.

Mitosis vs Meiosis

Observing Mitosis

Mitosis can be observed in root tip squash preparations (e.g. onion root tips), because the zone of cell division (meristem) behind the root cap contains many actively dividing cells. Staining with aceto-orcein or toluidine blue makes the chromosomes visible. Under a light microscope, you can identify each stage by the appearance and arrangement of chromosomes.

Mitotic index = number of cells in mitosis / total number of cells observed. A high mitotic index indicates rapid cell division, which could suggest a tumour tissue or a meristematic zone.

Summary

• Mitosis produces 2 genetically identical diploid daughter cells
• Cell cycle=interphase (G₁→S→G₂)+mitosis (PMAT)+cytokinesis
• During S phase: DNA replication (semi-conservative)→sister chromatids formed
• Prophase: chromosomes condense, nuclear envelope breaks down, spindle forms
• Metaphase: chromosomes align at equator; kinetochores attach to spindle from both poles
• Anaphase: centromeres split; sister chromatids pulled to opposite poles by shortening spindle fibres
• Telophase: nuclear envelopes reform; chromosomes decondense
• Cytokinesis: cleavage furrow in animals; cell plate in plants
• Checkpoints at G₁/S, G₂/M, and metaphase prevent errors
• Mitotic index = cells in mitosis ÷ total cells

AQA Exam Tips

• Describe each stage precisely: AQA will show a micrograph or diagram and ask you to name the stage and justify. Prophase = chromosomes visible, nuclear envelope absent, spindle forming. Metaphase = chromosomes at equator. Anaphase = chromatids moving to poles. Telophase = nuclear envelopes reforming.
• Genetic identity: when asked why daughter cells are genetically identical, say: chromosomes are replicated in S phase to form sister chromatids; sister chromatids are separated in anaphase; each daughter cell receives one chromatid from every chromosome.
• Purpose of mitosis: growth, repair of tissues, asexual reproduction - know all three.
• Mitotic index calculation: always show the fraction and multiply by 100 if asked for a percentage. A high mitotic index in tumour tissue is commonly asked.
• Cytokinesis differences: cleavage furrow (animals) vs cell plate (plants) - this is an AQA favourite.
• Cancer link: mutations in tumour suppressor genes (p53) or proto-oncogenes disrupt checkpoints → uncontrolled mitosis → tumour formation.
• Do not confuse chromatid and chromosome: before anaphase, each chromosome consists of two chromatids. After the centromere splits in anaphase, each chromatid is referred to as a chromosome.