Every trait you inherited — eye colour, height, even the way your cells repair themselves — traces back to a special kind of cell division called meiosis. Here's how it works, stage by stage, with the visuals to match.
Meiosis is the type of cell division that produces gametes — sperm and egg cells — each carrying half the usual number of chromosomes. It takes one diploid cell and turns it into four genetically unique haploid cells through two back-to-back rounds of division. If that sounds like a lot, don't worry: the whole process becomes intuitive once you see it laid out.
Meiosis I
Homologous chromosomes pair up, swap bits of DNA, then separate into two cells.
Meiosis II
Sister chromatids separate, producing four haploid cells ready to become gametes.
Variety
Crossing over and independent assortment make each gamete genetically unique.
Meiosis in a Nutshell
THE BIG PICTUREMeiosis takes a diploid cell (2n, two sets of chromosomes) and produces four haploid cells (n, one set each). In humans, that means starting with a cell containing 46 chromosomes and ending with four cells containing 23 chromosomes each.
It happens in two main phases: Meiosis I separates homologous chromosomes, and Meiosis II separates sister chromatids. The result is four cells, each genetically distinct from the parent and from each other.
Diploid (2n): two full sets of chromosomes. Haploid (n): one set. Homologous chromosomes: matching pairs, one from each parent. Sister chromatids: identical copies joined at the centromere.
Meiosis I: The Reduction Division
HOMOLOGUES SEPARATEMeiosis I is called the reduction division because it halves the chromosome number. It has four phases: prophase I, metaphase I, anaphase I, and telophase I.
Prophase I
The nuclear envelope breaks down, chromosomes condense, and — crucially — homologous chromosomes pair up to form a tetrad (also called a bivalent). While paired, they exchange segments of DNA in a process called crossing over. This is the first major source of genetic variation.
Metaphase I
Homologous pairs line up along the cell equator. The orientation of each pair is random — this is independent assortment, the second major source of variation. One homologue faces each pole, ready to be pulled apart.
Anaphase I
The homologous chromosomes are pulled apart to opposite poles. Unlike mitosis, sister chromatids stay attached at this stage. The cell begins to elongate.
Telophase I & Cytokinesis
Two new nuclei form, and the cell splits into two haploid daughter cells. Each still has chromosomes made of two sister chromatids, so they're not quite ready for fertilisation yet.
| Phase | What happens | Key point |
|---|---|---|
| Prophase I | Chromosomes condense, homologues pair, crossing over occurs | Crossing over = new allele combos |
| Metaphase I | Homologous pairs align at the equator | Independent assortment = more variety |
| Anaphase I | Homologues pulled to opposite poles | Sister chromatids stay together |
| Telophase I | Two haploid cells form | Chromosome number is halved |
Meiosis II: The Equational Division
SISTER CHROMATIDS SEPARATEMeiosis II looks a lot like mitosis, but it happens in two haploid cells at once. There's no DNA replication beforehand, and the result is four haploid cells.
Prophase II
The nuclear envelope breaks down again in each cell, and spindle fibres form, preparing to grab the chromosomes.
Metaphase II
Chromosomes line up single-file at the equator of each cell. Each chromosome still consists of two sister chromatids.
Anaphase II
Sister chromatids are finally pulled apart, moving to opposite poles of each cell. This is the step that ensures each resulting cell gets one copy of each chromosome.
Telophase II & Cytokinesis
Four haploid cells form, each with a single set of chromosomes. In males, all four become sperm. In females, only one becomes a viable egg — the other three become polar bodies that degenerate.
Why Genetic Variety Matters
Meiosis isn't just about halving chromosome numbers. It's a variation machine, and it has two main tricks:
- Crossing over (prophase I): homologous chromosomes swap segments, creating chromosomes that mix DNA from both parents.
- Independent assortment (metaphase I): each homologous pair lines up independently, so the combination of chromosomes in each gamete is random.
Together, these mechanisms mean that a single human can produce around 8 million genetically different gametes — before you even factor in crossing over.
This variation is why siblings (except identical twins) look different, why some people inherit a trait from a grandparent, and why populations can adapt to new environments over generations.
Meiosis vs Mitosis: The Key Differences
Mitosis is for growth and repair; meiosis is for reproduction. Here's the side-by-side you actually need.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth & repair | Gamete production |
| Divisions | One | Two |
| Daughter cells | 2, diploid | 4, haploid |
| Genetic identity | Identical to parent | Genetically unique |
| Crossing over | No | Yes (prophase I) |
| Where | Throughout the body | Reproductive organs |
Frequently Asked Questions
Quick answers to the questions people ask most about meiosis.