Mitosis vs meiosis: the difference, the phases and why it matters

What is mitosis?

Mitosis is the cell division in which one nucleus divides into two nuclei with identical chromosome sets, so that parent and daughter cells carry the same genetic material. In humans that means one cell with 46 chromosomes produces two cells with 46 chromosomes each.

Mitosis accounts for growth, repair and cell replacement. Skin renews itself, a wound heals and a child grows through mitosis. In single-celled organisms, mitosis is the whole of reproduction.

What is meiosis?

Meiosis is the cell division that halves the chromosome number and produces gametes – sperm and eggs in animals, spores in plants. One diploid cell with 46 chromosomes yields four haploid cells with 23 each, and those four differ genetically from each other and from the parent.

The halving is necessary for a simple reason: fertilisation fuses two gametes. Without it, the chromosome number would double every generation.

What is the difference between mitosis and meiosis?

The decisive difference is what lines up on the equator during metaphase. In mitosis each chromosome lines up individually and the sister chromatids are pulled apart. In meiosis I the homologous pairs line up alongside each other and whole chromosomes are pulled to opposite poles while sister chromatids stay together.

What are the phases of mitosis?

Mitosis is conventionally divided into four phases, followed by cytokinesis in which the cell itself is pinched in two.

Prophase. Chromatin condenses into visible chromosomes, each consisting of two identical sister chromatids joined at the centromere. The nuclear envelope begins to break down and the spindle forms.

Metaphase. Chromosomes line up individually along the cell’s equatorial plane, with spindle fibres attached at the kinetochore on each side.

Anaphase. Sister chromatids are pulled apart towards opposite poles. From this moment each chromatid counts as a chromosome in its own right.

Telophase. Chromosomes reach the poles, decondense, and new nuclear envelopes form. Cytokinesis then divides the cytoplasm into two cells.

What are the phases of meiosis?

Meiosis consists of two rounds of division sharing the same phase names, marked I and II. No DNA replication occurs between them – which is precisely why the chromosome number halves.

Prophase I is the longest and most important phase. Homologous chromosomes pair into bivalents, and at chiasmata they exchange segments in crossing over. This is where new combinations of alleles arise.

Metaphase I lines the pairs up on the equator. Anaphase I pulls whole homologues, not chromatids, to opposite poles. Telophase I gives two haploid cells in which each chromosome still has two chromatids.

Meiosis II resembles an ordinary mitosis: sister chromatids separate in anaphase II. The result is four haploid cells.

What is crossing over and why does it matter?

Crossing over is the physical exchange of DNA segments between homologous chromosomes in prophase I, and it is the main reason siblings are not copies of each other. A chromosome you inherited from your mother contains, after meiosis, stretches from both of her parents.

The effect is substantial. Without crossing over, each chromosome would be inherited as an undivided block and the number of possible combinations would be limited to how the chromosomes are sorted. With it, every chromosome becomes a unique mosaic.

Where does genetic variation come from?

Three mechanisms generate variation, and they multiply with each other. Crossing over reshuffles within chromosomes. Independent assortment in metaphase I determines which parent’s version of each pair ends up in which cell – with 23 pairs that gives more than eight million combinations per gamete, or 2 to the power of 23.

The third is fertilisation itself: which of millions of sperm meets which egg. Together, that is why two siblings with the same parents can be so different, while identical twins – which arise through mitosis after fertilisation – are genetically the same.

What happens when division goes wrong?

If a chromosome pair fails to separate correctly, known as non-disjunction, one daughter cell gains a chromosome and another loses one. Trisomy 21, Down syndrome, is the most common human example and usually stems from a separation error in meiosis I in the egg.

Errors at mitotic checkpoints have a different consequence: cells that divide despite damaged DNA. The cell cycle checkpoints, of which p53 is the best known player, exist to stop exactly that, and when they are disabled uncontrolled division is one of the mechanisms behind tumour growth.

How to learn the phases without mixing them up

Draw the phases by hand instead of reading about them. Placing four chromosomes on paper and following them through both divisions immediately exposes whether you understand what separates when – reading a passage does not. Start with two pairs, colour one pair in one shade and the other in another, and follow them all the way to four cells.

Then test yourself on real questions rather than rereading your notes, because retrieval builds stronger memory than recognition – the most robust finding in learning research, covered in our article on active recall. If your lecture notes are digital you can generate a quiz from them in Memmo and start with the questions straight away. It is worth comparing with the other core cell processes in our articles on photosynthesis and cellular respiration.