Mitosis Stages

Why This Matters

Mitosis is the foundation of how multicellular organisms grow, repair damaged tissue, and maintain themselves throughout life. When you're tested on cell division, you're really being asked to demonstrate your understanding of chromosome behavior, spindle mechanics, and the checkpoints that prevent errors. Every stage exists for a reason—to ensure genetic material is copied and distributed with precision.

Don't just memorize the order of phases. Instead, focus on what's happening to the chromosomes at each stage and why each step must occur before the next. The exam will ask you to identify stages from diagrams, explain what would happen if a step failed, and compare mitosis to meiosis. Know the mechanism behind each phase, and you'll be ready for anything.

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Preparation: Setting the Stage for Division

Before a cell can divide, it must duplicate everything it needs to create two functional daughter cells. This preparation phase accounts for about 90% of the cell cycle.

Interphase

• Not technically part of mitosis—this is when the cell grows, copies its DNA, and prepares all the materials needed for division
• Three sub-phases structure the work: G1 (cell growth and normal function), S (DNA synthesis—chromosomes are replicated), and G2 (final preparations and error-checking)
• DNA exists as loose chromatin—this relaxed form allows the replication machinery to access and copy genetic information

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Chromosome Condensation and Spindle Assembly

The transition into mitosis requires dramatic reorganization. Loose chromatin must compact into transportable units, and the machinery for moving chromosomes must assemble.

Prophase

• Chromatin condenses into visible chromosomes—each chromosome now consists of two identical sister chromatids joined at the centromere
• Nuclear envelope breaks down—this barrier must dissolve so spindle fibers can access and attach to chromosomes
• Centrosomes migrate to opposite poles—they organize the mitotic spindle, the apparatus that will physically separate genetic material

Prometophase

• Spindle fibers attach to kinetochores—these protein structures on centromeres serve as attachment points for microtubules
• Chromosomes begin chaotic movement—they're pushed and pulled as spindle fibers from both poles compete for attachment
• Nuclear envelope completely dissolves—fragments will be recycled to form new envelopes in daughter cells

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Alignment and Quality Control

This phase represents a critical checkpoint. The cell verifies that every chromosome is properly attached before committing to separation.

Metaphase

• Chromosomes align at the metaphase plate—this imaginary plane at the cell's equator ensures equal distribution to both sides
• Spindle attachment checkpoint activates—the cell will not proceed until every kinetochore has proper bipolar attachment (fibers from both poles)
• Sister chromatids remain joined—tension from opposing spindle fibers confirms correct attachment; this is the last moment chromosomes exist as pairs

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Separation: The Critical Division of Genetic Material

Anaphase is when the actual segregation occurs. Failure here causes aneuploidy—an incorrect chromosome number that can lead to cell death or disease.

Anaphase

• Sister chromatids separate at centromeres—the protein cohesin that held them together is cleaved by the enzyme separase
• Motor proteins walk chromatids toward poles—spindle fibers shorten as chromosomes are pulled apart at roughly 1 μm per minute
• Cell elongates through polar fiber sliding—non-kinetochore microtubules push poles apart, stretching the cell for eventual division

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Nuclear Reformation and Cytoplasmic Division

The final phases reverse the changes of prophase and physically create two separate cells. The nuclear compartment must reform before the cell can function normally.

Telophase

• Chromosomes decondense into chromatin—returning to the loose form allows genes to be transcribed again
• Nuclear envelopes reassemble—membrane fragments recruit around each chromosome cluster, re-establishing the nuclear compartment
• Spindle apparatus disassembles—microtubules depolymerize; their components will be recycled for the cytoskeleton

Cytokinesis

• Cytoplasm physically divides—this is the only stage that produces two separate cells; everything before reorganized one cell
• Animal cells use a cleavage furrow—a contractile ring of actin and myosin pinches the membrane inward like a drawstring
• Plant cells build a cell plate—vesicles fuse at the center to form a new cell wall because the rigid existing wall cannot pinch inward

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Quick Reference Table

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Self-Check Questions

1. Which two phases involve changes to the nuclear envelope, and what happens in each?

2. A cell is observed with chromosomes aligned in a single row at the center and spindle fibers attached to both sides of each chromosome. What phase is this, and what checkpoint is active?

3. Compare and contrast cytokinesis in animal cells versus plant cells. What structural difference explains why the mechanisms differ?

4. If the enzyme separase were inhibited, at which phase would mitosis arrest, and what would you observe in the cell?

5. A student claims that DNA replication occurs during prophase because that's when chromosomes become visible. Explain the error in this reasoning and identify when replication actually occurs.