Mitosis & the Cell Cycle
A complete, exam-ready walkthrough of the cell cycle and every stage of mitosis — interphase, prophase, metaphase, anaphase, telophase and cytokinesis — with clear, labelled diagrams. Written for A-level Biology across all major exam boards.
OCR A
OCR B
Edexcel
WJEC / Eduqas
What is mitosis?
Mitosis is a form of nuclear division that produces two genetically identical daughter nuclei, each with the same number of chromosomes as the parent cell. It is used for growth, tissue repair and replacement, and asexual reproduction.
Mitosis is only a small part of the wider cell cycle. Most of a cell’s life is spent in interphase, where the cell grows and copies its DNA before division begins. Mitosis itself is divided into four continuous stages — prophase, metaphase, anaphase and telophase — usually followed by cytokinesis, the division of the cytoplasm.
Mitosis produces genetically identical cells; meiosis produces genetically different cells with half the chromosome number. Don’t confuse the two — examiners frequently test the distinction.
The cell cycle
The cell cycle is the sequence of events between one cell division and the next. It has two main parts: a long interphase (subdivided into G1, S and G2) and a short mitotic (M) phase in which the cell actually divides.
Interphase
- G1 (first growth phase): the cell grows, makes proteins and organelles, and carries out its normal functions.
- S (synthesis phase): DNA is replicated, so each chromosome now consists of two identical sister chromatids joined at the centromere.
- G2 (second growth phase): the cell continues to grow and prepares for division; organelles such as mitochondria replicate.
Mitotic phase
- Mitosis: the nucleus divides (prophase → metaphase → anaphase → telophase).
- Cytokinesis: the cytoplasm divides to give two separate cells.
Interphase is not a “resting phase” — it is the most metabolically active part of the cycle, and the point at which DNA is copied.
The stages of mitosis
Mitosis is a continuous process, but it is described in four stages. Click each stage to expand the labelled diagram and the key events you need to recall.
1Interphase (before mitosis begins)DNA replicated, chromosomes not yet visible
Key events
- DNA has been replicated in the S phase — each chromosome is now two sister chromatids.
- Chromosomes are uncoiled and not visible; they exist as diffuse chromatin.
- The nuclear envelope and nucleolus are intact.
- Centrioles (in animal cells) have replicated.
2ProphaseChromosomes condense & become visible
Key events
- Chromosomes condense (coil and shorten) and become visible, each as two sister chromatids joined at a centromere.
- The centrioles move to opposite poles of the cell.
- Spindle fibres (the spindle apparatus) begin to form.
- By the end of prophase the nuclear envelope breaks down and the nucleolus disappears.
3MetaphaseChromosomes line up on the equator
Key events
- Chromosomes line up single-file along the metaphase plate (the equator of the cell).
- Spindle fibres from both poles attach to the centromere of each chromosome.
- The chromosomes are held under tension, ready to be separated.
“Metaphase = Middle.” Chromosomes are in the middle of the cell.
4AnaphaseSister chromatids pulled apart
Key events
- The centromeres divide and the sister chromatids separate.
- Spindle fibres shorten, pulling the chromatids to opposite poles centromere-first (giving a characteristic V-shape).
- This is an active, energy-requiring process (ATP from mitochondria).
“Anaphase = Apart.” The chromatids move apart. This is the stage examiners most often ask you to identify.
5TelophaseTwo new nuclei re-form
Key events
- The chromatids (now called chromosomes again) reach the poles and begin to uncoil.
- A new nuclear envelope forms around each group of chromosomes, and the nucleolus reappears.
- The spindle fibres break down. There are now two nuclei.
6CytokinesisThe cytoplasm divides
Key events
- The cytoplasm divides to produce two separate cells.
- In animal cells, the membrane pinches inwards forming a cleavage furrow.
- In plant cells, a cell plate forms across the middle and a new cell wall is laid down (there is no cleavage furrow).
Strictly, cytokinesis is not part of mitosis — mitosis is nuclear division only. Cytokinesis is the division of the whole cell.
DNA content & chromosome number
A very common exam question asks you to interpret a graph of DNA content during the cell cycle, or to state the number of chromosomes at each stage. The key idea: DNA content doubles during the S phase, then halves when the cell divides — but the chromosome number of each daughter cell is the same as the parent.
- A human body cell starts with 46 chromosomes and 1 unit of DNA.
- During the S phase, DNA is replicated: DNA content rises to 2 units. Each of the 46 chromosomes is now two sister chromatids — but it is still counted as 46 chromosomes, not 92.
- During anaphase, sister chromatids separate. Each daughter cell receives 46 chromosomes and returns to 1 unit of DNA.
Chromosome number counts centromeres. Two sister chromatids share one centromere, so a replicated chromosome still counts as one chromosome.
Mitosis calculations
Three calculations come up again and again in exams: the mitotic index, the time a cell spends in each stage, and the number of cells produced by repeated division. Click each to expand a worked example.
1The mitotic index
What it is
The mitotic index is the proportion of cells in a sample that are undergoing mitosis, expressed as a percentage (or as a decimal). It gives an indication of how much cell division — and therefore how much growth — is occurring in a tissue.
Worked example
A student examines a stained root-tip squash and counts 200 cells, of which 32 are visibly in a stage of mitosis.
The mitotic index tells you how much mitosis is occurring, but it does not take into account the size of the cells. Growth in an organism happens in two ways: by producing more cells (mitosis) and by cells getting bigger (cell elongation / expansion).
A tissue can grow considerably by cell elongation while its mitotic index stays low — for example, the zone just above a root tip, where cells stretch rather than divide. So a low mitotic index does not necessarily mean little growth, and the mitotic index alone is not a complete measure of growth.
Start: 64 cells, all equal size.
144 cells, all equal size — growth by mitosis only. More cells, so the mitotic index is high.
105 cells, some elongated (dark) — growth by mitosis and cell elongation. Fewer cells, so the mitotic index is lower even though real growth has occurred.
2Time spent in each stage of the cell cycle
The number of cells seen in a stage is directly proportional to the length of time a cell spends in that stage. So if you know the percentage of cells in a stage and the total length of the cell cycle, you can find the time spent in that stage.
Worked example
The cell cycle lasts 24 hours. In a sample the cells are distributed as follows:
| Stage | % of cells (normal) | % of cells (cancer) |
|---|---|---|
| Interphase | 82 | 45 |
| Prophase | 4 | 16 |
| Metaphase | 5 | 18 |
| Anaphase | 5 | 12 |
| Telophase | 4 | 9 |
Time in interphase for the normal cell:
A cell spending less time in interphase and more time in mitosis is dividing rapidly and uncontrollably — a feature of cancer cells. The same tissue also has a raised mitotic index.
3Number of cells produced by repeated division
Organisms such as bacteria divide by binary fission (and body cells by mitosis) so that the population doubles each division. If you know the starting number and the number of divisions, the final number is:
N = N₀ × 2ⁿ
N = final number of cells
N₀ = starting number of cells
n = number of divisions (generations)
Worked example
At time zero there are 2 bacteria. They divide once every 20 minutes. How many are there after 3 hours?
- Number of divisions: 3 hours = 180 minutes; 180 ÷ 20 = 9 divisions.
- Apply the formula:
N = 2 × 2⁹ = 2 × 512 = 1024
To find n from a population increase, rearrange with logs: n = log(N ÷ N₀) ÷ log 2. This links to the exponential-growth maths on the exam-board maths pages.
Test yourself
Choose a puzzle below and click an answer to check it. Use the sections above if you get stuck.
Frequently asked questions
What is the difference between mitosis and the cell cycle?
The cell cycle is the whole sequence of events between one division and the next — including the long interphase (G1, S, G2) where the cell grows and copies its DNA. Mitosis is just the short part of the cycle in which the nucleus divides.
What are the four stages of mitosis in order?
Prophase, Metaphase, Anaphase, Telophase — often remembered as PMAT. Cytokinesis (division of the cytoplasm) usually follows telophase but is not technically part of mitosis.
Why does mitosis produce genetically identical cells?
Because DNA is replicated exactly during the S phase, and the identical sister chromatids are separated so that each daughter cell receives one complete, identical copy of every chromosome. There is no crossing over or independent assortment as there is in meiosis.
How is cytokinesis different in plant and animal cells?
In animal cells the cell-surface membrane pinches inwards to form a cleavage furrow. In plant cells the rigid cell wall means this cannot happen — instead vesicles form a cell plate across the middle of the cell, which develops into a new cell wall and membrane.
What is the role of the spindle fibres?
Spindle fibres attach to the centromeres of the chromosomes at metaphase and, by shortening in anaphase, pull the separated sister chromatids to opposite poles of the cell, ensuring each daughter cell gets a complete set.
Mitosis & the Cell Cycle
A complete, exam-ready walkthrough of the cell cycle and every stage of mitosis — interphase, prophase, metaphase, anaphase, telophase and cytokinesis — with clear, labelled diagrams. Written for A-level Biology across all major exam boards.
What is mitosis?
Mitosis is a form of nuclear division that produces two genetically identical daughter nuclei, each with the same number of chromosomes as the parent cell. It is used for growth, tissue repair and replacement, and asexual reproduction.
Mitosis is only a small part of the wider cell cycle. Most of a cell’s life is spent in interphase, where the cell grows and copies its DNA before division begins. Mitosis itself is divided into four continuous stages — prophase, metaphase, anaphase and telophase — usually followed by cytokinesis, the division of the cytoplasm.
Mitosis produces genetically identical cells; meiosis produces genetically different cells with half the chromosome number. Don’t confuse the two — examiners frequently test the distinction.
The cell cycle
The cell cycle is the sequence of events between one cell division and the next. It has two main parts: a long interphase (subdivided into G1, S and G2) and a short mitotic (M) phase in which the cell actually divides.
Interphase
- G1 (first growth phase): the cell grows, makes proteins and organelles, and carries out its normal functions.
- S (synthesis phase): DNA is replicated, so each chromosome now consists of two identical sister chromatids joined at the centromere.
- G2 (second growth phase): the cell continues to grow and prepares for division; organelles such as mitochondria replicate.
Mitotic phase
- Mitosis: the nucleus divides (prophase → metaphase → anaphase → telophase).
- Cytokinesis: the cytoplasm divides to give two separate cells.
Interphase is not a “resting phase” — it is the most metabolically active part of the cycle, and the point at which DNA is copied.
The stages of mitosis
Mitosis is a continuous process, but it is described in four stages. Click each stage to expand the labelled diagram and the key events you need to recall.
1Interphase (before mitosis begins)DNA replicated, chromosomes not yet visible
Key events
- DNA has been replicated in the S phase — each chromosome is now two sister chromatids.
- Chromosomes are uncoiled and not visible; they exist as diffuse chromatin.
- The nuclear envelope and nucleolus are intact.
- Centrioles (in animal cells) have replicated.
2ProphaseChromosomes condense & become visible
Key events
- Chromosomes condense (coil and shorten) and become visible, each as two sister chromatids joined at a centromere.
- The centrioles move to opposite poles of the cell.
- Spindle fibres (the spindle apparatus) begin to form.
- By the end of prophase the nuclear envelope breaks down and the nucleolus disappears.
3MetaphaseChromosomes line up on the equator
Key events
- Chromosomes line up single-file along the metaphase plate (the equator of the cell).
- Spindle fibres from both poles attach to the centromere of each chromosome.
- The chromosomes are held under tension, ready to be separated.
“Metaphase = Middle.” Chromosomes are in the middle of the cell.
4AnaphaseSister chromatids pulled apart
Key events
- The centromeres divide and the sister chromatids separate.
- Spindle fibres shorten, pulling the chromatids to opposite poles centromere-first (giving a characteristic V-shape).
- This is an active, energy-requiring process (ATP from mitochondria).
“Anaphase = Apart.” The chromatids move apart. This is the stage examiners most often ask you to identify.
5TelophaseTwo new nuclei re-form
Key events
- The chromatids (now called chromosomes again) reach the poles and begin to uncoil.
- A new nuclear envelope forms around each group of chromosomes, and the nucleolus reappears.
- The spindle fibres break down. There are now two nuclei.
6CytokinesisThe cytoplasm divides
Key events
- The cytoplasm divides to produce two separate cells.
- In animal cells, the membrane pinches inwards forming a cleavage furrow.
- In plant cells, a cell plate forms across the middle and a new cell wall is laid down (there is no cleavage furrow).
Strictly, cytokinesis is not part of mitosis — mitosis is nuclear division only. Cytokinesis is the division of the whole cell.
DNA content & chromosome number
A very common exam question asks you to interpret a graph of DNA content during the cell cycle, or to state the number of chromosomes at each stage. The key idea: DNA content doubles during the S phase, then halves when the cell divides — but the chromosome number of each daughter cell is the same as the parent.
- A human body cell starts with 46 chromosomes and 1 unit of DNA.
- During the S phase, DNA is replicated: DNA content rises to 2 units. Each of the 46 chromosomes is now two sister chromatids — but it is still counted as 46 chromosomes, not 92.
- During anaphase, sister chromatids separate. Each daughter cell receives 46 chromosomes and returns to 1 unit of DNA.
Chromosome number counts centromeres. Two sister chromatids share one centromere, so a replicated chromosome still counts as one chromosome.
Mitosis calculations
Three calculations come up again and again in exams: the mitotic index, the time a cell spends in each stage, and the number of cells produced by repeated division. Click each to expand a worked example.
1The mitotic index
What it is
The mitotic index is the proportion of cells in a sample that are undergoing mitosis, expressed as a percentage (or as a decimal). It gives an indication of how much cell division — and therefore how much growth — is occurring in a tissue.
Worked example
A student examines a stained root-tip squash and counts 200 cells, of which 32 are visibly in a stage of mitosis.
The mitotic index tells you how much mitosis is occurring, but it does not take into account the size of the cells. Growth in an organism happens in two ways: by producing more cells (mitosis) and by cells getting bigger (cell elongation / expansion).
A tissue can grow considerably by cell elongation while its mitotic index stays low — for example, the zone just above a root tip, where cells stretch rather than divide. So a low mitotic index does not necessarily mean little growth, and the mitotic index alone is not a complete measure of growth.
Start: 64 cells, all equal size.
144 cells, all equal size — growth by mitosis only. More cells, so the mitotic index is high.
105 cells, some elongated (dark) — growth by mitosis and cell elongation. Fewer cells, so the mitotic index is lower even though real growth has occurred.
2Time spent in each stage of the cell cycle
The number of cells seen in a stage is directly proportional to the length of time a cell spends in that stage. So if you know the percentage of cells in a stage and the total length of the cell cycle, you can find the time spent in that stage.
Worked example
The cell cycle lasts 24 hours. In a sample the cells are distributed as follows:
| Stage | % of cells (normal) | % of cells (cancer) |
|---|---|---|
| Interphase | 82 | 45 |
| Prophase | 4 | 16 |
| Metaphase | 5 | 18 |
| Anaphase | 5 | 12 |
| Telophase | 4 | 9 |
Time in interphase for the normal cell:
A cell spending less time in interphase and more time in mitosis is dividing rapidly and uncontrollably — a feature of cancer cells. The same tissue also has a raised mitotic index.
3Number of cells produced by repeated division
Organisms such as bacteria divide by binary fission (and body cells by mitosis) so that the population doubles each division. If you know the starting number and the number of divisions, the final number is:
N = N₀ × 2ⁿ N = final number of cells N₀ = starting number of cells n = number of divisions (generations)Worked example
At time zero there are 2 bacteria. They divide once every 20 minutes. How many are there after 3 hours?
- Number of divisions: 3 hours = 180 minutes; 180 ÷ 20 = 9 divisions.
- Apply the formula:
To find n from a population increase, rearrange with logs: n = log(N ÷ N₀) ÷ log 2. This links to the exponential-growth maths on the exam-board maths pages.
Test yourself
Choose a puzzle below and click an answer to check it. Use the sections above if you get stuck.
Frequently asked questions
What is the difference between mitosis and the cell cycle?
The cell cycle is the whole sequence of events between one division and the next — including the long interphase (G1, S, G2) where the cell grows and copies its DNA. Mitosis is just the short part of the cycle in which the nucleus divides.
What are the four stages of mitosis in order?
Prophase, Metaphase, Anaphase, Telophase — often remembered as PMAT. Cytokinesis (division of the cytoplasm) usually follows telophase but is not technically part of mitosis.
Why does mitosis produce genetically identical cells?
Because DNA is replicated exactly during the S phase, and the identical sister chromatids are separated so that each daughter cell receives one complete, identical copy of every chromosome. There is no crossing over or independent assortment as there is in meiosis.
How is cytokinesis different in plant and animal cells?
In animal cells the cell-surface membrane pinches inwards to form a cleavage furrow. In plant cells the rigid cell wall means this cannot happen — instead vesicles form a cell plate across the middle of the cell, which develops into a new cell wall and membrane.
What is the role of the spindle fibres?
Spindle fibres attach to the centromeres of the chromosomes at metaphase and, by shortening in anaphase, pull the separated sister chromatids to opposite poles of the cell, ensuring each daughter cell gets a complete set.