Mitosis is really one continuous process, but it is far easier to learn in four snapshots. Each has one big event that gives it away, and if you can name that event you can name the phase — in an essay or under a microscope.
📘 What you need to know
Mitosis produces two genetically identical daughter nuclei, also identical to the parent nucleus and with the same chromosome number.
It occurs wherever identical nuclei are needed: embryonic development, growth, tissue repair and asexual reproduction.
The four phases are prophase, metaphase, anaphase, telophase — remember PMAT.
Prophase: chromosomes condense, centrosomes move to opposite poles, spindle fibres form, the nuclear envelope breaks down into vesicles and the nucleolus disappears.
Metaphase: chromosomes line up at the equator and spindle fibres attach to their centromeres via kinetochores, each sister chromatid attached to an opposite pole.
Anaphase: centromeres split, spindle fibres shorten, and chromatids are pulled to opposite poles.
Telophase: chromosomes decondense, nuclear envelopes re-form, spindle fibres break down and new nucleoli appear.
Why mitosis matters
Whenever a eukaryotic cell needs to produce nuclei that are genetically identical to the original, it uses mitosis. That covers a great deal: an embryo developing, an organism growing, a wound healing, dead skin cells being replaced, and organisms reproducing asexually.
The key outcome is that the genome is preserved exactly. Every cell in your body carries the same DNA precisely because mitosis copies it faithfully every time.
The four phases
Prophase
Chromosomes condense by supercoiling, and become visible when stained.
Each chromosome now consists of two identical sister chromatids, each containing one DNA molecule, joined at the centromere.
The two centrosomes (replicated in G2, just before prophase) move towards opposite poles.
Spindle fibres, made of protein microtubules, begin to emerge from the centrosomes.
The nuclear envelope breaks down into small vesicles, and the nucleolus disappears.
Metaphase
The centrosomes reach opposite poles and spindle fibres continue to extend from them.
Chromosomes line up at the equator of the spindle, also called the metaphase plate, so they are equidistant from both poles.
Spindle fibres reach the chromosomes and attach to the centromeres, using specific proteins called kinetochores.
Each sister chromatid is attached to a fibre from an opposite pole — which is what makes the next step work.
Anaphase
The sister chromatids separate at the centromere, which divides in two.
The spindle fibres shorten.
The separated chromatids — now called chromosomes — are pulled to opposite poles.
Telophase
Chromosomes arrive at the poles and begin to decondense.
Nuclear envelopes re-form around each set of chromosomes.
The spindle fibres break down.
New nucleoli form inside each nucleus.
Only four chromosomes are drawn here for clarity. Human cells have 46, and the colours simply show which parent each came from.
The anaphase V
Anaphase chromosomes have a distinctive look, and it comes straight from the mechanics. The spindle fibre is attached at the centromere, so that is the part that gets pulled first. The chromosome’s arms trail behind it through the cytoplasm, giving the characteristic V shape.
This is worth remembering for the practical: a V-shaped chromosome in a micrograph is a strong sign of anaphase.
Worked examples
WORKED EXAMPLE
Naming the phase
In a cell, the nuclear envelope has disappeared, the chromosomes are equidistant from both poles, and spindle fibres are attached to their centromeres. Name the phase and justify your answer.
Step 1: Use the strongest clue
“Equidistant from both poles” means the chromosomes are lined up at the equator.
Step 2: Check the supporting detail
Spindle fibres attached at the centromeres fits the same phase.
Step 3: Rule out the neighbours
In prophase the fibres have not attached yet; in anaphase the chromatids are already separating.
Metaphase“Justify” means quote the evidence from the stem, not just name the phase.
WORKED EXAMPLE
Counting through the phases
A human cell (46 chromosomes) is undergoing mitosis. State the number of chromosomes and chromatids present in the cell at metaphase, and the number of chromosomes at each pole at the end of anaphase.
Step 1: Metaphase
Each chromosome still has two chromatids joined at one centromere.
46 chromosomes, 92 chromatidsStep 2: End of anaphase
Centromeres have split, so all 92 chromatids are now chromosomes, half at each pole.
92 ÷ 2 = 46Metaphase: 46 chromosomes and 92 chromatids. Each pole: 46 chromosomesThe daughter cells end up with 46 — exactly what the parent had.
WORKED EXAMPLE
Explaining an error
In a cell, both sister chromatids of one chromosome become attached to spindle fibres from the same pole. Suggest the consequence.
Step 1: What normally happens
Each sister chromatid attaches to a fibre from an opposite pole, so they are pulled apart.
Step 2: What happens here
Both are pulled to the same pole.
Step 3: Consequence
One daughter nucleus gains an extra chromosome and the other is missing one, so they are no longer genetically identical.
Daughter cells with the wrong chromosome numberThis is the same underlying problem as non-disjunction in meiosis.
💡 Exam tip
Learn one headline event per phase. Condense, line up, split, re-form. Then add detail.
Say the spindle attaches at the centromere, via kinetochores — the precise wording earns marks.
In anaphase, be careful: the fibres shorten, they do not “snap” or “pull the cell apart”.
Cytokinesis is not a phase of mitosis. It follows telophase.
State the outcome of mitosis fully: two nuclei, genetically identical, with the same chromosome number.
Use PMAT to check your order before you start writing.
⚠ Common mix-up
Saying chromosomes replicate during prophase. Replication happened earlier, in S phase.
Confusing centromere and centrosome. The centromere joins chromatids; the centrosome organises the spindle at the pole.
Saying the nuclear envelope “disappears”. It breaks down into small vesicles and is rebuilt in telophase.
Putting metaphase and anaphase the wrong way round. Line up first, then separate.
Claiming mitosis produces two cells. It produces two nuclei; cytokinesis produces the cells.
Forgetting that mitosis maintains the chromosome number. It is not a halving division.
Up next: Mitosis (Skills) — theory into practice. You will be given a micrograph and asked which phase a cell is in, so let us look at exactly what each stage looks like down a microscope.
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