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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.
All living organisms are made of cells. Organisms need new cells for three main reasons:
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 is the series of events that takes place in a cell, leading to its division and duplication. It consists of three main stages:
During interphase, the cell prepares for division:
Interphase is the longest stage of the cell cycle, often taking up to 90% of the total time.
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.
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.
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.
| 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 |
Mitosis is essential because:
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.
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.
A human body cell has 46 chromosomes (the diploid number, written 2n = 46). Follow the number through the cell cycle:
| Stage | DNA content | Chromosome count | Number of chromatids per chromosome |
|---|---|---|---|
| Start of interphase | 2C | 46 | 1 |
| End of interphase (after S phase) | 4C | 46 | 2 (sister chromatids) |
| During mitosis (before anaphase) | 4C | 46 | 2 |
| After anaphase (chromatids separated) | 2C per cell | 46 per cell | 1 |
| After cytokinesis | 2C per cell | 46 per cell | 1 |
Common mistake: Students often say the chromosome number doubles during interphase. The DNA amount doubles, but the chromosome number stays the same — each chromosome now has two identical sister chromatids joined at the centromere.
graph LR
A["Interphase<br/>46 single chromosomes"] --> B["DNA replication<br/>46 chromosomes<br/>each with 2 chromatids"]
B --> C["Mitosis<br/>Chromatids pulled apart"]
C --> D["Daughter cell 1<br/>46 single chromosomes"]
C --> E["Daughter cell 2<br/>46 single chromosomes"]
Although mitosis is the main cell division process you must know, it is useful to compare it with related processes used in asexual reproduction:
| Process | Organism | Division mechanism | Outcome |
|---|---|---|---|
| Mitosis | Animals, plants, fungi | Nucleus divides; cytoplasm splits | Two identical diploid cells |
| Binary fission | Bacteria | Circular DNA copied; cell pinches in two | Two identical bacterial cells |
| Budding | Yeast, Hydra | Small outgrowth forms; nucleus divides by mitosis; bud detaches | New smaller genetically identical organism |
All three produce clones — offspring that are genetically identical to the parent because no fusion of gametes and no recombination occurs.
If one cell divides by mitosis once every 24 hours, how many cells will exist after 5 days, assuming no cell death?
The pattern follows 2ⁿ, where n is the number of divisions. In reality, most cells divide far more slowly than this (skin cells around every 27 days; liver cells only when damage is detected) because the cell cycle is tightly regulated by checkpoint proteins.
Common mistake: Students sometimes assume all body cells divide continuously. In fact, mature nerve cells and red blood cells cannot divide at all.
Cancer is caused by mutations in genes that control the cell cycle. Two important gene types are:
Risk factors include:
Exam Tip: If a question asks you to evaluate risk factors, explain whether the risk factor is causal (directly damages DNA) or correlational (associated with higher incidence). Smoking is causal; age is correlational.
Grade 3–4 answer: "Mitosis makes two new cells. It is used for growth and to repair damaged cells. The new cells are the same as the original cell."
Grade 5–6 answer: "Mitosis is a type of cell division that produces two genetically identical daughter cells, each with the same diploid number of chromosomes. Before mitosis, the cell goes through interphase, where DNA replicates. Mitosis is essential for growth, repair and asexual reproduction."
Grade 7–9 answer: "The cell cycle consists of interphase, mitosis and cytokinesis. During interphase, each chromosome is copied to form two sister chromatids joined at the centromere. During mitosis, spindle fibres pull the sister chromatids to opposite poles, so each daughter nucleus receives exactly one copy of each chromosome. The resulting daughter cells are diploid and genetically identical — a crucial point because any mutations in the parent cell's DNA will be inherited by both daughter cells. Because mitosis does not involve meiosis, there is no independent assortment or crossing over, so no genetic variation is introduced. When tumour suppressor genes such as p53 are mutated, the cell cycle is no longer halted at checkpoints and uncontrolled mitosis causes a malignant tumour that can metastasise via the bloodstream or lymphatic system."
The jump to Grade 7–9 uses precise terms (sister chromatids, centromere, spindle fibres, diploid, p53) and links mechanism to consequence.
Although this lesson focuses on mitosis, you will also study meiosis in Topic 3. Understanding both together helps avoid confusion.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 diploid | 4 haploid |
| Genetic variation | None — clones | Yes — independent assortment and crossing over |
| Used for | Growth, repair, asexual reproduction | Gamete formation (sperm, eggs) |
| Chromosomes per daughter cell (humans) | 46 (2n) | 23 (n) |
Exam Tip: If a question mentions gametes or fertilisation, the process is meiosis, not mitosis. If it mentions growth, repair or identical copies, the process is mitosis.
When gametes produced by meiosis fuse at fertilisation, the resulting zygote has 46 chromosomes (2n) again — 23 from each parent. The zygote then divides by mitosis to grow into an embryo. Alleles for traits are passed down, and combinations of alleles give rise to different genotypes (e.g. homozygous or heterozygous) and different phenotypes (e.g. brown eyes, blue eyes). A Punnett square can be used to predict the probabilities of offspring genotypes from a cross — a point revisited in later lessons.
Common mistake: Some students think mitosis creates genetic variation. It does not — only meiosis and fertilisation do. A stem cell dividing by mitosis and then differentiating produces a specialised cell with the same DNA as the original.
Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Biology Topic 2 Cells and control / Topic 3 Genetics — specifically CB2a Mitosis and the cell cycle, including interphase, chromosome replication and cancer as uncontrolled cell division. Assessed on Biology Paper 1.