Key Cell Cycle Phases

Why This Matters

The cell cycle is the foundation for understanding how organisms grow, repair damage, and reproduce—and it's absolutely central to molecular biology exams. You're being tested on your ability to explain how cells ensure accurate DNA replication, what checkpoints prevent errors from propagating, and why the precise choreography of chromosome separation matters. These concepts connect directly to topics like cancer biology, stem cell differentiation, and genetic inheritance.

Don't just memorize the phase names and their order. Know what molecular events define each phase, which checkpoints regulate transitions, and how errors at each stage lead to disease. When you see an FRQ about cell division gone wrong, you need to pinpoint exactly where the cycle failed and why that matters for the daughter cells.

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Interphase: The Growth and Preparation Stages

Most of a cell's life is spent in interphase—the period of growth, DNA replication, and preparation that precedes actual division. The cell must double its contents and accurately copy its genome before it can successfully divide.

G1 Phase (Gap 1)

• Primary growth phase—the cell increases in size and synthesizes proteins, RNA, and organelles needed for upcoming replication
• G1 checkpoint (Restriction Point) assesses cell size, nutrient availability, and DNA integrity before committing to division
• Cyclin D and CDK4/6 accumulate during this phase, driving progression toward S phase when conditions are favorable

S Phase (Synthesis)

• DNA replication occurs—each chromosome is duplicated, producing two identical sister chromatids joined at the centromere
• Replication origins fire in a coordinated pattern, with licensing factors ensuring each segment replicates exactly once
• Histone synthesis ramps up to package newly replicated DNA into chromatin structure

G2 Phase (Gap 2)

• Final preparation for mitosis—the cell continues growing and synthesizes proteins essential for chromosome segregation
• G2 checkpoint verifies complete DNA replication and scans for damage; ATM and ATR kinases halt progression if errors are detected
• Cyclin B and CDK1 (MPF) accumulate but remain inactive until checkpoint requirements are satisfied

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M Phase: Dividing the Genetic Material

Mitosis is the dramatic culmination of the cell cycle, where duplicated chromosomes are physically separated into two identical sets. The spindle apparatus provides the mechanical force, while checkpoint proteins ensure accuracy before irreversible steps occur.

Prophase

• Chromatin condenses into visible chromosomes—each consisting of two sister chromatids connected at the centromere
• Nuclear envelope breakdown begins as the lamin proteins are phosphorylated and disassembled
• Centrosomes migrate to opposite poles while nucleating microtubules that will form the mitotic spindle

Metaphase

• Chromosomes align at the metaphase plate—the cell's equatorial plane, equidistant from both spindle poles
• Kinetochore attachment is verified; spindle fibers from opposite poles must connect to sister chromatid kinetochores (bipolar attachment)
• Spindle assembly checkpoint (SAC) prevents anaphase until all chromosomes achieve proper tension and alignment

Anaphase

• Sister chromatids separate—cohesin proteins are cleaved by separase, allowing chromatids to become independent chromosomes
• Anaphase A involves kinetochore microtubules shortening to pull chromosomes poleward
• Anaphase B elongates the cell as polar microtubules slide apart, increasing spindle pole distance

Telophase

• Nuclear envelopes reform—vesicles fuse around each chromosome set as lamins are dephosphorylated and reassemble
• Chromosomes decondense back into chromatin, allowing transcription to resume
• Mitotic spindle disassembles as the cell prepares for physical separation of cytoplasm

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Cytokinesis: Completing Cell Division

Cytokinesis physically divides the cytoplasm into two daughter cells. The mechanism differs fundamentally between animal and plant cells due to the presence of a rigid cell wall in plants.

Cytokinesis

• Cleavage furrow formation in animal cells—a contractile ring of actin and myosin II pinches the membrane inward
• Cell plate formation in plant cells—Golgi-derived vesicles fuse at the cell's midplane, building a new cell wall from the inside out
• Timing overlaps with telophase—cytokinesis typically begins during late anaphase and completes after nuclear division finishes

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Interphase as a Collective Phase

Understanding interphase as a unified concept helps clarify what "dividing" versus "non-dividing" cells are actually doing.

Interphase

• Encompasses G1, S, and G2 phases—representing approximately 90% of the total cell cycle duration in actively dividing cells
• Metabolically active period—the cell performs its specialized functions while preparing for division
• G0 phase represents an exit from the cycle; quiescent cells like neurons may remain in G0 permanently

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Quick Reference Table

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Self-Check Questions

1. Which two phases both involve checkpoint assessment of DNA integrity, and what specific question does each checkpoint answer?

2. A cell treated with a drug that prevents cyclin B degradation would arrest at which phase transition, and why?

3. Compare and contrast the mechanisms of cytokinesis in animal versus plant cells—what structural constraint explains the difference?

4. If the spindle assembly checkpoint failed to function, during which phase would errors occur, and what would be the consequence for daughter cells?

5. A student observes a cell with chromosomes aligned at the center and spindle fibers attached to kinetochores. Which phase is this, and what must happen before the cell can proceed to the next stage?