Class 9 Biology | Updated for NCERT 2026-27 | Reading Time: 7 minutes
Did you know that right now, while you're reading this, millions of cells in your body are dividing to create new cells?
Your skin cells are constantly replacing old ones, your hair is growing, and cuts on your skin are healing - all thanks
to cell division. Let's explore how this amazing process keeps you alive and healthy!
What is Cell Division and Why is it Needed?
Cell division is the process by which a parent cell divides into two or more daughter cells.
Simple Definition
Cell Division: The process where one cell splits into two new cells.
Why Do Cells Need to Divide?
Cell division is essential for life. Here's why:
1. Growth
You started as a single fertilised egg (zygote)
Through millions of cell divisions, you grew into a baby, then a child, and now a teenager
A baby has about 26 billion cells; an adult has 37 trillion cells!
All this growth happens because of cell division
2. Repair and Healing
When you get a cut or wound, cells around it divide to heal the injury
Broken bones heal when bone cells divide and form new tissue
Damaged liver tissue can regenerate through cell division
3. Replacement of Old or Dead Cells
Your body constantly replaces worn-out cells
Every day, you lose about 1% of your total cells (that's hundreds of billions of cells!)
New cells produced by division replace the dead ones
Examples of Cell Replacement
Skin cells: Completely replaced every 2-4 weeks
Red blood cells: Replaced every 120 days
Stomach lining cells: Replaced every 2-3 days
Hair follicle cells: Constantly dividing to make hair grow
Without cell division, you couldn't grow, heal from injuries, or even stay alive!
What is Mitosis?
Mitosis is the most common type of cell division that occurs in the body.
Definition
Mitosis: A type of cell division where one parent cell divides to produce two daughter cells that
are genetically identical to the parent cell.
Key Features of Mitosis:
One becomes two: One parent cell → Two daughter cells
Genetically identical: Daughter cells have the exact same DNA as the parent
Same chromosome number: The daughter cells have the exact same number of chromosomes as the parent. (For example, in humans, if a parent cell has 46 chromosomes, each daughter cell will also have 46).
Occurs in body cells: Happens in all somatic (body) cells, not reproductive cells
[Figure: Mitosis Process]
Insert diagram showing one parent cell dividing into two identical daughter cells through mitosis
Mitosis produces two genetically identical daughter cells from one parent cell
Where Does Mitosis Occur?
Mitosis occurs in all somatic cells (body cells) of multicellular organisms:
Skin cells: Constantly dividing to replace dead surface cells
Bone marrow cells: Producing new blood cells
Root tip cells in plants: Helping roots grow longer
Shoot tip cells in plants: Helping stems and leaves grow
Intestinal lining cells: Replacing worn-out cells
Note: Mitosis does NOT occur in reproductive cells (sperm and egg). Those are formed by a different
type of division called meiosis. (Unlike mitosis, meiosis produces four daughter cells with only half the number of chromosomes!)
Importance of Mitosis
Mitosis is crucial for life. Here's why:
1. Normal Growth
Enables organisms to grow from a single cell to millions/trillions of cells
Maintains body size by adding new cells
Plants grow taller and develop new branches through mitosis
Produces offspring genetically identical to the parent
Think About This
If you cut a planarian (flatworm) into pieces, each piece can regenerate into a complete new organism through
mitosis! Each new planarian is a clone of the original.
The Cell Cycle — A Controlled Process
Cell division doesn't happen randomly. It follows a controlled and orderly sequence called the
cell cycle.
What is the Cell Cycle?
The cell cycle is the series of events that take place in a cell leading to its division and duplication. It ensures
cells divide at the right time and in the right way.
Why Must Cell Division Be Controlled?
Prevents chaos: Uncontrolled division would create problems
Ensures DNA is copied correctly: Each daughter cell must get complete genetic information
Maintains tissue balance: Right number of cells in each tissue
The cell cycle has checkpoints that monitor whether the cell is ready to divide. If something is wrong
(damaged DNA, insufficient nutrients), division is stopped.
What Happens if Mitosis Goes Wrong?
When the normal control mechanisms of cell division fail, serious problems can occur:
1. Uncontrolled Cell Division → Tumours
If cells divide without control, they form a mass called a tumour
Benign tumours: Non-cancerous, don't spread to other parts
Malignant tumours (cancer): Cancerous, can invade other tissues and spread
Cancer occurs when cell cycle checkpoints fail and cells divide uncontrollably
2. Abnormal Chromosome Numbers
If chromosomes don't separate properly during mitosis, daughter cells may have wrong chromosome numbers
Cells with abnormal chromosome numbers usually die or function poorly
Can lead to genetic disorders in body tissues
Important Note
This is why cancer treatments often target rapidly dividing cells - to stop the uncontrolled mitosis. However,
these treatments also affect normal rapidly dividing cells (hair follicles, intestinal lining), which is why cancer
patients experience side effects like hair loss.
Observing Mitosis — Activity 2.5 (Onion Root Tip)
NCERT Activity 2.5 allows you to observe cells undergoing mitosis under a microscope using onion root tips.
Why Onion Root Tips?
Root tips are regions of active growth
Cells here divide rapidly by mitosis
Easy to obtain and prepare
Cells are large and clearly visible under microscope
What You Observe:
Many cells in different stages of mitosis
Some cells with visible chromosomes (dividing cells)
Some cells in resting stage (not dividing)
Clear evidence that cells are constantly dividing in growing regions
[Figure: Onion root tip cells showing mitosis]
Insert microscope image showing onion root tip cells in various stages of mitosis
Onion root tip cells observed under microscope showing different stages of mitosis
🔬 Did You Know? (The 4 Stages)
While looking under the microscope, scientists divide the continuous process of mitosis into four main stages for easier study: Prophase, Metaphase, Anaphase, and Telophase. (An easy way to remember the order is the acronym PMAT!).
This activity proves that mitosis actually occurs and that root growth happens through continuous cell division.
Key Terms to Remember
Cell Division
Process by which a parent cell divides into two or more daughter cells.
Mitosis
Cell division producing two genetically identical daughter cells with the same chromosome number as parent.
Daughter Cells
New cells formed as a result of cell division.
Somatic Cells
Body cells (non-reproductive cells) where mitosis occurs; e.g., skin cells, liver cells.
Chromosomes
Thread-like structures in the nucleus containing DNA; carry genetic information.
Cell Cycle
Controlled and orderly sequence of events from one cell division to the next.
Tumour
Mass of cells formed by uncontrolled cell division.
Asexual Reproduction
Reproduction involving only one parent, producing genetically identical offspring through mitosis.
5 Important Questions with Answers
Q1. What is mitosis? (Short Answer)
Mitosis is a type of cell division in which one parent cell divides to produce two daughter
cells that are genetically identical to the parent cell and to each other. Both daughter cells have the same
number of chromosomes as the parent cell.
Q2. Why is cell division necessary? Explain with three reasons. (Long Answer)
Cell division is essential for life. Here are three main reasons:
1. Growth:
All multicellular organisms grow from a single cell (zygote) through repeated cell divisions
A baby grows into an adult because cells keep dividing and increasing in number
Plants grow taller and develop new branches through cell division in root tips and shoot tips
2. Repair and Healing:
When you get injured (cut, wound, broken bone), cells around the damaged area divide rapidly
New cells replace damaged or dead cells
This heals the wound and restores normal tissue function
Example: Skin cells divide to close a cut; bone cells divide to heal a fracture
3. Replacement of Old or Worn-Out Cells:
Cells have a limited lifespan and eventually die
Every day, your body loses billions of cells (about 1% of total)
Cell division produces new cells to replace the dead ones
Examples: Skin cells replaced every 2-4 weeks, red blood cells replaced every 120 days
Q3. Where does mitosis occur in the body? (Short Answer)
Mitosis occurs in all somatic cells (body cells) of multicellular organisms.
Examples:
Skin cells
Bone marrow cells (producing blood cells)
Root tip and shoot tip cells in plants
Liver cells, intestinal cells, and other body tissues
Note: Mitosis does NOT occur in reproductive cells (sperm and egg cells).
Q4. What happens when cell division is not controlled? Explain. (Long Answer)
Cell division is normally a controlled process regulated by the cell cycle. When this control is lost,
serious problems can occur:
1. Formation of Tumours:
If cells divide without control, they keep multiplying and form a mass called a tumour
Benign tumours: These are non-cancerous. They grow slowly and don't spread to other parts
of the body. They can usually be removed surgically.
Malignant tumours (Cancer): These are cancerous. They grow rapidly, invade nearby tissues,
and can spread to distant parts of the body through blood or lymph (metastasis).
Cancer occurs when cell cycle checkpoints fail and cells ignore signals to stop dividing
2. Abnormal Chromosome Numbers:
If chromosomes don't separate properly during mitosis, daughter cells may receive wrong number of chromosomes
These cells usually cannot function normally
May lead to cell death or tissue malfunction
3. Tissue Damage:
Uncontrolled division disrupts normal tissue structure
Affects organ function
Can be life-threatening if vital organs are affected
Example: Skin cancer begins when skin cells start dividing uncontrollably due to UV radiation
damaging DNA and breaking cell cycle control mechanisms.
Q5. How is mitosis important for plants? Give examples. (Long Answer)
Mitosis is extremely important for plants. Here's how:
1. Growth and Development:
Plants grow throughout their life through mitosis
Root tips: Active mitosis makes roots grow longer, helping plants absorb more water and minerals
Shoot tips: Mitosis in shoot tips makes stems grow taller and produce new leaves and branches
This is why you can observe mitosis easily in onion root tips (Activity 2.5)
2. Repair and Regeneration:
When a plant is damaged (broken branch, wounded stem), cells divide by mitosis to repair the damage
Some plants can regenerate entire new plants from small pieces through mitosis
Example: If you cut a rose stem and plant it, new roots develop through mitosis
3. Asexual Reproduction:
Many plants reproduce asexually using mitosis
Vegetative propagation: New plants grow from stems, roots, or leaves through mitosis
Examples:
Potato tubers produce new plants
Bryophyllum leaves develop plantlets on their margins
Strawberry plants produce runners that develop into new plants
All offspring are genetically identical to the parent (clones)
4. Fruit and Flower Development:
Mitosis helps in the development of flowers and fruits, increasing in size through cell division.
