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. You're being tested on more than just "what happens when." The exam expects you 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 the sequence Prophase → Metaphase → Anaphase → Telophase. Know what each stage accomplishes mechanically, how the 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

Before mitosis begins, the cell must prepare. 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, duplicates organelles, and synthesizes proteins needed for DNA replication; this is where the G1/S checkpoint determines if the cell should commit to division
• S phase is when DNA replication occurs, converting each chromosome into two sister chromatids joined at the centromere
• G2 phase involves final preparation including centrosome replication and ATP production; the G2/M checkpoint ensures DNA replication is complete and undamaged before mitosis begins

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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 prevents tangling during separation
• Centrosomes migrate to opposite poles of the cell and begin assembling the mitotic spindle from microtubules
• Nuclear envelope breakdown begins and the nucleolus disassembles, giving spindle fibers access to chromosomes

Prometaphase

• Nuclear envelope completely disintegrates, marking the transition from prophase and allowing full spindle-chromosome interaction
• Kinetochore-microtubule attachment occurs as spindle fibers connect to kinetochores—protein structures assembled at each chromosome's centromere
• Chromosomes move erratically toward the cell's center as opposing spindle forces balance out; the spindle assembly checkpoint (SAC) monitors attachment quality

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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 means each chromosome connects to spindle fibers from both poles, creating tension that signals proper attachment
• The spindle assembly checkpoint (SAC) halts progression until all kinetochores are properly attached; failure here causes nondisjunction and aneuploidy

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Separation: Sister Chromatids Become Chromosomes

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

Anaphase

• Separase enzyme cleaves cohesin proteins holding sister chromatids together, allowing them to separate at the centromere
• Sister chromatids move to opposite poles as motor proteins walk along shortening kinetochore microtubules—each chromatid is now an individual chromosome
• Cell elongation occurs as non-kinetochore microtubules push the poles apart, preparing the cell for division into two compartments

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Rebuilding and Division: Two Cells Emerge

The final stages reverse early mitotic events and physically divide the cell. Telophase essentially runs prophase backward, 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 from vesicles of the old envelope, creating two distinct nuclei
• 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
• Animal cells use a cleavage furrow—a contractile ring of actin and myosin pinches the membrane inward
• Plant cells build a cell plate from vesicles at the cell's center, which matures into a new cell wall

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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?

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?