Class 9 Biology | Updated for NCERT 2026-27 | Reading Time: 7 minutes
You look like your parents but you're not an exact copy of either of them. You have your mother's eyes but your
father's nose. How does this happen? The answer lies in a special type of cell division called meiosis
that creates unique combinations of genes in every individual. Let's explore this fascinating process!
What is Meiosis?
Meiosis is a special type of cell division that produces gametes (sex cells) for sexual reproduction.
Simple Definition
Meiosis: A type of cell division that produces four daughter cells, each with half the number of
chromosomes as the parent cell. These daughter cells are called gametes (sperm or egg cells).
Key Features of Meiosis:
One becomes four: One parent cell → Four daughter cells (gametes)
Half chromosomes: Each gamete has half the chromosome number of the parent cell
Genetically different: Each gamete is genetically unique (not identical to parent or each other)
Occurs only in reproductive organs: Not in body cells
Purpose: Produces sex cells for sexual reproduction
Important to Remember
Gametes are sex cells: sperm (male) and eggs/ova (female). They combine during fertilisation to
form a new organism.
Where Does Meiosis Occur?
Meiosis occurs ONLY in reproductive organs where gametes (sex cells) are produced.
In Animals:
Males:
Testes: Meiosis produces sperm cells
Millions of sperm are produced daily through meiosis
Each sperm has half the chromosomes (23 in humans)
Females:
Ovaries: Meiosis produces egg cells (ova)
One egg is released each month (in humans)
Each egg has half the chromosomes (23 in humans)
In Plants:
Male Reproductive Part:
Anthers: Meiosis produces pollen grains
Pollen grains contain male gametes
Female Reproductive Part:
Ovary (in flower): Meiosis produces egg cells
Egg cells are present inside ovules
[Figure: Location of meiosis in animals and plants]
Insert diagram showing testes and ovaries in animals, anthers and ovaries in flowering plants
Meiosis occurs in reproductive organs: testes and ovaries in animals; anthers and ovaries in plants
Remember the Difference
Mitosis → Happens in ALL body cells (skin, liver, roots) Meiosis → Happens ONLY in reproductive organs (testes, ovaries, anthers)
How Does Meiosis Work? (Simplified for Class 9)
Meiosis is more complex than mitosis because the parent cell divides twice, not once.
The Two-Step Process:
Step 1: First Division (Meiosis I) — Reduction Division
The parent cell has the full set of chromosomes (e.g., 46 in humans)
Chromosomes pair up and then separate
The cell divides into two daughter cells
Key point: Each daughter cell now has half the chromosome number (e.g., 23 in humans)
This is called "reduction division" because chromosome number is reduced
Step 2: Second Division (Meiosis II) — Similar to Mitosis
Each of the two cells from Step 1 divides again
This division is similar to mitosis
No further reduction in chromosome number
Results in four daughter cells
Final Result:
One parent cell → Four gametes
Each gamete has half the DNA and chromosomes of the parent cell
All four gametes are genetically different from each other
Meiosis: One parent cell divides twice to produce four gametes, each with half the chromosomes
Stage
What Happens
Number of Cells
Chromosomes per Cell
Start
Parent cell
1
46 (full set)
After Meiosis I
First division complete
2
23 each (half)
After Meiosis II
Second division complete
4
23 each (half)
Why Do Gametes Have Half the Chromosomes?
This is a brilliant design of nature! Here's why:
The Chromosome Restoration System:
Parent cells have full chromosome set:
In humans: 46 chromosomes in body cells
Gametes have half:
Sperm: 23 chromosomes
Egg: 23 chromosomes
During fertilisation:
Sperm (23) + Egg (23) = Zygote (46)
Original chromosome number is restored!
What if gametes had full chromosomes?
If sperm had 46 chromosomes and egg had 46 chromosomes, the baby would have 92 chromosomes! In the next generation,
it would be 184, then 368... This would be disastrous. Meiosis prevents this by halving the chromosome number,
which is then restored during fertilisation.
Simple Math
Parent cells: 46 chromosomes
Gametes (after meiosis): 23 chromosomes
Fertilisation: 23 (sperm) + 23 (egg) = 46 (baby)
Result: Baby has same chromosome number as parents!
Importance of Meiosis
Meiosis is crucial for sexual reproduction and genetic diversity. Here's why:
1. Gamete Formation
Meiosis is the ONLY way to produce gametes (sperm and egg cells)
Without meiosis, sexual reproduction would be impossible
Ensures each gamete has the correct chromosome number (half)
2. Maintains Chromosome Number Across Generations
Prevents doubling of chromosome number in each generation
Keeps chromosome number constant in a species
Humans always have 46 chromosomes because of meiosis + fertilisation
3. Genetic Variation and Diversity
Each gamete produced is genetically unique
During meiosis, chromosomes shuffle and exchange segments
This creates new combinations of genes
Result: Every child is unique!
Why Children Resemble Parents but Are Not Identical:
You inherit half your genes from mother (through egg)
You inherit half your genes from father (through sperm)
Meiosis ensures each gamete has a unique mix of genes
So you have traits from both parents in a unique combination
This is why siblings look similar but not identical (unless identical twins)
4. Evolution and Survival
Genetic diversity created by meiosis helps species adapt to changing environments
Different gene combinations mean some individuals may survive better in new conditions
This drives evolution over time
Think About This
Meiosis creates so many unique combinations that (except for identical twins) no two humans have ever been or will
ever be genetically identical! This is the beauty of genetic diversity.
What Happens if Meiosis Goes Wrong?
Sometimes errors occur during meiosis, leading to serious consequences:
1. Abnormal Chromosome Numbers in Gametes
If chromosomes don't separate properly, a gamete may get extra or missing chromosomes
Example: Gamete with 24 chromosomes instead of 23 (in humans)
2. Genetic Disorders
When abnormal gametes combine during fertilisation, the offspring may have genetic disorders:
Down Syndrome: Extra chromosome 21 (47 total instead of 46)
Turner Syndrome: Missing one X chromosome in females
Klinefelter Syndrome: Extra X chromosome in males
3. Developmental Problems
Embryo may not develop properly
May lead to miscarriage in early pregnancy
Physical or mental developmental issues
4. Reduced Fertility
If meiosis produces mostly abnormal gametes, fertility is reduced
Difficulty in conceiving or maintaining pregnancy
Important Note
Most errors in meiosis are caught by the body's quality control mechanisms. Abnormal embryos often don't survive
past early development. This is nature's way of ensuring healthy offspring.
Cell division producing four gametes with half the chromosome number of parent cell.
Gametes
Sex cells (sperm in males, eggs in females) produced by meiosis for sexual reproduction.
Fertilisation
Fusion of male and female gametes to form a zygote; restores full chromosome number.
Chromosomes
Thread-like structures containing DNA that carry genetic information.
Genetic Diversity
Variation in genes among individuals; created by meiosis through gene shuffling.
Zygote
Fertilised egg formed when sperm and egg combine; has full chromosome number.
Reduction Division
First division in meiosis where chromosome number is reduced to half.
Sexual Reproduction
Reproduction involving fusion of male and female gametes; produces genetically unique offspring.
5 Important Questions with Answers
Q1. Define meiosis. (Short Answer)
Meiosis is a type of cell division that occurs in reproductive organs to produce gametes (sex cells).
In this process, one parent cell divides twice to produce four daughter cells, each with half the number of
chromosomes as the parent cell.
Q2. Where does meiosis occur in animals and plants? (Short Answer)
In Animals:
Males: Testes (produces sperm cells)
Females: Ovaries (produces egg cells)
In Plants:
Male part: Anthers (produces pollen grains)
Female part: Ovary of flower (produces egg cells)
Q3. Why do gametes have half the number of chromosomes? Explain with an example. (Long Answer)
Gametes have half the number of chromosomes to maintain a constant chromosome number in a species across generations.
Why this is necessary:
Body cells have a full set of chromosomes (in humans: 46)
During sexual reproduction, two gametes (sperm and egg) fuse together in fertilisation
If both gametes had 46 chromosomes, the offspring would have 92 chromosomes
In the next generation, it would become 184, then 368... leading to chaos
To prevent this, meiosis reduces chromosome number to half in gametes
Example in humans:
Body cells: 46 chromosomes
Sperm (after meiosis): 23 chromosomes
Egg (after meiosis): 23 chromosomes
Fertilisation: 23 + 23 = 46 chromosomes in baby
Result: Baby has the same chromosome number as parents
Conclusion: The halving of chromosomes in gametes ensures that when they combine during fertilisation,
the original chromosome number is restored. This keeps the chromosome number constant generation after generation.
Q4. What is the importance of meiosis? (Long Answer)
Meiosis is extremely important for sexual reproduction. Here's why:
1. Formation of Gametes:
Meiosis is the only process that produces gametes (sperm and egg cells)
Without meiosis, sexual reproduction would be impossible
Ensures each gamete has exactly half the chromosomes needed
2. Maintains Chromosome Number:
By reducing chromosome number to half in gametes, meiosis prevents doubling with each generation
During fertilisation, the full chromosome number is restored
This keeps chromosome number constant in a species (humans always have 46)
3. Creates Genetic Variation:
Each gamete produced by meiosis is genetically unique
Chromosomes shuffle and exchange genetic material during meiosis
This creates new combinations of genes in every gamete
Result: Every child is unique (except identical twins)
You inherit half genes from mother and half from father in a unique combination
4. Helps Species Survive and Evolve:
Genetic diversity created by meiosis helps populations adapt to changing environments
Different gene combinations mean some individuals may survive better under new conditions
This genetic variation is the raw material for evolution
Q5. How is meiosis different from mitosis? Give any four differences. (Long Answer)
Four main differences between meiosis and mitosis:
Feature
Mitosis
Meiosis
1. Where it occurs
All body (somatic) cells
Only in reproductive organs (testes, ovaries, anthers)
2. Number of divisions
One division
Two divisions
3. Number of daughter cells
Two daughter cells
Four daughter cells (gametes)
4. Chromosome number
Daughter cells have same number as parent (46 in humans)
Daughter cells have half the number of parent (23 in humans)
Additional differences:
Purpose: Mitosis is for growth and repair; Meiosis is for gamete formation
