Mitosis and Meiosis
Two ways a cell divides, each with a different purpose. Mitosis makes copies; meiosis makes gametes — and shuffles the genome in the process.
Primary sources: NCBI Bookshelf: Cell Division · Khan Academy AP Biology
01 · Stages of mitosis
PMAT: four stages, two identical cells.
Mitosis divides one diploid cell into two genetically identical diploid daughter cells. It underpins growth, repair and asexual reproduction.
The cell grows (G1), replicates all its DNA (S phase) and grows further (G2). Not technically part of mitosis but essential preparation.
Chromosomes condense and become visible. The mitotic spindle begins to form from centrioles. The nuclear envelope starts to break down.
Chromosomes align along the cell equator (metaphase plate). Spindle fibres attach to each chromosome's centromere from opposite poles.
Sister chromatids are pulled to opposite poles of the cell. The cell elongates. Each pole now has a complete set of chromosomes.
Nuclear envelopes reform around each set of chromosomes. Chromosomes de-condense. Cytokinesis divides the cytoplasm into two daughter cells.
02 · Meiosis
Two divisions, four haploid cells.
Meiosis consists of two successive divisions — Meiosis I and Meiosis II — producing four haploid cells from one diploid parent. It occurs only in the gonads (testes and ovaries in animals) to produce gametes (sperm and eggs).
- Meiosis I (reductional division) — homologous chromosome pairs line up and then separate, halving the chromosome number from 2n to n. Crossing over occurs during prophase I, exchanging segments between homologous chromosomes.
- Meiosis II (equational division) — similar to mitosis; sister chromatids separate in each of the two cells from Meiosis I, giving four haploid daughter cells.
The key outcome is four genetically unique haploid cells — unlike mitosis, which produces two identical copies. Genetic variation is introduced by crossing over and by the random orientation of bivalents at metaphase I (independent assortment). This variation is the raw material for natural selection — see our natural selection guide.
03 · Genetic variation
Meiosis shuffles the genome.
Two mechanisms in meiosis generate variation beyond simple segregation:
- Crossing over (recombination) — during prophase I, homologous chromosomes pair up as bivalents. Non-sister chromatids exchange segments at points called chiasmata. Each chiasma shuffles alleles to create recombinant chromosomes with new combinations of alleles.
- Independent assortment — at metaphase I, each bivalent orients randomly toward either pole. With 23 chromosome pairs, humans can produce 2²³ (8.4 million) distinct chromosome combinations from this alone — before crossing over.
This variation is crucial for evolution. Sexual reproduction continually produces new genotype combinations on which natural selection can act. See our DNA, genes and heredity guide for how alleles and genotypes work, and our mitosis glossary entry for cell cycle checkpoints.
04 · Cell cycle control
Checkpoints prevent errors from multiplying.
The cell cycle has built-in quality checkpoints. At each, regulatory proteins assess whether conditions are right to proceed:
- G1 checkpoint — is the cell large enough? Is the DNA undamaged? If not, the cell pauses or undergoes apoptosis.
- G2 checkpoint — has all DNA been accurately replicated? Any double-strand breaks trigger arrest.
- Spindle assembly checkpoint (M) — are all chromosomes correctly attached to spindle fibres before anaphase begins?
The tumour suppressor protein p53 is central to the G1 checkpoint. Mutations in p53 are found in more than half of human cancers — allowing cells with damaged DNA to divide uncontrollably. CRISPR tools are being investigated to restore p53 function. See our CRISPR article and our mitosis glossary entry.
Questions about mitosis and meiosis
- Mitosis produces two daughter cells with the same chromosome number as the parent (diploid → diploid). It is used for growth, repair and asexual reproduction. Meiosis halves the chromosome number (diploid → haploid) and introduces genetic variation through crossing over and independent assortment. See our mitosis glossary entry for a concise summary.
- The cell first prepares during Interphase (DNA replication in S phase). Mitosis then has four stages: Prophase (chromosomes condense), Metaphase (chromosomes align at the equator), Anaphase (sister chromatids pulled apart) and Telophase (nuclear envelopes reform). Cytokinesis follows to divide the cytoplasm. The mnemonic PMAT helps.
- Crossing over (recombination) in prophase I exchanges segments between homologous chromosomes, creating new allele combinations. Independent assortment means each pair of homologous chromosomes separates independently, producing 2²³ (over 8 million) possible chromosome combinations in human gametes — before fertilisation. See our DNA, genes and heredity guide.
- If gametes were diploid (like body cells) then fertilisation would create tetraploid offspring (4n), and each subsequent generation would double the chromosome number. Meiosis prevents this by producing haploid gametes. Errors in meiosis (non-disjunction) can result in gametes with an extra or missing chromosome — as in Down syndrome (trisomy 21). See NCBI on meiosis.
Understand the cell first
Mitosis and meiosis happen inside cells. The cells and organelles guide covers the nucleus, chromosomes and organelles involved.