Mitosis and the Cell Cycle

This lesson covers the stages of the cell cycle, including mitosis, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the stages of the cell cycle, explain the importance of mitosis and understand why organisms need to produce new cells.

Why Do Cells Divide?

All living organisms are made of cells. Organisms need new cells for three main reasons:

Growth — multicellular organisms grow by increasing the number of cells they contain.
Repair — damaged or worn-out cells must be replaced by identical copies.
Replacement — some cells have a limited lifespan (e.g. red blood cells last about 120 days) and must be continually replaced.

Before a cell can divide, it must first make copies of its contents so that each new daughter cell receives everything it needs to function.

Exam Tip: Never say cells divide "to reproduce" when the question asks about multicellular organisms. Cell division for growth and repair is different from reproduction. Asexual reproduction also uses mitosis, but the context of the question matters.

The Cell Cycle

The cell cycle is the series of events that takes place in a cell, leading to its division and duplication. It consists of three main stages:

Stage 1 — Interphase (Cell Growth)

During interphase, the cell prepares for division:

The cell grows and increases in size.
The number of organelles (e.g. mitochondria, ribosomes) increases.
The cell's DNA replicates — each chromosome is copied to form two identical copies called sister chromatids, joined at a centromere.

Interphase is the longest stage of the cell cycle, often taking up to 90% of the total time.

Stage 2 — Mitosis (Nuclear Division)

During mitosis, the nucleus divides. The copied chromosomes are pulled apart so that each new nucleus receives exactly the same set of chromosomes as the original cell.

Stage 3 — Cytokinesis (Cell Division)

During cytokinesis, the cytoplasm and cell membrane divide to form two identical daughter cells. In plant cells, a new cell wall also forms between the two daughter cells.

graph LR
    A[Interphase<br/>Cell grows<br/>DNA replicates<br/>Organelles increase] --> B[Mitosis<br/>Nucleus divides<br/>Chromosomes separate]
    B --> C[Cytokinesis<br/>Cytoplasm divides<br/>Two daughter cells formed]
    C --> A

Exam Tip: Many students forget that interphase is part of the cell cycle. Do not call interphase a "resting phase" — the cell is extremely active during this period, growing and replicating its DNA.

The Stages of Mitosis

Although you do not need to know the names of each sub-stage (prophase, metaphase, anaphase, telophase) for the Combined Science specification, it helps to understand the overall sequence:

Stage	What Happens
Early mitosis	Chromosomes condense and become visible; the nuclear membrane breaks down
Middle mitosis	Chromosomes line up along the centre (equator) of the cell; spindle fibres attach to centromeres
Late mitosis	Spindle fibres pull the sister chromatids apart to opposite ends (poles) of the cell
End of mitosis	Two new nuclear membranes form around each set of chromosomes

The key outcome is that each daughter cell has an identical set of chromosomes to the parent cell.

Key Features of Mitosis

Mitosis produces two daughter cells from one parent cell.
Each daughter cell is genetically identical to the parent cell and to each other — they are clones.
The daughter cells have the same number of chromosomes as the parent cell (the diploid number).
In humans, body cells contain 46 chromosomes (23 pairs), and after mitosis each daughter cell also contains 46 chromosomes.

Feature	Mitosis
Number of divisions	1
Number of daughter cells	2
Genetic variation	None — daughter cells are identical
Chromosome number	Same as parent (diploid)
Used for	Growth, repair, replacement, asexual reproduction

Importance of Mitosis

Mitosis is essential because:

It ensures genetic consistency — every body cell has the same DNA.
It allows organisms to grow from a single fertilised egg into a multicellular organism.
It replaces cells that are damaged or worn out (e.g. skin cells, blood cells).
It is the basis of asexual reproduction in some organisms (e.g. strawberry plants producing runners, bacteria dividing by binary fission).

Cancer and Uncontrolled Cell Division

Sometimes the control mechanisms of the cell cycle break down. When cells divide in an uncontrolled way, a mass of abnormal cells called a tumour can form.

Benign tumours — grow slowly, do not invade other tissues, are not cancerous.
Malignant tumours — grow rapidly, invade surrounding tissues and can metastasise (spread to other parts of the body through the blood or lymphatic system).

Cancer is a disease caused by uncontrolled cell division due to changes (mutations) in genes that regulate the cell cycle.

Exam Tip: If asked about cancer, always link it to uncontrolled mitosis or uncontrolled cell division. Do not simply say "cells divide quickly" — emphasise that the normal controls have broken down.

Summary

The cell cycle consists of interphase (growth and DNA replication), mitosis (nuclear division) and cytokinesis (cytoplasm division).
Interphase is the longest stage — the cell grows, organelles increase and DNA is replicated.
Mitosis produces two genetically identical daughter cells with the same number of chromosomes as the parent cell.
Mitosis is important for growth, repair and asexual reproduction.
Cancer results from uncontrolled cell division due to mutations affecting cell cycle regulation.