Mitosis and meiosis - what is the difference? Cell division step by step

The idea: copying and shuffling

Cells divide for two reasons. The first is that the body needs new cells - to grow, to heal a cut, to replace skin. The new cells should be just like the old ones, so the cell copies its genetic material and gives each daughter cell a complete set. That is mitosis.

The second reason is sexual reproduction. An egg and a sperm fuse into one cell, so each of them can carry only half the chromosomes - otherwise the number would double in every generation. And that half is different every time, because the chromosomes get shuffled. That is meiosis.

Before you start: chromosomes come in pairs

A human cell has 46 chromosomes, which make 23 pairs. In each pair one chromosome comes from the mother and the other from the father. Two such chromosomes are homologous: they carry the same genes, though often in slightly different versions. A cell with the double set is called diploid (2n), one with a single set haploid (n).

Before a cell divides, it copies its DNA during interphase - in the S phase. From then on, each chromosome is made of two identical sister chromatids, joined at the centromere.

In the animations the cell has only 2n = 4 chromosomes: a long pair and a short pair. The teal ones come from the mother, the amber ones from the father. During interphase chromosomes are only loosely packed and you cannot make them out under a light microscope, but we draw them so you can follow what happens to them. Go through the animations with the buttons or the arrow keys.

Mitosis step by step

One 2n cell has become two 2n cells with identical DNA. Each got one chromatid of every chromosome.

Meiosis step by step

Meiosis is two divisions in a row, with no copying of DNA in between. In the first, the homologous chromosomes separate; in the second, the sister chromatids do.

One 2n cell has become four n cells. No two are alike: each has a different mix of the mother’s and the father’s chromosomes.

Try it yourself: how many different gametes?

In metaphase I each pair can face one of two ways - with the mother’s chromosome towards one pole or towards the other. That decides what each new cell gets. Turn the pairs or line them up at random, and try to find every possible set. Then use the slider to see how fast the number grows with more pairs.

With $n$ pairs of chromosomes there are $2^n$ possible sets. For 3 pairs that is 8; for the 23 human pairs it is 8,388,608. Crossing over adds new combinations of genes within the chromosomes themselves, so it is extremely unlikely that two gametes carry exactly the same set of genes.

Mitosis and meiosis side by side

How to count chromosomes and chromatids

The key rule: the number of chromosomes is the number of centromeres. A chromosome can have one chromatid (before the S phase, and after the chromatids separate) or two (from the S phase until they separate).

Example: a cell with 2n = 8.

Some textbooks also use the letter c, as in 2n4c: the number before n is how many sets of chromosomes the cell has, and the number before c how many copies of DNA. A cell in metaphase of mitosis is 2n4c, a cell after meiosis I is 1n2c, and a gamete is 1n1c.

Your turn

Dog cells have 2n = 78 chromosomes. How many chromosomes does a dog’s gamete have? How many chromatids are there in one cell just after the first division of meiosis?

Common mistakes

• Mixing up sister chromatids and homologous chromosomes. Sister chromatids are two identical copies of one chromosome, joined at the centromere. Homologous chromosomes are two different chromosomes of the same pair - one from the mother, one from the father.
• “The number of chromosomes drops in meiosis II.” It drops in meiosis I, when the homologues separate. Meiosis II works like mitosis: it separates sister chromatids, and each cell keeps n chromosomes.
• “Cells after meiosis I are diploid because they have a lot of DNA.” They are haploid: they have n chromosomes, only each one still has two chromatids.
• Counting chromatids instead of chromosomes. In metaphase of mitosis a human cell has 46 chromosomes, not 92 - chromosomes are counted by their centromeres.
• “Crossing over happens in mitosis.” Crossing over is a feature of meiosis: it happens in prophase I, when the homologues are paired up. In mitosis the homologues do not pair, and swaps between them are rare - usually a side effect of repairing damaged DNA.
• “Gametes are always made by meiosis.” In animals, yes - but not in every organism. The parasite Toxoplasma gondii is haploid for most of its life, so its gametes form without meiosis, from cells that multiply by mitosis; meiosis only happens after fertilisation. In the 2026 Polish matura only 17% of students solved the task built on this life cycle; many automatically linked gametes with meiosis (CKE report, in Polish).

In exams

In the Polish biology matura, cell division is a separate section of the exam requirements. The questions can be tricky: in 2026 only 17% of students solved a task that asked them to apply what they knew about mitosis and meiosis to a parasite’s life cycle (CKE report, in Polish). It is worth practising on cells with a number other than 46 - like the ones in the animations - and on life cycles other than the human one.

Test yourself

Flashcards for review

Flip a card and say whether you knew the answer. The ones you did not know come back at the end of the deck. Why it pays to come back to them in a few days is explained in the guide Spaced repetition.