Difficulty: Introductory | Prerequisites: Cell Division Foundations notes (vocabulary, cell cycle overview).
Mitosis is how your body grows from a single fertilised egg into trillions of cells, and how it repairs damaged tissue throughout your life. It is the division of the nucleus into two genetically identical copies, followed by cytokinesis (splitting the cytoplasm). Every somatic cell in your body was produced by mitosis.
This topic builds directly on the cell cycle and chromosome vocabulary from the Foundations notes. If you are not comfortable with the terms diploid, chromatid and centromere, review those first.
Mitosis takes one diploid cell and produces two genetically identical diploid daughter cells. It proceeds through four stages (prophase, metaphase, anaphase, telophase), followed by cytokinesis. The mnemonic PMAT helps you remember the order. The purpose is growth, repair and asexual reproduction.
Prophase
The first stage of mitosis. Chromatin condenses into visible chromosomes, the nuclear envelope breaks down, centrioles move to opposite poles and spindle fibres begin to form. Think of it as the cell packing up its office before the move.
Metaphase
The stage in which replicated chromosomes (each consisting of two sister chromatids) line up along the metaphase plate, the imaginary equator of the cell. Spindle fibres from each pole attach to the centromere of each chromosome.
Anaphase
The stage in which sister chromatids are pulled apart at the centromere and moved to opposite poles by the spindle fibres. Each separated chromatid is now considered its own chromosome.
Telophase
The final stage of mitosis. Chromosomes de-condense back into chromatin, nuclear envelopes re-form around each set of chromosomes, and the cell now has two distinct nuclei.
Cytokinesis
The division of the cytoplasm that follows mitosis, physically splitting the cell into two daughter cells. In animal cells this involves a cleavage furrow; in plant cells, a cell plate.
Spindle fibres (microtubules)
Protein filaments that extend from the centrioles and attach to chromosomes at the centromere. They are the machinery that physically pulls chromatids apart during anaphase.
Metaphase plate (equatorial plate)
The imaginary plane running through the middle of the cell where chromosomes align during metaphase. In simple terms, it is the halfway line.
Centrioles
Small cylindrical organelles that move to opposite poles of the cell and organise the spindle fibres. Found in animal cells.
Mitosis is always preceded by interphase (G1, S, G2). By the time a cell enters prophase, its DNA has already been replicated in S phase and it has two copies of every chromosome, held together as sister chromatids.
Chromatin condenses into tightly coiled chromosomes, making them visible under a microscope.
The nuclear envelope breaks down and disappears.
Centrioles migrate to opposite poles of the cell.
Spindle fibres (microtubules) begin to form between the two poles.
Prophase is the longest phase of mitosis.
Replicated chromosomes (each consisting of two sister chromatids joined at the centromere) line up along the metaphase plate.
Spindle fibres from each pole attach to the centromere of each chromosome.
This alignment ensures that when the chromatids separate, each pole will receive one complete set.
The centromeres split.
Spindle fibres shorten, pulling sister chromatids apart toward opposite poles.
Once separated, each chromatid is now called a chromosome in its own right.
This is the critical step: errors here (non-disjunction) can lead to daughter cells with the wrong number of chromosomes.
Chromosomes arrive at opposite poles and begin to de-condense back into chromatin.
A new nuclear envelope forms around each set of chromosomes.
The cell now contains two distinct nuclei, each with a full diploid set of chromosomes.
The cytoplasm divides, producing two separate daughter cells.
In animal cells, a cleavage furrow pinches the cell in two.
In plant cells, a cell plate forms across the middle and new cell wall material is deposited.
The result: two genetically identical diploid cells, each capable of entering the cell cycle again.
Cancer is, at its simplest, uncontrolled cell division. Normal cells respond to growth-regulating signals; cancer cells do not, and they form masses called malignant tumours.
One way cancer can arise is through mistakes during anaphase. If sister chromatids fail to separate correctly (non-disjunction), daughter cells end up with the wrong number of chromosomes, a condition called aneuploidy. Aneuploidy disrupts normal gene regulation and is a hallmark of many cancers.
Students often say "the cell divides in mitosis." Mitosis is the division of the nucleus only. The cell as a whole divides during cytokinesis, which is a separate (though closely linked) process.
A frequent error is listing interphase as a stage of mitosis. Interphase precedes mitosis but is not part of it. The stages of mitosis are prophase, metaphase, anaphase and telophase.
Students sometimes believe that after mitosis, the two daughter cells cannot divide again. They can. Diploid daughter cells are fully capable of re-entering the cell cycle at G1.
Some students confuse what separates in anaphase of mitosis with what separates in anaphase I of meiosis. In mitosis, sister chromatids separate. In meiosis I, homologous chromosomes separate. This distinction is heavily tested.
Know the PMAT mnemonic (Prophase, Metaphase, Anaphase, Telophase) and what happens at each stage.
Be able to identify stages from a diagram or micrograph. Look for: condensed chromosomes but intact nucleus = late prophase; chromosomes lined up at the middle = metaphase; V-shaped chromosomes being pulled apart = anaphase; two forming nuclei = telophase.
Understand that mitosis produces two identical diploid cells. If a question asks how many cells result, or whether they are identical, this is the answer.
Know the connection between anaphase errors and cancer (aneuploidy).
Be clear on what separates in mitosis (sister chromatids) versus meiosis I (homologous chromosomes).
True or false: Prophase is the shortest stage of mitosis.
Fill in the blank: During metaphase, chromosomes line up along the ____.
True or false: After mitosis, both daughter cells are diploid.
Fill in the blank: In anaphase, ____ are pulled to opposite poles.
True or false: Cytokinesis is considered a stage of mitosis.
Answers: 1. False (prophase is the longest). 2. Metaphase plate. 3. True. 4. Sister chromatids. 5. False (cytokinesis follows mitosis but is a separate process).
Q: List the four stages of mitosis in order and state the key event of each.
A: Prophase (chromatin condenses, nuclear envelope breaks down, spindle forms), Metaphase (chromosomes align at the metaphase plate), Anaphase (sister chromatids separate and move to opposite poles), Telophase (chromosomes de-condense, nuclear envelopes re-form).
Q: A human somatic cell undergoes mitosis. How many chromosomes does each daughter cell contain?
A: 46 (the same as the parent cell, because mitosis preserves the diploid number).
Q: What structure attaches spindle fibres to chromosomes, and at what stage does this attachment occur?
A: The centromere. Spindle fibres attach during metaphase.
Q: Explain how an error in anaphase could contribute to cancer.
A: If sister chromatids fail to separate properly (non-disjunction), daughter cells receive an abnormal number of chromosomes (aneuploidy). This can disrupt genes that regulate cell growth, potentially leading to uncontrolled division and tumour formation.
Q: Compare cytokinesis in animal cells and plant cells.
A: In animal cells, a cleavage furrow pinches the cell membrane inward until the cell splits in two. In plant cells, a cell plate forms at the midline and new cell wall material is deposited, because the rigid cell wall prevents pinching.
Mitosis connects directly to meiosis: meiosis II is mechanically identical to mitosis (sister chromatids separate), so understanding one makes the other easier.
It also connects to cancer biology and cell-cycle regulation. The checkpoints that control when a cell may proceed from G1 to S, and from G2 into mitosis, are a major topic in later units.
Developmental biology relies heavily on mitosis. Embryonic development, tissue differentiation and wound healing all depend on precise, regulated mitotic divisions.
mitosis, PMAT, prophase, metaphase, anaphase, telophase, cytokinesis, cell division, nuclear division, spindle fibres, spindle apparatus, microtubules, centrioles, centromere, metaphase plate, equatorial plate, sister chromatids, cleavage furrow, cell plate, diploid daughter cells, somatic cell division, asexual reproduction, cell growth and repair, non-disjunction, aneuploidy, cancer and mitosis, BIO 101, intro biology