What is meiosis?

Most cells in your body have two sets of chromosomes, one from each parent. A cell like this is called diploid (written 2n). Meiosis makes haploid cells (n), with just one set. When a haploid sperm fertilises a haploid egg, the two sets join and the new cell is diploid again: 23 + 23 = 46.

What is the purpose of meiosis?

Meiosis has two jobs, and both matter for sexual reproduction.

1. Halving the chromosome number. If sperm and eggs had 46 chromosomes each, a baby would have 92, and the number would double every generation. Meiosis halves it, so fertilisation restores the right number.
2. Making genetic variation. Each gamete gets a different mix of the parent's chromosomes and genes. That is why brothers and sisters with the same parents look different, and it gives a species the variety that natural selection works on.

Meiosis stages

Before meiosis begins, the cell copies all its DNA during interphase, just as it does before mitosis. Each chromosome then has two identical copies, called sister chromatids, joined at a point called the centromere. Then come two divisions, meiosis I and meiosis II, each with four stages: prophase, metaphase, anaphase and telophase.

Meiosis I: separating the pairs

• Prophase I: chromosomes coil up and become visible. Each chromosome finds its partner, the matching chromosome from the other parent, and they pair up closely. Matching pairs are called homologous chromosomes. While paired, they swap sections of DNA. This is crossing over, and the points where they swap are called chiasmata.
• Metaphase I: the homologous pairs line up across the middle of the cell. Which side each chromosome of a pair faces is random, and each pair lines up independently of the others.
• Anaphase I: spindle fibres pull the pairs apart. One whole chromosome from each pair goes to each end of the cell. The sister chromatids stay joined.
• Telophase I and cytokinesis: the cell splits in two. Each new cell is haploid, with one chromosome from each pair, but each chromosome still has two chromatids.

Meiosis II: separating the chromatids

• Prophase II: a new spindle forms in each of the two cells. There is no DNA copying between the two divisions.
• Metaphase II: the chromosomes line up singly across the middle of each cell.
• Anaphase II: the sister chromatids are pulled apart to opposite ends. From now on, each chromatid counts as a chromosome of its own.
• Telophase II and cytokinesis: both cells divide, giving four haploid cells in total.

A quick way to remember it: meiosis I separates partners, meiosis II separates copies. Meiosis II works much like mitosis, but in haploid cells.

How meiosis creates genetic variation

Three things shuffle the genes, so almost every gamete is unique.

1. Independent assortment

In metaphase I, each of the 23 pairs lines up with either the mother's or the father's chromosome on a given side, at random. Each pair has 2 ways to line up, so the number of possible combinations is 2²³ = 8,388,608. One person can make more than 8 million chromosome combinations in their gametes, before counting anything else.

2. Crossing over

In prophase I, paired chromosomes swap sections. A chromosome that came from your mother can leave meiosis carrying some of your father's genes too. Crossing over happens at different points each time, which multiplies the possible combinations far beyond 8 million.

3. Random fertilisation

Any sperm can fertilise any egg. Without crossing over, two parents could already produce 2²³ × 2²³ = about 70 trillion different chromosome combinations. With crossing over, the number is far larger, which is why no two people, apart from identical twins, share the same genome.

Meiosis vs mitosis

Mitosis is the everyday division your body uses to grow and repair itself. Meiosis is the special division for making sex cells.

Test yourself on the differences: decide whether each statement describes mitosis or meiosis, then press Mark to check it in our Genetics and DNA course.

Meiosis and inheritance

Meiosis is why Punnett squares work. A gene can come in different versions, called alleles, and you carry two alleles of each gene, one on each chromosome of a pair. When the pair separates in anaphase I, the two alleles go to different cells. So every gamete gets exactly one allele of each gene, and which one is down to chance.

Take a parent with one allele for brown eyes, B, and one for blue, b. Half their gametes carry B and half carry b. If two parents who are both Bb have a child, the chance of the child getting b from both is ½ × ½ = ¼, so there is a 1 in 4 chance of blue eyes in this simple model. Real eye colour depends on several genes, but the principle, one allele from each parent at random, comes straight from meiosis.

Genes on different chromosomes are shuffled independently, thanks to independent assortment. Genes that sit near each other on one chromosome usually travel as a group, unless crossing over separates them. Our Genetics and DNA course builds from alleles to Punnett squares and genetic engineering.

Where does meiosis happen?

• In men, meiosis happens in the testes. Each cell that goes through meiosis makes four sperm, and sperm production carries on throughout adult life.
• In women, it happens in the ovaries, and the divisions are uneven. Most of the cytoplasm goes to one cell, which becomes the egg, and the other cells, called polar bodies, break down. Egg cells start meiosis before birth, then pause in prophase I for years or decades. Meiosis II only finishes if the egg is fertilised.
• In flowering plants, meiosis happens in the anthers, where it makes the cells that develop into pollen grains, and in the ovules.

When meiosis goes wrong

Sometimes a pair of chromosomes, or a pair of chromatids, fails to separate. This is called non-disjunction. One gamete ends up with an extra chromosome and another with one missing. If a gamete with an extra copy of chromosome 21 is fertilised, the baby has three copies, 47 chromosomes in total, which causes Down syndrome. Non-disjunction becomes more likely with the mother's age, possibly because egg cells spend so long paused partway through meiosis.

Worked examples

Example 1: chromosome numbers

A cat's body cells have 38 chromosomes. How many are in a cat's egg cell, and in a fertilised egg? Meiosis halves the number, so an egg cell has 38 ÷ 2 = 19. Fertilisation joins it with a sperm that also has 19: 19 + 19 = 38, back to diploid.

Example 2: counting combinations

A fruit fly has 8 chromosomes, in 4 pairs. Ignoring crossing over, how many different chromosome combinations can its gametes have? Each pair can line up 2 ways, so 2 × 2 × 2 × 2 = 2⁴ = 16. The rule is 2ⁿ, where n is the number of pairs, not the number of chromosomes.

Example 3: following the DNA

A cell has an amount of DNA x just before it copies its DNA. How much does each cell have at each point of meiosis?

• After copying in interphase: 2x, because every chromosome now has two chromatids.
• After meiosis I: x in each of the two cells, because the pairs have been split.
• After meiosis II: x/2 in each of the four cells, because the chromatids have been split.

This pattern, 2x then x then x/2, often appears as a graph in exams. Each drop marks one of the two divisions.

Common mistakes

• Saying meiosis makes two cells. It makes four. Mitosis makes two.
• Mixing up what separates when. Homologous pairs separate in anaphase I. Sister chromatids separate in anaphase II.
• Thinking DNA is copied again before meiosis II. It is copied once, before meiosis I, and never in between.
• Calling the cells after meiosis I diploid. They are already haploid: they have one chromosome from each pair, even though each chromosome still has two chromatids.
• Using 2 × 23 instead of 2²³. Independent assortment gives 2 to the power of the number of pairs, which is 8,388,608 for humans, not 46.
• Spelling them alike. A memory aid: meiosis makes me, because it makes the sex cells that combine to make a new person.

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