Cells and organisation · GCSE Biology
Meiosis
Explain meiosis for GCSE Biology: two divisions, four genetically different haploid gametes, 46 to 23 chromosomes, and why fertilisation then restores the diploid number.
Meiosis: two divisions, four cells, haploid, genetically different. It makes sperm and eggs (and plant gametes). Mitosis is the identical-body-cell process — do not mix them.
The important bits
What you need to know
- 1
Meiosis is cell division that produces gametes (sex cells). In humans it occurs in the testes and ovaries (and in plants in anthers and ovules).
- 2
It involves two divisions. One diploid parent cell (46 chromosomes) yields four haploid cells (23 chromosomes each).
- 3
Daughter cells are genetically different from each other and from the parent because of independent assortment of chromosomes and crossing over (Triple may require those names; Combined needs “genetically different”).
- 4
Fertilisation fuses two haploid gametes to restore the diploid number (23 + 23 = 46) in the zygote. The zygote then grows by mitosis.
- 5
Sexual reproduction therefore produces variation: meiosis shuffles alleles, fertilisation combines two parents. That variation is the raw material for natural selection.
- 6
Asexual reproduction uses mitosis and produces clones. Meiosis is not used to grow a plant from a cutting or to heal skin.
- 7
Sex chromosomes: eggs carry X; sperm carry X or Y. Fertilisation XX female, XY male — a 50% chance, shown on a Punnett square.
- 8
If chromosome number were not halved, fertilisation would double it every generation (92, 184…). Meiosis exists to keep the number stable.
Quotations worth analysing
Short evidence. Real method.
“Meiosis produces four genetically different haploid gametes.”
Four, different, haploid, gametes: each word is a mark. Mitosis is two, identical, diploid, body cells.
“Gametes have half the number of chromosomes of a body cell.”
Human numbers: 23 in a gamete, 46 in a body cell. Write the numbers when the question is human.
“Fertilisation restores the diploid number.”
Meiosis halves; fertilisation doubles back. Then mitosis grows the organism. Keep the three processes in that order.
Go deeper
Why two divisions, not one?
DNA is still replicated once, so the cell starts meiosis with doubled DNA, as in mitosis. The first division separates homologous pairs (the matching chromosomes you inherited one from each parent), so each cell is already haploid in terms of pairs. The second division separates sister chromatids, producing four cells. Without the pair-separation step you would not halve the chromosome number correctly. Combined papers rarely demand stage names; they do demand the outcome: four haploid genetically different gametes. If a diagram shows pairing of homologous chromosomes, it is meiosis, not mitosis. Mitosis never pairs homologues in that way. Use that diagram clue when the labels have been removed.
Go deeper
Where does the genetic difference come from?
Independent assortment: which chromosome of each pair goes to which pole is random, so many combinations of maternal and paternal chromosomes are possible. Crossing over (Higher/Triple): chromatids of a homologous pair swap sections, creating new allele combinations on a chromosome. Mutation can add extra change but is not the main GCSE story for meiosis. Fertilisation then multiplies variation because any sperm can fuse with any egg. That is why siblings (except identical twins) are not clones. Identical twins come from one zygote that later splits by mitosis — they are not the product of two identical meioses. In the exam, “meiosis makes cells different so we are all unique” is woolly; name haploid gametes and fertilisation mixing alleles.
Go deeper
How does meiosis sit next to mitosis in a life-cycle question?
Adult body (diploid) → meiosis in gonads → gametes (haploid) → fertilisation → zygote (diploid) → mitosis → embryo and adult. Plants add pollen and ovules but the logic is the same. Asexual plants skip meiosis and fertilisation and use mitosis only. When a question gives chromosome numbers, check they halve at meiosis and restore at fertilisation. A common trap is a cell with 12 chromosomes producing gametes with 12 — that would be mitosis. Gametes should show 6. Another trap is saying the zygote divides by meiosis to grow; growth is mitosis. Keep the verbs attached to the right process: meiosis produces, fertilisation fuses, mitosis grows.
See the idea in action
A human cell with 46 chromosomes produces sperm. DNA is replicated, then meiosis I and II produce four sperm cells with 23 chromosomes each. The four sperm are genetically different. At fertilisation a sperm (23) fuses with an egg (23) to make a zygote with 46 chromosomes. That zygote grows into a baby by mitosis, which keeps 46 chromosomes and makes identical body cells. Writing “sperm are made by mitosis with 46 chromosomes” would reverse both processes.
Exam technique
Turn knowledge into marks
Memorise the mitosis/meiosis table: 2 vs 4 cells, identical vs different, diploid vs haploid, growth vs gametes. Quote 46 and 23 for humans. After fertilisation, growth is mitosis again.
Common mistakes
Do not give these marks away
- 01
Saying meiosis makes two identical cells, or that it is used for growth and repair.
- 02
Forgetting the second division, or writing that gametes are diploid.
- 03
Claiming the zygote grows by meiosis, or that fertilisation halves the chromosome number.
Which statement about meiosis is correct?
AIt produces two genetically identical diploid cells for growth
BIt produces four genetically different haploid gametes
CIt produces clones of the parent body cell
DIt doubles the chromosome number to 92 in humans
Show the answer
It produces four genetically different haploid gametes. Meiosis is two divisions giving four different haploid sex cells. Two identical diploid cells is mitosis. Chromosome number is halved, not doubled.
Quick questions
If this is the bit you searched
What is meiosis in GCSE Biology?
Cell division that makes gametes. It produces four genetically different haploid cells with half the chromosome number of the parent.
How is meiosis different from mitosis?
Mitosis: two identical diploid body cells for growth and repair. Meiosis: four different haploid gametes for sexual reproduction.
Why must gametes be haploid?
So fertilisation restores the diploid number. If gametes were diploid, chromosome number would double every generation.
Where does meiosis happen in humans?
In the testes and ovaries, producing sperm and eggs with 23 chromosomes each.