Stages of Mitosis

Why This Matters

Mitosis is the engine of growth, repair, and asexual reproduction in eukaryotes, and it's a cornerstone of AP Biology's Unit 4 on Cell Communication and Cell Cycle. The exam expects more than just naming stages in order. You need to understand how the cell cycle is regulated, why checkpoints exist, and what goes wrong when mitosis fails (think: cancer, nondisjunction, aneuploidy). Every stage connects to broader concepts like cyclin-CDK regulation, spindle assembly checkpoints, and the molecular machinery that ensures faithful chromosome segregation.

Don't just memorize Prophase → Metaphase → Anaphase → Telophase. Know what each stage accomplishes mechanically, how stages relate to checkpoints and regulation, and why errors at specific stages lead to specific consequences. FRQs love asking you to compare mitosis and meiosis or explain what happens when the spindle assembly checkpoint fails, so think in terms of mechanisms and outcomes, not just vocabulary.

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Preparation: Interphase Sets the Stage

Interphase isn't part of mitosis itself, but it's essential context. This is when DNA replication occurs and the cell stockpiles the materials needed for division.

Interphase (G1, S, G2)

• G1 phase: The cell grows, produces organelles, and synthesizes proteins needed for DNA replication. The G1/S checkpoint acts here, where the cell evaluates whether conditions (cell size, nutrient availability, growth factor signals) are favorable enough to commit to division. Cyclin D accumulates and activates CDK4/6, which phosphorylates the Rb protein, releasing its inhibition of transcription factors (like E2F) that drive S phase entry.
• S phase: DNA replication occurs, converting each chromosome into two sister chromatids joined at the centromere by cohesin proteins. After S phase, a cell that started with 46 chromosomes still has 46 chromosomes, but each one now consists of two identical chromatids. The DNA content doubles (from $2n$ to $4n$ in terms of DNA mass), even though the chromosome number stays the same.
• G2 phase: Final preparation, including completion of centrosome duplication (which began in S phase; each centrosome now has two centrioles) and ATP stockpiling. The G2/M checkpoint verifies that DNA replication is complete and undamaged before mitosis begins. Cyclin B paired with CDK1 (together called MPF, or maturation-promoting factor) drives entry into mitosis.

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Chromosome Condensation and Spindle Assembly

The first stages of mitosis focus on packaging genetic material for transport and building the machinery to move it. Chromatin must condense into portable chromosomes, and the spindle apparatus must form to pull them apart.

Prophase

• Chromatin condenses into visible chromosomes, each consisting of two sister chromatids. This compaction, driven by condensin proteins, prevents tangling during separation.
• Centrosomes migrate to opposite poles of the cell and begin assembling the mitotic spindle from microtubules (made of $\alpha$- and $\beta$-tubulin dimers).
• The nucleolus disassembles, and nuclear envelope breakdown begins as MPF phosphorylates nuclear lamins.

Prometaphase

• The nuclear envelope completely disintegrates, allowing full spindle-chromosome interaction.
• Kinetochore-microtubule attachment occurs as spindle fibers connect to kinetochores, protein structures assembled at each chromosome's centromere. Each sister chromatid has its own kinetochore, so a replicated chromosome has two.
• Chromosomes move erratically toward the cell's center as opposing spindle forces balance out. The spindle assembly checkpoint (SAC) begins monitoring attachment quality here and continues into metaphase.

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Alignment and the Critical Checkpoint

Proper alignment ensures each daughter cell receives exactly one copy of each chromosome. The metaphase checkpoint is the cell's last chance to catch attachment errors before separation becomes irreversible.

Metaphase

• Chromosomes align at the metaphase plate, the cell's equatorial plane, with sister chromatids facing opposite poles.
• Bipolar attachment (also called amphitelic attachment) means each chromosome's two kinetochores connect to spindle fibers from opposite poles, creating tension that signals proper attachment.
• The spindle assembly checkpoint (SAC) halts progression until every single kinetochore is properly attached and under tension. Unattached kinetochores produce a "wait" signal by generating the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex (APC/C). If the SAC fails, chromosomes can missegregate, causing nondisjunction and aneuploidy (abnormal chromosome number), a hallmark of many cancers.

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Separation: Sister Chromatids Become Chromosomes

Anaphase is the point of no return. Once cohesin proteins are cleaved, separation is irreversible, and the cell is committed to producing two daughter cells.

Anaphase

Once the SAC is satisfied, it releases its inhibition of the APC/C. The APC/C then triggers a cascade:

1. APC/C ubiquitinates securin, tagging it for degradation by the proteasome. Securin had been inhibiting the enzyme separase.
2. Freed separase cleaves cohesin proteins holding sister chromatids together at the centromere.
3. Sister chromatids move to opposite poles as motor proteins (dynein) walk along shortening kinetochore microtubules (Anaphase A). Each chromatid is now considered an individual chromosome.
4. Cell elongation occurs as non-kinetochore (polar) microtubules slide past each other, pushed apart by kinesin motor proteins, stretching the cell (Anaphase B).

The APC/C also ubiquitinates Cyclin B, leading to its degradation. This drops CDK1 activity and is what allows the cell to exit mitosis and enter telophase. If you're asked on an FRQ how the cell transitions out of mitosis, Cyclin B destruction by the APC/C is the key event.

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Rebuilding and Division: Two Cells Emerge

The final stages reverse early mitotic events and physically divide the cell. Telophase runs prophase in reverse, while cytokinesis differs dramatically between animal and plant cells.

Telophase

• Chromosomes decondense back into chromatin as the cell prepares to resume normal gene expression.
• Nuclear envelopes re-form around each chromosome set as nuclear lamins are dephosphorylated and membrane vesicles reassemble, creating two distinct nuclei. Nucleoli also reappear.
• The spindle apparatus disassembles as the cell transitions from mitosis to cytokinesis.

Cytokinesis

Cytoplasmic division produces two genetically identical daughter cells, each with a complete set of chromosomes and organelles. The mechanism depends on cell type:

• Animal cells use a cleavage furrow: a contractile ring of actin microfilaments and myosin II assembles just beneath the plasma membrane at the former metaphase plate. This ring contracts and pinches the cell inward until it splits in two.
• Plant cells can't pinch inward because of their rigid cell wall. Instead, Golgi-derived vesicles carrying cell wall materials gather at the center of the cell and fuse to form a cell plate. The cell plate grows outward until it reaches the existing cell wall, then matures into a new cell wall with plasma membrane on each side.

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Quick Reference Table

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Self-Check Questions

1. Which two stages involve changes to the nuclear envelope, and how do those changes differ?

2. At which stage does the spindle assembly checkpoint (SAC) operate, and what consequence results if it fails?

3. Compare the alignment of chromosomes at the metaphase plate in mitosis versus Metaphase I of meiosis. What is the key structural difference?

4. What molecular event makes anaphase irreversible, and which proteins are involved? Trace the pathway from APC/C activation to chromatid separation.

5. If an FRQ asks you to explain why plant and animal cytokinesis differ mechanically, which structures and proteins would you discuss for each cell type, and in which direction does each mechanism proceed?

6. How does the APC/C contribute to mitotic exit beyond triggering chromatid separation? Which protein does it target, and what's the downstream effect?