Meiosis and Mitosis: GCSE Biology Comparison

Every cell in your body came from one fertilised egg; every gamete you produce carries a unique shuffle of your parents' genes. Mitosis copies cells faithfully for growth and repair; meiosis halves the chromosome number and generates the variation that drives evolution — two processes, two entirely different purposes.

What is mitosis and when does it occur?

Mitosis is the type of cell division used for growth, repair of tissues, and asexual reproduction. It produces two daughter cells that are:

Genetically identical to the parent cell and to each other

Diploid — they contain the full complement of paired chromosomes (46 in humans: 23 pairs)

Mitosis occurs in virtually all body (somatic) cells throughout the organism's life. Examples include:

Skin cells dividing to replace cells shed from the surface

Red blood cells produced in bone marrow

Liver cells regenerating after damage

Growth at the tips of plant roots and shoots

The key point is that mitosis is a maintenance process — it keeps the body running and its chromosome number constant.

What is meiosis and when does it occur?

Meiosis is the cell division that produces gametes (sex cells): sperm in males and eggs (ova) in females. It produces four daughter cells that are:

Genetically different from the parent cell and from each other

Haploid — they contain only one chromosome from each homologous pair (23 chromosomes in human gametes)

Meiosis occurs only in the gonads — the testes (producing sperm) and ovaries (producing eggs). It is the foundation of sexual reproduction: when two haploid gametes fuse at fertilisation, the resulting zygote has the correct diploid number of 46. Without meiosis, chromosome numbers would double every generation.

How do mitosis and meiosis compare?

Feature | Mitosis | Meiosis
Purpose | Growth, repair, asexual reproduction | Production of gametes for sexual reproduction
Where it occurs | Body (somatic) cells throughout the organism | Gonads only (testes and ovaries)
Number of divisions | 1 | 2 (meiosis I then meiosis II)
Number of daughter cells produced | 2 | 4
Ploidy of daughter cells | Diploid (2n) | Haploid (n)
Identical to parent cell? | Yes — genetically identical | No — genetically varied
Genetic variation produced? | No (barring mutation) | Yes — crossing over and independent assortment
Stages | Prophase, Metaphase, Anaphase, Telophase (PMAT) | PMAT I then PMAT II

What are the stages of mitosis?

Mitosis is divided into four main stages (remembered as PMAT):

Prophase — chromosomes condense and become visible; the nuclear envelope breaks down; spindle fibres begin to form.

Metaphase — chromosomes line up along the equator (middle) of the cell; spindle fibres attach to the centromere of each chromosome.

Anaphase — spindle fibres shorten, pulling sister chromatids apart to opposite poles of the cell.

Telophase — nuclear envelopes reform around each set of chromosomes; the cell then divides (cytokinesis) to produce two genetically identical daughter cells.

Before mitosis begins, the cell enters interphase, during which it grows and copies all its DNA — so that each daughter cell receives a complete set of genetic information.

What makes meiosis produce genetic variation?

Meiosis generates genetic variation through two mechanisms:

1. Crossing over (recombination) During meiosis I, homologous chromosomes (the pair inherited from each parent) line up alongside each other and exchange segments of DNA at points called chiasmata. The resulting chromosomes are a mixture of DNA from both grandparents — neither purely maternal nor purely paternal. Each gamete therefore carries a unique recombined chromosome.

2. Independent assortment During meiosis I, each pair of homologous chromosomes lines up independently of every other pair. For each of the 23 pairs in humans, the maternal or paternal chromosome can go to either pole randomly. This alone gives 2²³ = over 8 million possible chromosome combinations in the gametes — before crossing over is even considered.

Together, these two mechanisms mean no two gametes are genetically identical, which is why (unless you are an identical twin) no two humans share the same genome.

Why does life need both types of cell division?

Mitosis and meiosis serve entirely different purposes, and multicellular sexually reproducing life requires both:

Without mitosis, a fertilised egg could never grow into a multicellular organism, tissues could not repair themselves, and growth would be impossible. Every cell in your body (except gametes) is a product of mitosis from the original zygote.

Without meiosis, sexual reproduction would be impossible. The chromosome number would double every generation (46 → 92 → 184...) — which is almost always fatal. Meiosis is also the engine of genetic variation: the shuffling of alleles it produces is the raw material for natural selection and evolution.

The two processes are complementary: meiosis generates the diverse gametes, fertilisation combines them, and mitosis builds the new organism.

Frequently asked questions

What is the main difference between meiosis and mitosis?

Mitosis produces two genetically identical diploid daughter cells for growth and repair. Meiosis produces four genetically varied haploid daughter cells for sexual reproduction. Mitosis involves one round of division; meiosis involves two. The key functional distinction is purpose: mitosis maintains the body and keeps the chromosome number constant, while meiosis enables reproduction and generates the genetic diversity that drives evolution.

How many chromosomes do human gametes contain?

Human gametes (sperm and eggs) are haploid and contain 23 chromosomes — one from each of the 23 homologous pairs found in body cells. When a sperm fertilises an egg, the resulting zygote receives 23 from each parent, restoring the diploid number of 46. This halving during meiosis is essential: without it, the chromosome number would double with every generation of reproduction.

What is crossing over and why does it produce genetic variation?

Crossing over is the exchange of DNA segments between homologous chromosomes during meiosis I. The chromosomes wrap around each other at points called chiasmata and swap equivalent sections of DNA. This shuffles genetic information between the chromosomes a person inherited from each of their parents, so each gamete produced carries a unique recombined combination of alleles. Crossing over, combined with independent assortment, means that the number of genetically distinct gametes a single human can produce is astronomically large.

Why does meiosis produce haploid cells but mitosis produces diploid cells?

Meiosis must produce haploid cells because they are destined to fuse with another gamete at fertilisation — if both gametes were diploid, the zygote would have double the correct chromosome number, which is almost always lethal. Mitosis does not need to halve the chromosome number because the resulting cells remain in the same organism and must function alongside all other body cells, which are diploid. Each process is precisely suited to its biological role.

For Socratic GCSE biology with Professor Darwin — zooming from organism to cell to chromosome and back — visit aitutors.me.