Cell Cycle & Division

Chapter Summary

The cell cycle comprises Interphase (G₀, G₁, S, G₂) and M phase (mitosis or meiosis). Use the selector below to explore each phase’s diagram and key events. Afterwards, dive into regulation, checkpoints, and clinical relevance.

Select Phase:

G₁ (Growth)

Cell increases in size, synthesizes proteins and organelles. G₁/S checkpoint checks DNA integrity before replication.

Interphase in Depth

Interphase is when the cell prepares for division:

G₀: Quiescent state—cells perform normal functions (e.g., neurons).
G₁: Cell growth, protein/organelle synthesis, G₁/S DNA damage checkpoint.
S: DNA replication—each chromosome duplicates into two sister chromatids.
G₂: Final growth and repair, G₂/M checkpoint ensures complete replication.

Cell Cycle Regulation & Checkpoints

Progression through the cycle is tightly controlled by:

Cyclins & CDKs: Cyclin-dependent kinases activate phase transitions.
CDK inhibitors: Block kinase activity under damage/stress.
Tumor suppressors: p53 halts cycle if DNA damage is detected.

Key checkpoints:

G₁/S: checks DNA integrity before synthesis.
G₂/M: ensures complete replication before mitosis.
Spindle: verifies chromosome attachment at metaphase.

Clinical Correlations

Cancer: Loss of checkpoint control (e.g., mutated p53) → uncontrolled proliferation.
Nondisjunction: Meiosis errors → aneuploidies (e.g., Down syndrome in Meiosis I).
CDK Inhibitors as Drugs: Used in targeted cancer therapies to restore checkpoint control.

Key Terms

Checkpoint: control mechanism monitoring cycle progression
Sister Chromatids: identical copies formed during S phase
Ploidy: number of chromosome sets (2n = diploid; n = haploid)
Cyclin-Dependent Kinase (CDK): enzyme driving cell cycle transitions

Quiz Yourself

1. Which phase includes the DNA damage checkpoint?

G₁ S G₂

2. Loss of which protein often leads to cancer?

Cyclin p53 CDK

3. Nondisjunction in Meiosis I can cause which condition?

Down syndrome Klinefelter syndrome Turner syndrome