Difficulty: Introductory | Prerequisites: Part 1 of these notes (Cell Division and Interphase). You should already understand the cell cycle phases (G1, S, G2), what sister chromatids are, and the difference between prokaryotic and eukaryotic division.
Once a cell has copied its DNA during interphase, it needs to distribute those copies accurately and then physically split in two. That is the job of mitosis (nuclear division) and cytokinesis (cytoplasmic division). This topic covers the five stages of mitosis, the machinery that moves chromosomes (the mitotic spindle), and how animal and plant cells divide their cytoplasm differently. Mistakes here cause cells with the wrong number of chromosomes, which links directly to cancer and genetic disorders covered later in the course.
Mitosis has five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Chromosomes condense, line up at the cell's equator, and are pulled apart by the mitotic spindle. Cytokinesis then splits the cytoplasm: animal cells pinch inward (cleavage furrow), plant cells build a new wall down the middle (cell plate).
Mitosis
The division of the cell's nucleus into two genetically identical nuclei. In simple terms, it is the process that ensures each daughter cell receives a complete copy of every chromosome.
Cytokinesis
The division of the cytoplasm following mitosis, producing two separate daughter cells. Think of it as the physical splitting once the nuclear contents have been sorted.
Prophase
The first stage of mitosis. Chromosomes condense (become visible), the nuclear envelope begins to break down, and the mitotic spindle starts forming.
Prometaphase
The stage after prophase in which the nuclear envelope fully disintegrates, kinetochores form at each centromere, and spindle fibres attach to the kinetochores.
Metaphase
The stage in which all chromosomes align along the metaphase plate (the cell's equator), with kinetochore microtubules attached from both poles.
Anaphase
The stage in which cohesin proteins are cleaved by the enzyme separase, allowing sister chromatids to separate and move toward opposite poles of the cell.
Telophase
The final stage of mitosis. Nuclear envelopes re-form around each set of chromosomes, the chromosomes decondense back into chromatin, the nucleolus reappears, and the spindle breaks down.
Mitotic spindle
The structure of microtubules that separates the chromosomes during mitosis. It assembles from the centrosomes (microtubule organising centres, or MTOCs).
Centrosome (MTOC)
The microtubule organising centre. It replicates during interphase, and the two copies migrate to opposite poles of the cell to anchor the spindle.
Kinetochore
A protein complex that forms at the centromere of each chromatid and serves as the attachment point for spindle microtubules. Think of it as the handle the spindle grabs onto.
Kinetochore microtubules
Spindle fibres that attach directly to the kinetochore. These are the ropes that pull the chromatids apart.
Polar (non-kinetochore) microtubules
Spindle fibres from opposite poles that overlap at the cell's centre and push against each other, elongating the cell.
Astral microtubules
Spindle fibres that anchor the centrosome to the cell membrane at each pole.
Centrioles
Structures found only in animal cells that sit within the centrosome and help organise spindle formation.
Separase
The enzyme that cleaves cohesin proteins during anaphase, releasing the sister chromatids.
Cleavage furrow
The indentation formed by a contractile ring of microfilaments that pinches an animal cell in two during cytokinesis.
Cell plate
The structure that forms during cytokinesis in plant cells, built from vesicles that fuse at the centre of the cell and eventually become a new cell wall.
Kinesin
A motor protein that generally moves cargo from the centre of the cell toward the periphery (toward the plus end of microtubules).
Dynein
A motor protein that generally moves cargo from the periphery toward the centre (toward the minus end of microtubules). Dynein accounts for roughly 75% of the force that moves chromatids during anaphase.
Mitosis is the portion of the cell cycle in which the duplicated chromosomes are separated and distributed into two identical nuclei. It follows interphase and is itself followed by cytokinesis. The five stages, in order: prophase, prometaphase, metaphase, anaphase, telophase. A useful mnemonic: Please Pass Me Another Taco.
Prophase
Chromatin condenses into visible chromosomes (each consisting of two sister chromatids, the "dyads").
The mitotic spindle begins to form as microtubules extend from the centrosomes.
The centrosomes start migrating toward opposite poles of the cell.
Kinetochores begin to appear at the centromere of each chromosome.
The nucleolus disappears.
Prometaphase
The nuclear envelope breaks apart (disintegrates into fragments).
Kinetochores are now fully formed at the centromere of each sister chromatid.
Spindle microtubules from opposite poles attach to the kinetochores. Each sister chromatid's kinetochore connects to microtubules from one pole, so the pair is linked to both poles.
Non-kinetochore (polar) microtubules extend across the cell and begin overlapping at the centre.
Metaphase
All chromosomes (as dyads of sister chromatids) align along the metaphase plate, the imaginary plane at the cell's equator.
The centromere-kinetochore complexes are under equal tension from microtubules on both sides.
This alignment is critical: the cell checks (via the spindle assembly checkpoint) that every kinetochore is properly attached before proceeding.
Anaphase
The enzyme separase cleaves the cohesin proteins holding the sister chromatids together.
The chromatids separate (chromatid disjunction) and are pulled toward opposite poles along kinetochore microtubules.
Once separated, each chromatid is now called a chromosome.
The force that moves them comes from two sources:
Roughly 75% from the motor protein dynein "walking" the chromosome along the microtubule (the "Pac-Man" mechanism, with the microtubule depolymerising behind the motor protein).
Roughly 25% from shortening of the microtubule at the centrosome end.
The polar microtubules push against each other, elongating the cell.
Telophase
The separated chromosomes arrive at opposite poles.
A new nuclear envelope re-forms around each set of chromosomes.
The chromosomes decondense back into chromatin.
The nucleolus reappears in each new nucleus.
The mitotic spindle disassembles.
Non-kinetochore microtubules from opposite poles overlap and push against each other, helping to elongate the cell further.
Cytokinesis
Cytokinesis is the physical division of the cytoplasm, producing two separate daughter cells. It typically overlaps with telophase.
In animal cells: a contractile ring of microfilaments (actin and myosin) forms just beneath the plasma membrane at the cell's equator. The ring contracts, pulling the membrane inward and creating a cleavage furrow. The furrow deepens until the cell is pinched in two.
In plant cells: vesicles from the Golgi apparatus gather at the centre of the cell and fuse to form a cell plate. The cell plate grows outward until it reaches the existing cell wall, dividing the cell. A new cell wall then forms on each side of the plate.
The key difference: animal cells pinch from the outside in; plant cells build a wall from the inside out.
The mitotic spindle is the machine that physically moves chromosomes. It is built from microtubules, which are polymers of the protein tubulin.
Spindle assembly
The centrosome (MTOC) is the organising centre. It replicates during S phase of interphase.
During prophase, the two centrosomes migrate to opposite poles of the cell, trailing microtubules between them.
In animal cells, each centrosome contains a pair of centrioles. Plant cells lack centrioles but still form functional spindles.
Three types of spindle microtubules
Kinetochore microtubules: attach to the kinetochore at each chromatid's centromere. These are the fibres that pull chromatids to the poles during anaphase.
Polar (non-kinetochore) microtubules: extend from opposite poles and overlap at the cell centre. They push against each other to elongate the cell.
Astral microtubules: radiate outward from each centrosome to the cell membrane, anchoring the spindle in position.
Motor proteins
Microtubules serve as tracks for two families of motor proteins:
Kinesin walks toward the plus end (generally from the cell centre toward the periphery).
Dynein walks toward the minus end (generally from the periphery toward the cell centre).
During anaphase, motor proteins on the kinetochore (primarily dynein) "walk" the chromosome along the microtubule. As the motor protein advances, the microtubule behind it depolymerises (the "Pac-Man" mechanism). About 75% of the force comes from dynein walking; about 25% comes from microtubule shortening at the centrosome end.
Students often confuse "centromere" and "centrosome." The centromere is the region on a chromosome where sister chromatids are joined. The centrosome is the organelle that organises the spindle. Different structures, different roles.
Some students think cytokinesis is part of mitosis. Mitosis is nuclear division only (prophase through telophase). Cytokinesis is a separate, overlapping process.
Students sometimes think the cleavage furrow is a plant cell structure because "cleavage" sounds like cutting. The cleavage furrow is animal cells only. Plant cells form a cell plate.
Many students forget prometaphase entirely and list only four stages. For BIO 311C, there are five stages: prophase, prometaphase, metaphase, anaphase, telophase.
Be able to list all five stages of mitosis in order and describe the key event of each.
Know which structures are present at each stage: when does the nuclear envelope break down? When does it re-form? When do kinetochores appear?
Understand the three types of spindle microtubules and the role of each.
Be able to compare cytokinesis in animal cells (cleavage furrow, contractile ring) vs. plant cells (cell plate, vesicles).
Know the role of separase and cohesin in anaphase.
The 75/25 split for chromatid movement (dynein vs. microtubule shortening) is the kind of detail that turns up in multiple-choice questions.
True or false: The nuclear envelope is intact during metaphase. (False, it breaks down during prometaphase.)
Fill in the blank: The enzyme that cleaves cohesin during anaphase is called ___. (separase)
True or false: Plant cells divide their cytoplasm using a cleavage furrow. (False, plant cells use a cell plate.)
Fill in the blank: Centrioles are found only in ___ cells. (animal)
True or false: Kinetochore microtubules attach to the centromere via the kinetochore protein complex. (True)
Q: List the five stages of mitosis in order.
A: Prophase, prometaphase, metaphase, anaphase, telophase.
Q: What happens to the nuclear envelope during prometaphase, and when does it reappear?
A: The nuclear envelope disintegrates during prometaphase, allowing spindle fibres to access the chromosomes. It re-forms during telophase around each set of separated chromosomes.
Q: Name the three types of spindle microtubules and state the function of each.
A: Kinetochore microtubules attach to kinetochores and pull chromatids to the poles. Polar (non-kinetochore) microtubules overlap at the centre and push against each other to elongate the cell. Astral microtubules anchor the spindle to the cell membrane.
Q: Compare cytokinesis in animal and plant cells.
A: Animal cells use a contractile ring of microfilaments that pinches inward to form a cleavage furrow. Plant cells form a cell plate from Golgi-derived vesicles that fuse at the cell centre and grow outward to become a new cell wall.
Q: Describe the "Pac-Man" mechanism of chromosome movement during anaphase.
A: Motor proteins (mainly dynein) on the kinetochore walk the chromosome along the microtubule toward the centrosome. As the motor protein advances, the microtubule depolymerises behind it. About 75% of the force is from dynein; about 25% is from microtubule shortening at the centrosome end.
Q: What is the role of separase in anaphase?
A: Separase is an enzyme that cleaves cohesin, the protein complex holding sister chromatids together. Once cohesin is cut, the sister chromatids are free to move to opposite poles.
Mitosis connects back to Part 1 of these notes (interphase and DNA replication) and forward to meiosis, where many of the same structures (spindle, kinetochore, cohesin) are reused but with key differences that produce genetic variation. Understanding how the spindle checkpoint works also feeds directly into the cancer biology unit: most cancers involve mutations in genes that regulate the cell cycle or the spindle assembly checkpoint.
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