Cell Cycle – Interphase (G₁, S, G₂) Comprehensive Bullet-Point Notes

The eukaryotic cell cycle is a repeating sequence of growth, DNA replication, and division that produces two genetically identical daughter cells.
Major phases to master:
• G₁, S, G₂ – collectively called Interphase (≈ 90\% of the total cycle)
• M Phase (Mitosis) – prophase, metaphase, anaphase, telophase
• Cytokinesis – physical separation of cytoplasm after nuclear division
Core idea: cells alternate between long periods of preparation (interphase) and a short period of actual division (M + cytokinesis).
Analogy (p. 3): just as different-aged people have different heights, tissues reach different sizes because individual cells grow before they divide.

Definition: Longest portion of the cycle; interval between two consecutive mitoses.
Primary objective: Mass increase, genome duplication, error checking, and accumulation of materials for the next division.
Nick-name from slides: “PREPARATION PHASE.”
Split into three biochemically distinct sub-phases:
1. G₁ (Gap 1) – cell growth & macromolecule synthesis
2. S (Synthesis) – DNA, centrosome, and histone duplication
3. G₂ (Gap 2) – final growth + quality control for mitosis/meiosis
Typical order: G₁ \rightarrow S \rightarrow G₂, after which the cell either
• enters M phase to divide, or
• withdraws into G₀ (resting stage).

Point of exit: Cells that fail G₁ checkpoint or receive differentiation signals leave the cycle from late G₁.
Two biological flavors:
• Quiescent (reversible) – “paused movie” state; conserving energy, awaiting growth factors; e.g., liver, memory T-cells.
• Senescent (irreversible) – permanent arrest due to age or stress; initially tumor-suppressive but can promote inflammation/cancer in aged tissue.
Examples: Neurons and skeletal muscle fibers dwell almost lifelong in G₀.
Clinical angle: Manipulating quiescence is crucial for regenerative medicine; modulating senescence is a rising anti-aging and oncology strategy.

Main activities (p. 9):
• Intensive cell growth – increase cytoplasmic volume & organelles.
• mRNA & protein synthesis needed for upcoming DNA replication.
Chromosome status: one chromatid per chromosome (human karyotype = 46).
G₁ Checkpoints (p. 11 & 15):
• DNA-damage checkpoint mediated by p53.
– Detects lesions, halts cycle, initiates DNA repair.
– Irreparable damage ⇒ apoptosis (programmed death).
• Restriction point (R-point): evaluates nutrients, size, and external mitogens; determines commit to S phase or entry into G₀.
Failure consequences (example, p. 33):
• Insufficient growth ⇒ incomplete replication, genome instability.
• Loss of p53 ⇒ uncontrolled proliferation, cancer initiation.

Core event: Complete, semiconservative DNA replication, producing two identical sister chromatids per original chromosome (diagram p. 19–22).
• Human: 46 \to 92 chromatids during this window.
Simultaneous tasks:
• Centrosome duplication – forms two microtubule-organizing centers for mitosis (p. 23).
• Histone synthesis – provides protein spools for packaging the newly made DNA (p. 24).
S-phase DNA-damage checkpoint (p. 25):
• Detects replication errors or breaks; stalls fork progression until resolved.
Outcome: Successfully replicated & packaged genome allows transition to G₂ (p. 26).

Main activities (p. 27–29):
• Continued cell growth; synthesis of proteins specifically required for mitosis (e.g., tubulin, condensins).
• Verification that DNA replication is complete and error-free.
G₂/M Checkpoint (p. 32):
• “Double-checks” duplicated chromosomes for breaks/mis-pairings.
• Engages repair pathways; blocks mitotic entry if defects detected.
Biological stakes: Skipping G₂ surveillance risks aneuploidy, a hallmark of many cancers.

Major labeled parts:
• Telomeres – repetitive end sequences protecting chromosomes.
• Centromere – constricted region housing kinetochore during mitosis.
• Short (p) & Long (q) arms – designated relative to centromere.
• Sister chromatids – identical DNA copies post-replication.
Significance: Understanding structure helps connect replication sites, segregation mechanics, and telomere-linked aging.

p53 accumulation is a central response to DNA damage.
• Outcome 1: Cell-cycle arrest ⇢ repair ⇢ survival.
• Outcome 2: Apoptosis ⇢ removal of potentially malignant cells.
Dysregulation:
• Hyperactive apoptosis ⇒ neurodegeneration, immunodeficiency.
• Suppressed apoptosis ⇒ tumor progression, autoimmune flare.
Ethical / therapeutic dimension: Drug designs often aim to re-activate p53 or controlled apoptosis in cancer cells while sparing healthy tissue.

Height analogy (p. 3) links visible organismal growth to microscopic cell doubling.
Historical tie-in: The discovery of the restriction point paralleled the emergence of cancer biology; many oncogenes act by overriding checkpoints.
Laboratory relevance: S-phase labeling with BrdU or EdU is a common diagnostic test for proliferation rate in biopsy samples.

G₁ – Cell grows; synthesizes RNA & proteins; evaluates environment; decides between S or G₀.
S – Entire genome, centrosome, and histone content duplicated.
G₂ – Growth continues; DNA integrity verified; machinery assembled for mitosis.

G₁: Missing growth factors ⇒ small cell size, replication stress.
S: Failed replication fork stabilization ⇒ double-strand breaks.
G₂: Unrepaired damage ⇒ chromosomal non-disjunction, aneuploidy.
M (beyond scope but contextual): Mis-attached kinetochores ⇒ daughter cells with unequal chromosomes.

Interphase length ≈ \text{90\%} of total cycle time.
Human diploid set: 46 chromosomes (2n) duplicating to 92 chromatids.
Kinetic formula (conceptual): \text{Cell number after } n \text{ divisions} = 2^{n}.

Draw the circular “pie” diagram: label G₁ \,(40\%), S\,(39\%), G₂\,(19\%), M\,(2\%) (typical mammalian cycle).
Associate checkpoints with guardian molecules (p53, ATM/ATR, Chk1/2).
Practice predicting outcomes when checkpoints fail – a common exam essay theme.