Before a cell can split, its nucleus has to. There are only two ways that happens — mitosis and meiosis — and both depend on work done earlier, while the cell looked as though it was doing nothing at all.
📘 What you need to know
A eukaryotic nucleus divides by mitosis or by meiosis. The nucleus divides before the cell, so no anucleate cells are made.
Mitosis gives genetically identical nuclei. It is used for growth, repair, replacement and asexual reproduction, and keeps the chromosome number the same (usually diploid, 2n).
Meiosis gives genetically different nuclei with half the chromosome number (haploid, n). It makes gametes and generates genetic diversity.
DNA replicates during interphase, giving two identical chromatids joined at the centromere — a pair of sister chromatids.
DNA is packed by winding around histones into nucleosomes, forming chromatin, which supercoils in prophase to make visible chromosomes.
Chromosomes are moved by microtubules made of tubulin, which lengthen and shorten, and by motor proteins.
Two ways to divide a nucleus
Feature
Mitosis
Meiosis
Daughter nuclei
Genetically identical to each other and to the parent
Genetically different from each other and from the parent
Chromosome number
Maintained — usually diploid (2n)
Halved — haploid (n)
Number of nuclei produced
Two
Four
What it is used for
Growth, repair of damaged tissue, replacement of cells, asexual reproduction
Producing gametes, and generating genetic diversity
A quick way to keep them straight: mitosis is for building and maintaining a body, meiosis is for making the next one. If the answer needs identical cells, it is mitosis. If it needs variety, it is meiosis.
Replication has to happen first
Neither type of nuclear division can start until the DNA has been copied. That copying happens during interphase, the part of the cell cycle before nuclear division begins.
Replication produces two identical DNA strands, each one called a chromatid.
The two chromatids are held together at a narrow point called the centromere.
Because they are identical copies, they are called sister chromatids. This is the familiar X shape.
During anaphase the centromere splits and the two chromatids are pulled apart, one to each daughter cell. From that moment they are called chromosomes again.
Chromatid or chromosome is not about size or shape — it is about whether the copies are still joined at a centromere.
Fitting the DNA in
A single DNA molecule can be more than 50 000 µm long. A human nucleus averages less than 5 µm across. So the DNA is around 10 000 times longer than the container it has to fit into. Packing is not optional.
🧩 How the DNA gets packed
The DNA winds around proteins called histones. Each length of DNA coiled around eight histone proteins forms a nucleosome.
Strings of nucleosomes make up the loosely coiled complex called chromatin. This is how the DNA sits for most of the cell cycle.
During prophase, the chromatin is condensed by supercoiling — the molecule is coiled repeatedly on itself.
The result is a chromosome short and thick enough to be moved around, and visible under a light microscope once stained.
Chromatin and chromosome are the same DNA at different levels of coiling. That is why chromosomes only become visible when division is about to start.
Moving the chromosomes
Condensed chromosomes still have to be dragged into position. That job belongs to microtubules and microtubule motors.
Microtubules are fibres made of the protein tubulin, and they form part of the cell’s cytoskeleton.
They can lengthen and shorten, which is what actually moves the chromosomes.
Two forms of tubulin, alpha and beta, join to form dimers — a dimer is a compound of two subunits. Dimers are added to or removed from the ends of a microtubule to change its length.
Motor proteins carry the chromosomes along the microtubules towards the equator, ready for division.
Why this matters for the exam: when you describe metaphase and anaphase later, “the spindle fibres shorten” is really tubulin dimers being removed from the ends. Knowing that turns a memorised phrase into something you can explain.
Worked examples
WORKED EXAMPLE
How much packing is needed?
A DNA molecule is 50 000 µm long. The nucleus it must fit into is 5 µm across. Calculate how many times longer the DNA is than the nucleus is wide.
Step 1: Check the units match
Both are in micrometres, so no conversion is needed.
Step 2: Divide50 000 ÷ 5 = 10 00010 000 times longerAlways check units first. If one figure had been in mm, the answer would be out by a factor of 1000.
WORKED EXAMPLE
Chromatid or chromosome?
A cell is at the start of anaphase. Explain why the structures being pulled towards the poles are described as chromosomes and not chromatids.
Step 1: What changed
At the start of anaphase the centromere splits.
Step 2: Why the name changes
Chromatids are only called chromatids while they are joined to their identical copy at a centromere.
Step 3: State the outcome
Once separated, each is an independent structure with its own DNA molecule, so it is a chromosome.
Separated at the centromere, so chromosomesThe DNA itself has not changed at all — only what it is attached to.
WORKED EXAMPLE
Choose the right division
State which type of nuclear division is used in each case, and give a reason. (a) Healing a cut in the skin. (b) Producing sperm cells in the testes.
(a) Healing a cut
New cells must be identical to the ones they replace, and the chromosome number must stay the same.
Mitosis(b) Producing sperm
Gametes must be haploid, so that fertilisation restores the diploid number.
Meiosis(a) mitosis, (b) meiosisNever just name the process — the reason is usually where the second mark sits.
💡 Exam tip
Say nuclear division when you mean the nucleus and cell division when you mean the whole cell. Examiners notice.
Learn the replication point: DNA is copied in interphase, before division begins — never during mitosis itself.
Use 2n and n notation. It is quick to write and shows you understand diploid and haploid.
Remember the nucleosome detail: DNA wound around eight histone proteins.
If asked how chromosomes move, name microtubules, tubulin and motor proteins rather than just “spindle fibres”.
For “why does the nucleus divide first”, the answer is to avoid producing anucleate cells.
⚠ Common mix-up
Chromatid, sister chromatids and chromosome. Two joined copies are sister chromatids; once the centromere splits, each is a chromosome.
Thinking DNA replicates during mitosis. It replicates beforehand, in interphase.
Saying chromosomes are always visible. For most of the cell cycle the DNA is uncondensed chromatin and cannot be seen as separate chromosomes.
Confusing chromatin with chromosome. Same DNA, different level of coiling.
Assuming mitosis always means diploid cells. It maintains whatever number the parent had — usually 2n, but not always.
Thinking the spindle pushes chromosomes. Microtubules shorten and pull, helped by motor proteins.
Up next: Mitosis — the four phases in order, what happens to the chromosomes in each, and the details examiners expect you to name.
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