MEIOSIS is a type of cell division that produces GAMETES (sex cells — sperm and eggs in animals; pollen and ovules in plants).
Key features of meiosis:
Produces FOUR daughter cells (not two like mitosis).
Each daughter cell has HALF the chromosome number of the parent cell.
In humans: body cells have 46 chromosomes (23 pairs); gametes have 23 chromosomes (one from each pair).
This is called the HAPLOID number (23) vs DIPLOID number (46).
When two gametes fuse at fertilisation:
Sperm (23) + Egg (23) = Zygote (46) — full diploid number restored.
This ensures the chromosome number stays constant from generation to generation.
How Meiosis Works
MEIOSIS involves two divisions:
FIRST DIVISION (Meiosis I):
Chromosomes replicate (as in mitosis).
Homologous pairs line up together.
Pairs are SEPARATED — one chromosome from each pair goes to each new cell.
Produces 2 cells, each with 23 chromosomes (but each is a double-stranded copy).
SECOND DIVISION (Meiosis II):
The two cells divide again.
The double-stranded chromosomes split — one strand to each cell.
Produces 4 haploid cells, each with 23 single chromosomes.
GENETIC VARIATION:
During Meiosis I, CROSSING OVER occurs — chromosomes exchange sections of DNA.
This creates new combinations of alleles — novel genetic variation.
Random SHUFFLING of which chromosome from each pair goes to each cell also creates variation.
This is why siblings (same parents) are not genetically identical.
Meiosis vs Mitosis
MEIOSIS vs MITOSIS — key differences:
MITOSIS:
Produces 2 genetically IDENTICAL daughter cells.
Daughter cells are DIPLOID (same chromosome number as parent).
Used for growth, repair, asexual reproduction.
MEIOSIS:
Produces 4 genetically DIFFERENT daughter cells.
Daughter cells are HAPLOID (half the chromosome number).
Used only for gamete production (in gonads — testes and ovaries in humans).
IMPORTANCE OF MEIOSIS:
Ensures gametes have half the chromosome number — so fertilisation restores the full number.
Creates genetic variation among offspring — important for evolution and adaptation.
Variation means populations can respond to changing environments.
⚠️ Common Mistake
Meiosis produces FOUR cells, not two. Mitosis produces two. Meiosis produces genetically DIFFERENT cells — not identical. The daughter cells are HAPLOID (half chromosome number). Students often confuse meiosis with mitosis — remember: Meiosis = gametes, Mix up alleles, 4 cells.
📌 Key Note
Meiosis: produces 4 haploid gametes (23 chromosomes in humans). Two divisions. Crossing over creates new allele combinations → genetic variation. Meiosis I: separates homologous pairs. Meiosis II: separates chromatids. Variation = essential for evolution.
🎯 Matching Activity — Meiosis vs Mitosis
Match each feature to meiosis, mitosis, or both. — drag the symbols on the right to match the component names on the left.
Meiosis only
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Mitosis only
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Meiosis only
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Both
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Meiosis only
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DNA replication occurs before division begins
Daughter cells are haploid — half the chromosome number
Produces 4 daughter cells — all genetically different
Crossing over between homologous chromosomes creates new allele combinations
Produces 2 daughter cells — genetically identical to parent
🔬 Triple Science Only
Meiosis (4.6.1.2) is biology-only — not in Combined Science. Includes the detail of two meiotic divisions, crossing over as a source of genetic variation, and the significance of producing haploid gametes.
🎯 Test Yourself
Question 1 of 2
1. How many chromosomes does a human gamete contain, and why?
2. Why does meiosis produce genetically varied gametes rather than identical ones?
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