Difficulty: Introductory | Prerequisites: Cell Cycle Overview and Phases study notes
This set of notes covers the mechanics of how a cell actually divides: the five stages of mitosis, how the mitotic spindle separates chromosomes, the differences between cytokinesis in animal and plant cells, and how bacterial binary fission compares with eukaryotic division. You should be comfortable with the cell cycle phases (G1, S, G2, M) and key structures (centromere, kinetochore, centrosome) before working through this.
Mitosis splits the nucleus in five stages (prophase, prometaphase, metaphase, anaphase, telophase), each defined by what the chromosomes and spindle are doing. Cytokinesis then divides the cytoplasm, using a cleavage furrow in animal cells or a cell plate in plant cells. Bacteria skip mitosis entirely and divide by binary fission, using a ring of FtsZ protein instead of a spindle.
Prophase
The first stage of mitosis. Chromatin condenses into visible chromosomes, each consisting of two sister chromatids. The mitotic spindle begins to form from tubulin subunits, and cohesins hold the chromatids together. Think of it as the "packaging and prep" stage.
Prometaphase
The nuclear envelope fragments, allowing spindle microtubules to enter the nuclear region and attach to kinetochores on the chromosomes. In simple terms, the barrier comes down and the spindle grabs hold.
Metaphase
Chromosomes align along the metaphase plate (the cell's equator). Kinetochores are motionless relative to the poles. This is the "line them up" stage, and the spindle checkpoint fires here.
Anaphase
Cohesin proteins break down, centromeres split, and sister chromatids separate into individual chromosomes. Kinetochore microtubules shorten, pulling the chromosomes toward opposite poles, while nonkinetochore microtubules lengthen, pushing the poles apart and elongating the cell.
Telophase
Chromosomes arrive at the poles and begin to de-condense. Nuclear envelopes re-form around each set of chromosomes. Spindle microtubules disassemble. Cytokinesis typically begins during this stage.
Cohesin
A protein complex that holds sister chromatids together from S phase through to anaphase. Its breakdown is what allows the chromatids to separate.
Tubulin
The protein subunit from which microtubules are built. Tubulin assembles into spindle microtubules during prophase.
Kinetochore microtubules
Spindle fibres that attach to kinetochores on the chromosomes. They shorten during anaphase to pull chromosomes poleward.
Nonkinetochore microtubules
Spindle fibres that do not attach to chromosomes. They overlap at the cell's midline and lengthen during anaphase, pushing the two poles apart and elongating the cell.
Cleavage furrow
The inward pinching of the plasma membrane in animal cells during cytokinesis, formed by a ring of actin microfilaments contracting. This physically splits the cytoplasm in two.
Cell plate
The structure that forms during cytokinesis in plant cells. Vesicles from the Golgi apparatus coalesce along microtubules at the plane of division, building a new plasma membrane and cell wall between the two daughter cells.
Binary fission
The method of cell division in bacteria. The single circular chromosome replicates, the copies move apart, and a ring of FtsZ protein constricts the cell at the midpoint, dividing it into two daughter cells. No mitotic spindle is involved.
FtsZ
A bacterial protein structurally similar to eukaryotic tubulin. It forms a ring at the division site that constricts to pinch the cell in two during binary fission.
Prophase: Chromatin condenses into distinct chromosomes (each made of two sister chromatids joined by cohesins). Tubulins begin assembling into spindle microtubules from the centrosomes. Nucleoli disappear.
Prometaphase: The nuclear envelope breaks apart. Spindle microtubules extend into the former nuclear region and attach to kinetochores on the chromosomes. This attachment is why the nuclear envelope must disappear: it is a physical barrier between the spindle and the chromosomes.
Metaphase: All chromosomes are aligned at the metaphase plate, with kinetochores motionless relative to the poles. The spindle checkpoint verifies that every chromosome is properly attached before the cell proceeds.
Anaphase: Cohesins are cleaved, centromeres split, and each sister chromatid becomes an independent chromosome. Kinetochore microtubules shorten, pulling chromosomes toward the poles. Simultaneously, nonkinetochore microtubules lengthen and slide past each other, pushing the poles apart and elongating the cell. This is the stage where chromatids become chromosomes.
Telophase: Chromosomes reach the poles and begin de-condensing back into chromatin. Nuclear envelopes re-form. Spindle microtubules (both kinetochore and nonkinetochore) disassemble. The cell now contains two complete nuclei.
The mitotic spindle is the machinery that separates chromosomes. It is built from microtubules extending from the two centrosomes.
During prophase, microtubules lengthen as tubulin subunits are added.
By metaphase, kinetochore microtubules have attached to each pair of sister chromatids, and nonkinetochore microtubules from opposite poles overlap at the metaphase plate.
During anaphase, kinetochore microtubules shorten (pulling chromosomes poleward) while nonkinetochore microtubules lengthen (pushing poles apart). Motor proteins at the kinetochores use ATP to walk along the microtubules.
By the end of telophase, all spindle microtubules have disassembled.
A key exam scenario: if motor proteins in the kinetochores are inhibited but the kinetochore remains attached and nonkinetochore microtubules are unaffected, the cell will still elongate (nonkinetochore microtubules push the poles apart), sister chromatids will still separate (cohesins are still cleaved), but chromosomes will not move to the poles (motor proteins cannot pull them along the microtubules).
Animal cells divide the cytoplasm by constriction. A contractile ring of actin microfilaments pinches the plasma membrane inward, forming a cleavage furrow that deepens until the cell splits in two.
Plant cells cannot pinch inward because of their rigid cell wall. Instead, vesicles from the Golgi apparatus move along microtubules to the plane of division and coalesce to form a cell plate. The cell plate grows outward, fusing with the parent cell's plasma membrane. Membrane from the vesicles becomes the new plasma membrane, and materials inside the vesicles form the new cell wall between the daughter cells.
Key distinctions for exams:
Bacteria and animal cells both divide by constriction of a protein ring (FtsZ in bacteria, actin in animals). Plant cells do not.
Plant cells use a cell plate. Animal cells and bacteria do not.
Tubulin or tubulin-like molecules (FtsZ) function in both binary fission and plant cytokinesis, but not in animal cell cytokinesis (which relies on actin, not tubulin).
Bacteria have a single, circular chromosome and no mitotic spindle. In both bacteria and eukaryotes, chromosomes replicate before division, and the replicated copies separate by attaching to structural features of the cell.
Differences that are eukaryote-only: chromosomes condense before separation; the nuclear envelope fragments to permit spindle attachment; two copies of each chromosome are joined at centromeres as sister chromatids before separating.
Bacteria-only: chromosome separation begins at the origin of replication on the DNA.
Students often mix up prometaphase and metaphase. Prometaphase is when the envelope breaks down and microtubules first attach; metaphase is when all chromosomes are lined up and motionless at the plate. Attachment happens first, alignment second.
Students often think chromatids become chromosomes during telophase. They become chromosomes during anaphase, when the centromere splits. By telophase they are already individual chromosomes and are de-condensing.
Students sometimes believe plant cells divide by constriction like animal cells. They do not. The cell wall prevents constriction, so plant cells build a cell plate from the inside out.
Students often assume DNA condensation tracks with sister chromatid presence. It does not. Sister chromatids exist from S phase onward, but DNA does not condense until prophase. Condensation and chromatid presence are independent.
Know which events define each stage of mitosis. Fill-in-the-blank questions matching events to stages are extremely common.
Understand the spindle's two types of microtubules and what each does during anaphase (kinetochore shorten, nonkinetochore lengthen). Scenario questions about inhibitors or drugs that block one type are a favourite.
Be able to distinguish animal cytokinesis (cleavage furrow, actin) from plant cytokinesis (cell plate, Golgi vesicles). This is often tested as a "select all that apply" question.
Know the bacterial vs eukaryotic division comparison table. What is shared (replication before division, attachment to cell structures) and what differs (condensation, nuclear envelope, spindle) appears regularly.
Motor proteins at kinetochores require ATP. This is a straightforward recall question that turns up often.
Fill in the blank: Spindle fibres attach to kinetochores during ______. (Prometaphase.)
True or false: During anaphase, nonkinetochore microtubules shorten. (False. They lengthen.)
Fill in the blank: In plant cells, cytokinesis is accomplished by the formation of a ______. (Cell plate.)
True or false: Bacteria use a mitotic spindle to separate their chromosomes. (False. They use FtsZ protein rings.)
Fill in the blank: During prophase, a homologous pair of chromosomes consists of 2 chromosomes and ______ chromatids. (4.)
Q: During which phase of mitosis do chromatids become chromosomes?
A: Anaphase. When cohesins are cleaved and centromeres split, each sister chromatid becomes an independent chromosome.
Q: What is the correct chromosomal condition for one daughter nucleus at telophase of mitosis, starting from a diploid cell with 2 pairs of homologous chromosomes (2n = 4)?
A: Each daughter nucleus has 4 chromosomes (2n = 4), one of each type from each homologous pair, all un-duplicated (no sister chromatids). The daughter cell is genetically identical to the parent.
Q: If motor proteins in the kinetochores are inhibited during metaphase but the kinetochore remains attached to the spindle, and nonkinetochore microtubules are unaffected, will the cell elongate? Will sister chromatids separate? Will chromosomes move to the poles?
A: The cell will elongate (nonkinetochore microtubules still push the poles apart). Sister chromatids will still separate (cohesin breakdown is independent of motor proteins). Chromosomes will not move to the poles (the motor proteins that walk them along kinetochore microtubules are blocked).
Q: Which of the following statements are true of cytokinesis in plant cells? (a) Microfilaments form a contractile ring. (b) The plasma membrane grows inward to join the cell plate. (c) The cell plate consists of new plasma membrane and cell wall. (d) Golgi vesicles coalesce at the plane of division.
A: (c) and (d). Plant cytokinesis uses Golgi-derived vesicles that fuse into a cell plate, which grows outward to meet the existing membrane. No contractile ring of microfilaments is involved, and the membrane does not grow inward.
Q: Motor proteins require which molecule to function in moving chromosomes toward the poles?
A: ATP, as an energy source.
Mitosis, prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis, cleavage furrow, cell plate, mitotic spindle, kinetochore microtubules, nonkinetochore microtubules, cohesin, tubulin, centrosome, motor proteins, ATP, binary fission, FtsZ, bacterial cell division, plant cell division, animal cell division, spindle assembly checkpoint, metaphase plate, chromosome separation, sister chromatid separation
Connections to other topics: The mitotic spindle connects to the cytoskeleton (microtubules, actin filaments, motor proteins), which is its own unit in most intro biology courses. Cytokinesis differences between plant and animal cells tie into cell structure and the role of the cell wall. Binary fission links to bacterial genetics and evolution. Errors in chromosome separation (nondisjunction) connect to meiosis and genetic disorders such as Down syndrome.