Mitosis is one continuous process, but it is much easier to learn in four steps. Get the order right, know what the chromosomes are doing in each step, and name the structures properly — that is most of the marks on this topic.
📘 What you need to know
Mitosis produces two genetically identical daughter nuclei, identical to the parent nucleus and with the same chromosome number.
It happens whenever identical nuclei are needed: embryonic development, growth, tissue repair and asexual reproduction.
The four phases in order are prophase, metaphase, anaphase, telophase — remember PMAT.
Prophase: chromosomes condense, centrosomes move to opposite poles, spindle forms, nuclear envelope and nucleolus break down.
Metaphase: chromosomes line up at the equator and spindle fibres attach to the centromeres at kinetochores.
Anaphase: centromeres split, spindle fibres shorten, chromatids are pulled to opposite poles. Telophase: chromosomes decondense and nuclear envelopes reform.
What mitosis is for
Mitosis is the type of nuclear division used whenever a eukaryotic cell needs to produce nuclei that are exact copies. The daughter nuclei have the same number of chromosomes as the parent, and the same genes.
That is exactly what you want for growing, for healing a wound, for replacing worn-out cells, and for organisms that reproduce asexually. Nothing about those jobs calls for variety.
Learn the orderProphase →Metaphase →Anaphase →Telophase
Follow one chromosome through all four panels. It condenses, lines up, is split at the centromere, then unwinds again inside a brand new nucleus.
Prophase
Chromosomes condense, so they become visible when stained.
Each chromosome consists of two identical sister chromatids joined at the centromere, each containing one DNA molecule.
The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles of the nucleus.
Spindle fibres, which are protein microtubules, begin to emerge from the centrosomes. In animal cells each centrosome contains two centrioles.
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 the two poles.
Spindle fibres reach the chromosomes and attach to the centromeres. The attachment involves specific proteins called kinetochores.
Each sister chromatid is attached to a fibre coming from the opposite pole — which is what makes the next step work.
That last point is the one worth understanding rather than memorising. Because the two chromatids are pulled from opposite directions, each daughter cell is guaranteed to get exactly one copy of every chromosome.
Anaphase
The sister chromatids separate at the centromere, which divides in two.
The spindle fibres shorten.
The separated chromatids — now called chromosomes again — are pulled to opposite poles, centromere first.
Telophase
Chromosomes arrive at opposite poles and begin to decondense (uncoil).
Nuclear envelopes reform around each of the two sets of chromosomes.
The spindle fibres break down.
New nucleoli form inside each nucleus.
Nuclear division is now complete: two genetically identical nuclei. Cytokinesis follows, splitting the cytoplasm to give two separate daughter cells.
Phase
Chromosomes
Other structures
Prophase
Condense and become visible, each as two sister chromatids
Envelope breaks down, nucleolus disappears, spindle begins to form
Metaphase
Line up along the equator
Spindle fibres attach to centromeres at kinetochores
Anaphase
Centromeres split; chromatids pulled to opposite poles
Spindle fibres shorten
Telophase
Arrive at the poles and decondense
Nuclear envelopes reform, nucleoli reappear, spindle breaks down
Worked examples
WORKED EXAMPLE
Name the phase
Name the phase of mitosis in which each of the following happens. (a) The nuclear envelope breaks into small vesicles. (b) Chromosomes are equidistant from the two poles. (c) Each centromere divides in two.
(a) Envelope breaks downProphase(b) Equidistant from the poles
That is the description of lining up at the equator.
Metaphase(c) Centromere dividesAnaphaseProphase, metaphase, anaphaseLook for the one structure the statement is about, then match it to your PMAT list.
WORKED EXAMPLE
Count the DNA
A cell has 8 chromosomes before mitosis begins. (a) How many chromatids are present during metaphase? (b) How many chromosomes will each daughter nucleus contain?
(a) At metaphase
Each of the 8 chromosomes still consists of two sister chromatids.
8 × 2 = 16 chromatids(b) In each daughter nucleus
Mitosis maintains the chromosome number, and each daughter gets one chromatid from every chromosome.
8 chromosomes16 chromatids, then 8 chromosomes eachThe chromosome number does not double at replication — the number of chromatids does.
WORKED EXAMPLE
Explain the effect of a drug
A drug prevents spindle fibres from forming. Predict and explain its effect on mitosis.
Step 1: What the spindle does
Spindle fibres attach to the centromeres and line the chromosomes up at the equator, then shorten to pull the chromatids apart.
Step 2: What goes wrong without it
Chromosomes cannot be arranged at the equator, so metaphase cannot be completed and the chromatids are never separated.
Step 3: The outcome
Mitosis stops before anaphase, so no identical daughter nuclei are produced.
Division halts around metaphaseDrugs like this are used in cancer treatment, because they stop rapidly dividing cells.
💡 Exam tip
PMAT gets you the order every time. Write it in the margin before you start a mitosis question.
Say equator or metaphase plate, and poles rather than “ends” or “sides”.
Name the kinetochores when describing how spindle fibres attach. It is the detail that separates good answers.
Be careful with chromosome counting: replication doubles the chromatids, not the chromosome number.
Do not include cytokinesis as a phase of mitosis. Mitosis is nuclear division; cytokinesis comes after.
For a “describe mitosis” question, give the phases in order with two or three points each rather than everything you know about one phase.
⚠ Common mix-up
Calling cytokinesis the fifth phase of mitosis. It is a separate process that follows nuclear division.
Saying chromosomes line up in metaphase in “a line down the middle of the nucleus”. By metaphase the nuclear envelope has gone.
Mixing up centrosome, centromere and centriole. Centrosomes organise the spindle; the centromere joins sister chromatids; centrioles sit inside animal centrosomes.
Thinking the chromosome number halves in mitosis. It is maintained — halving happens in meiosis.
Writing that chromosomes “split” in anaphase. The centromere splits and the chromatids separate.
Forgetting telophase details. Envelope reforming, spindle breaking down and nucleoli reappearing are all creditworthy.
Up next: Mitosis (Skills) — how to spot each phase in a real micrograph, and how to calculate a mitotic index.
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