Biology · January 2026

Mitosis and Meiosis: Cell Division Explained

Cells divide for two fundamentally different reasons: to grow and repair (mitosis) and to produce sex cells for reproduction (meiosis). The two processes share some machinery but produce very different results.

Before Division: The Cell Cycle and DNA Replication

Before any division, a cell must duplicate its DNA. This happens during S phase (synthesis phase) of the cell cycle. A human somatic (body) cell contains 46 chromosomes — 23 pairs. After S phase, the cell contains 92 chromatids: each chromosome has been copied into two identical sister chromatids joined at the centromere. It is these duplicated chromosomes that are then separated during cell division.

Mitosis: Copying for Growth and Repair

Mitosis produces two daughter cells that are genetically identical to the parent cell and to each other. It is used for growth, tissue maintenance, and asexual reproduction in some organisms. Mitosis has four main stages, following an initial condensation of chromosomes:

Prophase: Chromosomes condense and become visible under a microscope. The nuclear envelope breaks down. Spindle fibres (made of tubulin protein) begin to form from the centrosomes.
Metaphase: Chromosomes are pulled to the cell's equatorial plane (the metaphase plate) by spindle fibres attaching to the centromere of each chromosome. Each chromosome's two sister chromatids face opposite poles.
Anaphase: The sister chromatids are pulled apart toward opposite poles of the cell by the shortening spindle fibres. Each pole now has a complete set of 46 chromosomes.
Telophase: Nuclear envelopes re-form around each set of chromosomes. Chromosomes decondense. Cytokinesis (division of the cytoplasm) follows, producing two separate daughter cells.

The result: 2 diploid (2n = 46 chromosome) cells, identical to the parent.

Meiosis: Halving the Genome for Sexual Reproduction

Meiosis produces gametes (sperm and egg cells) with half the parent cell's chromosome number. When two gametes fuse at fertilisation, the original chromosome number is restored. Meiosis consists of two successive rounds of division — meiosis I and meiosis II — with no additional DNA replication between them.

Meiosis I — Separating Homologous Pairs

Meiosis I is the reduction division: it separates the 23 pairs of homologous chromosomes (one maternal, one paternal) into two cells.

Prophase I: Homologous chromosomes pair up in a process called synapsis, forming structures called bivalents (or tetrads — four chromatids together). During this phase, homologous chromosomes exchange segments of DNA in a process called crossing over (or recombination), which generates new combinations of genetic material.
Metaphase I: Bivalents align on the metaphase plate. The orientation of each pair is random — either the maternal or the paternal chromosome can face either pole. This is called independent assortment.
Anaphase I: Homologous chromosomes (each still consisting of two sister chromatids) are pulled to opposite poles. Sister chromatids remain joined.
Telophase I / Cytokinesis: Two cells form, each with 23 chromosomes — but each chromosome is still a pair of sister chromatids. These cells are haploid (n = 23) but each chromosome is still double-stranded.

Meiosis II — Separating Sister Chromatids

Meiosis II resembles mitosis but starts with haploid cells. Spindle fibres form, chromosomes align, and sister chromatids are separated to opposite poles. Cytokinesis produces four haploid daughter cells — the gametes. In males, all four become sperm. In females, one becomes the egg and three become small, non-functional polar bodies.

Sources of Genetic Variation in Meiosis

Meiosis generates genetic diversity through two mechanisms. Crossing over in prophase I reshuffles alleles between homologous chromosomes, creating new combinations. Independent assortment in metaphase I means each pair of homologs lines up randomly — with 23 pairs, this produces 2²³ (about 8 million) possible combinations of chromosomes in the gametes, before even accounting for crossing over. This is a major reason offspring differ genetically from their parents and from each other.

Mitosis vs. Meiosis at a Glance

Purpose: Mitosis — growth and repair. Meiosis — production of gametes.
Number of divisions: Mitosis — one. Meiosis — two.
Number of daughter cells: Mitosis — 2. Meiosis — 4.
Ploidy of daughter cells: Mitosis — diploid (2n). Meiosis — haploid (n).
Genetic identity of daughters: Mitosis — identical to parent. Meiosis — genetically diverse.
Crossing over: Mitosis — does not occur. Meiosis — occurs in prophase I.
Where it occurs: Mitosis — all somatic cells. Meiosis — reproductive organs (testes and ovaries in animals).

Errors in Cell Division

When chromosomes fail to separate correctly during meiosis — a condition called non-disjunction — gametes end up with the wrong number of chromosomes. A gamete with an extra chromosome that fertilises a normal gamete produces a zygote with three copies of that chromosome (trisomy). Down syndrome (trisomy 21) results from non-disjunction of chromosome 21 during meiosis. Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13) are also survivable trisomies. Monosomy (missing one chromosome) is generally lethal, with the exception of Turner syndrome (45,X in females).

Errors during mitosis — mutations or chromosome abnormalities in somatic cells — can lead to uncontrolled cell division, which is the basis of cancer.

Summary

Mitosis divides a diploid cell into two genetically identical diploid daughter cells for growth and repair. Meiosis, which involves two sequential divisions without an intervening S phase, produces four genetically unique haploid gametes. Crossing over during meiosis I and random independent assortment of chromosome pairs generate vast genetic diversity. Non-disjunction errors in meiosis are the cause of chromosomal conditions such as Down syndrome. Together, mitosis and meiosis allow multicellular organisms both to grow as individuals and to reproduce sexually.