AP Biology: Cell Cycle Phases (G1, S, G2, M) and Checkpoints (G1/S, G2/M, Spindle)

Cell Cycle Phases (G1, S, G2, M) and Checkpoints (G1/S, G2/M, Spindle)

Concept Summary

• G? phase: Primary growth phase where the cell increases in size and synthesizes proteins; prepares for DNA replication.
• S phase: DNA synthesis occurs, doubling the genome (2n-4n); critical for genetic continuity.
• G? phase: Secondary growth phase where organelles replicate and microtubules assemble; prepares for mitosis.
• M phase (Mitosis): Nuclear division (prophase-metaphase-anaphase-telophase) followed by cytokinesis; ensures identical daughter cells.
• Checkpoints (G?/S, G?/M, Spindle): Regulatory control points that verify DNA integrity, replication completion, and spindle attachment before progression.

Core Questions

WHAT (definitional)

Q: What is the G?/S checkpoint? A: The restriction point where the cell commits to DNA replication if nutrients, growth signals, and DNA integrity are verified. Trap/Clarification: It is not a physical barrier but a molecular decision point (e.g., p53/Rb proteins).

Q: What defines the S phase? A: The phase where semiconservative DNA replication occurs, producing sister chromatids held by cohesins. Trap/Clarification: Chromosome number does not double (ploidy remains 2n); DNA content doubles (C-2C).

WHY (causal/explanatory)

Q: Why is the G?/M checkpoint critical? A: It ensures DNA replication is complete and damage-free before mitosis; prevents aneuploidy. Trap/Clarification: It does not check spindle formation (that’s the spindle checkpoint).

Q: Why do cells arrest at the spindle checkpoint? A: To prevent chromosome missegregation by ensuring all kinetochores are properly attached to spindle microtubules. Trap/Clarification: Arrest occurs during metaphase, not before.

HOW (process/application)

Q: How does the G?/S checkpoint regulate progression? A: Cyclin D-CDK4/6 phosphorylates Rb, releasing E2F to activate S-phase genes. Trap/Clarification: Rb is inactive when phosphorylated (not the other way around).

Q: How is DNA damage detected at the G?/M checkpoint? A: ATM/ATR kinases activate Chk1/Chk2, which inhibit CDC25 phosphatase, blocking cyclin B-CDK1 activation. Trap/Clarification: CDC25 activates CDK1; its inhibition halts the cycle.

CAN (conditions/possibilities)

Q: Can a cell skip G? and enter S phase directly? A: No; G? is mandatory for growth and checkpoint verification (e.g., embryonic cells bypass G? but still require S-phase prep). Trap/Clarification: "Rapid division"-skipping G?; it’s just a shortened G?.

Q: Under what conditions does the spindle checkpoint fail? A: If kinetochores lack tension or attachment (e.g., due to microtubule poisons like colchicine or mutations in Mad2). Trap/Clarification: Failure leads to nondisjunction, not apoptosis (unless severe).

Quick Facts & Traps

• Fact: G? phase is a non-dividing state (e.g., neurons); cells exit the cycle from G?.
• Trap: "G? is just a longer G?"-Reality: G? focuses on mitotic prep (e.g., microtubule organizing centers duplicate).
• Fact: Cyclin-CDK complexes drive phase transitions; cyclins are degraded after use (e.g., cyclin B in anaphase).
• Trap: "Checkpoints are phases"-Reality: They are molecular pauses within phases (e.g., G?/S is in G?).
• Fact: Spindle checkpoint monitors bipolar attachment; unattached kinetochores trigger the mitotic checkpoint complex (MCC).
• Trap: "DNA damage always causes apoptosis"-Reality: Cells may repair (e.g., p53-mediated arrest) or senesce.

Rapid-Fire True/False

• Statement: The S phase increases chromosome number from 2n to 4n. Answer: FALSE Why the common mistake happens: Confuses DNA content (2C-4C) with chromosome number (remains 2n until anaphase).

• Statement: The G?/M checkpoint ensures spindle fibers are properly formed. Answer: FALSE Why the common mistake happens: Spindle formation is checked during M phase (spindle checkpoint), not G?/M.

• Statement: A cell with damaged DNA can bypass the G?/S checkpoint if growth signals are strong. Answer: FALSE Why the common mistake happens: Overlooks p53’s role in halting the cycle regardless of growth signals.