A-Level Biology 3.14 3.15 3.16 CP6

Mitosis

Cell division for growth and asexual reproduction

🎯 Learning Objectives

🔬

What is Mitosis?

Core Concept

Mitosis is the process of nuclear division that produces two genetically identical daughter cells. It's essential for growth, repair, and asexual reproduction in organisms.

Before mitosis begins, the cell spends most of its life in interphase. During interphase:

  • DNA replicates to form identical copies
  • New proteins and organelles are synthesised
  • ATP production increases to provide energy for division
  • The cell grows to prepare for producing two new cells

⚡ Key Point: Interphase is NOT a resting phase – the cell is extremely active, preparing for division.

A cell in interphase – chromatin threads visible, nucleolus present, centrioles paired

🧠 Remember the Stages
Please Make Another Tea
Prophase → Metaphase → Anaphase → Telophase
✓

Quick Check

Retrieval Practice

What happens to DNA during interphase?

A
DNA condenses into visible chromosomes
B
DNA replicates to form identical copies
C
DNA is broken down and recycled
D
DNA moves to opposite poles of the cell
🔄

The Stages of Mitosis

PMAT

Mitosis is a continuous process, but scientists divide it into four distinct stages for easier study. The German cytologist Walther Flemming (1843-1905) first described this process as the "dance of the chromosomes".

P
Prophase
  • Chromosomes coil up and become visible
  • Each chromosome consists of two sister chromatids joined at the centromere
  • Nucleolus breaks down
  • Centrioles move to opposite poles
  • Spindle fibres begin to form
M
Metaphase
  • Nuclear membrane breaks down completely
  • Centrioles at opposite poles
  • Spindle fibres fully formed
  • Chromosomes line up at the metaphase plate (equator)
  • Each centromere attaches to a spindle fibre
A
Anaphase
  • Centromeres split
  • Sister chromatids separate – they are now individual chromosomes
  • Spindle fibres pull chromosomes to opposite poles
  • This happens quickly (minutes)
  • Motor proteins "walk" along microtubules
T
Telophase
  • Spindle fibres break down
  • Nuclear envelopes reform around each set
  • Nucleoli and centrioles reform
  • Chromosomes uncoil and become less visible
  • Two nuclei now present
⚠️ EXAM HINT

The spindle is what moves the chromosomes. It must form before the chromosomes start to move. Questions often ask about the sequence of events – make sure you know that spindle formation happens in prophase/metaphase, not anaphase.

🔢

Put in Order

Sequencing Challenge

Arrange these events in the correct order (click to place):

1
Click an option below...
2
Click an option below...
3
Click an option below...
4
Click an option below...
Centromeres split and chromatids separate
Chromosomes condense and become visible
Nuclear envelopes reform
Chromosomes line up at the equator
🔀

Cytokinesis

Final Division

After mitosis divides the nucleus, cytokinesis divides the cytoplasm to create two separate daughter cells.

📌 Result: Two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.

🐾
Animal Cells
  • A ring of contractile fibres forms
  • Fibres tighten like a belt around the middle
  • Cell "pinches" inward (like a drawstring bag)
  • Forms a cleavage furrow
  • Eventually splits into two cells
🌱
Plant Cells
  • Cannot pinch inward (rigid cell wall)
  • Golgi vesicles collect at equator
  • Vesicles fuse to form cell plate
  • Cellulose wall builds up from inside
  • Plasmodesmata form in small gaps
⭐

Why is Mitosis Important?

Mitosis is essential for several biological processes:

🌱 Growth
Organisms grow by increasing their number of cells through mitosis
🩹 Repair
Damaged tissues are replaced by new cells produced through mitosis
🔄 Asexual Reproduction
Some organisms reproduce by mitosis, creating genetically identical clones
⚠️ Disadvantage
No genetic variation – if conditions change, all individuals may be affected equally
✓

Quick Check

Retrieval Practice

How does cytokinesis differ between animal and plant cells?

A
Animal cells form a cell plate; plant cells pinch inward
B
Animal cells pinch inward; plant cells form a cell plate
C
Both cell types pinch inward from the outside
D
Neither cell type undergoes cytokinesis
📊

Mitotic Index

Calculation

The mitotic index tells us how actively cells are dividing in a tissue sample. It's an important measurement in both research and medicine.

Mitotic Index = Number of cells in mitosis⁄Total number of cells

Cells in mitosis can be identified by visible chromosomes. In interphase, the genetic material appears as diffuse chromatin threads.

📝

Worked Example

Question: A microscope image of root tip cells shows 25 cells in total. 12 of these cells have visible chromosomes. Calculate the mitotic index.

1
Identify the values

Cells in mitosis = 12 (those with visible chromosomes)
Total cells = 25

2
Apply the formula
Mitotic Index = 12 ÷ 25
Mitotic Index = 0.48
3
Interpret the result

48% of cells are undergoing mitosis – this indicates highly active cell division, typical of growth regions like root tips.

🏥

Clinical Application

The mitotic index is used to identify cancerous tissue. Cancer cells divide uncontrollably, so tumours have a much higher mitotic index than normal tissue.

Doctors can also use the mitotic index to monitor whether cancer treatment is working – a falling mitotic index indicates the treatment is reducing cell division.

⚠️ EXAM HINT

Mathematical skills are an important part of science! Simple calculations like the mitotic index will often appear in examination papers. Always show your working.

🧮

Practice Calculation

Apply the Formula

A tissue sample contains 80 cells. 20 cells show visible chromosomes. What is the mitotic index?

A
0.20
B
0.25
C
0.40
D
4.00
📝

Exam Practice

Test Yourself

Practice with these exam-style questions. Use the support features if you need help structuring your answer.

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Multiple Choice

1 mark each

1. At which stage of mitosis do the centromeres split?

A
Prophase
B
Metaphase
C
Anaphase
D
Telophase

2. Which structure is responsible for moving chromosomes during mitosis?

A
Nuclear membrane
B
Spindle fibres
C
Cell membrane
D
Nucleolus
📝 Explain Question
4 marks

Explain why mitosis is important for growth in multicellular organisms.

Structure your answer:
  • Define what mitosis produces
  • Explain how this relates to growth
  • Mention genetic consistency
  • Give a specific example
genetically identical daughter cells increase cell number same chromosomes
⚠️ Common mistakes
  • Confusing mitosis with meiosis
  • Forgetting to mention "genetically identical"
  • Not explaining the link to growth
📝 Describe Question
6 marks

Describe the events that occur during prophase and metaphase of mitosis.

Structure your answer:
  • Start with prophase events (3 marks worth)
  • Then metaphase events (3 marks worth)
  • Use specific terminology
  • Describe in correct sequence
chromosomes condense sister chromatids centromere nucleolus disappears spindle fibres centrioles metaphase plate equator
⚠️ Common mistakes
  • Mixing up the sequence of events
  • Using "chromosome" when you mean "chromatid"
  • Forgetting the spindle must form before chromosomes line up
  • Not mentioning that the nuclear membrane breaks down
🎯

Lesson Summary

Review

Well done for completing this lesson on mitosis! Let's review the key points.

🔬 Key Concepts

  • Mitosis produces two genetically identical daughter cells
  • Interphase is the preparation phase (DNA replication)
  • Four stages: PMAT (Prophase, Metaphase, Anaphase, Telophase)
  • Cytokinesis divides the cytoplasm

📊 Calculations

  • Mitotic Index = cells in mitosis ÷ total cells
  • Higher index = more active division
  • Used to identify cancerous tissue
  • Look for visible chromosomes to identify dividing cells

🔑 Key Terms

  • Chromatid: One copy of a replicated chromosome
  • Centromere: Region joining sister chromatids
  • Spindle: Fibres that move chromosomes
  • Metaphase plate: Cell equator where chromosomes align

⭐ Importance

  • Growth – increasing cell number
  • Repair – replacing damaged cells
  • Asexual reproduction – creating clones
  • Maintains chromosome number
Interphase
The phase between cell divisions when DNA replicates and the cell prepares for mitosis
Chromatid
One of the two identical copies of DNA making up a replicated chromosome, joined at the centromere
Centromere
The region of a chromosome where sister chromatids are joined and where spindle fibres attach
Spindle fibres
Protein structures (microtubules) that move chromosomes during cell division
Centrioles
Organelles that organise the spindle fibres in animal cells (found at poles)
Cytokinesis
Division of the cytoplasm following mitosis to form two separate cells
Clones
Genetically identical organisms produced by asexual reproduction
Mitotic Index
The ratio of cells in mitosis to total cells; indicates how actively a tissue is dividing
🧠 Final Review
Please Make Another Tea
You now know the dance of the chromosomes! 🎉
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