11.2 The Eukaryotic Cell Cycle, Mitosis, Checkpoints & Apoptosis
Key Takeaways
The eukaryotic cell cycle progresses through Interphase (G1 growth, G0 quiescence, S phase DNA replication from 2n/2c to 2n/4c, and G2 pre-mitotic check) and M phase (mitosis and cytokinesis), driven by oscillating cyclins activating constant cyclin-dependent kinases (CDKs).
The G1/S restriction point is governed by the Rb-E2F switch: hypophosphorylated Rb sequesters E2F to block S-phase entry; mitogen-stimulated Cyclin D-CDK4/6 and Cyclin E-CDK2 hyperphosphorylate Rb, releasing active E2F to transcribe DNA replication enzymes.
DNA damage arrests the cell cycle through the p53 surveillance axis: ATM/ATR kinases stabilize p53 by blocking MDM2 binding; p53 transcriptionally upregulates p21 (CIP1/WAF1), a universal CDK inhibitor that halts G1/S and G2/M transitions.
The Spindle Assembly Checkpoint (SAC) arrests metaphase until all kinetochores achieve amphitelic bipolar attachment; checkpoint satisfaction releases active APC/C-Cdc20 to degrade securin, liberating separase to cleave cohesin rings and trigger anaphase.
Programmed cell death (apoptosis) executes non-inflammatory suicide via intrinsic (mitochondrial Bax/Bak pores, Cytochrome c release, Apaf-1 apoptosome, caspase-9) or extrinsic (FasL/TNF death receptor, FADD, caspase-8) cascades converging on executioner caspases-3/7, whereas necrosis involves ATP exhaustion, oncosis, membrane lysis, and severe inflammation.
11.2 The Eukaryotic Cell Cycle, Mitosis, Checkpoints & Apoptosis
Note
Foundational Standard: The cell cycle balances faithful genomic replication with equational segregation. The MCAT focuses on cyclin-CDK heterodimers, the tumor suppressor circuitry (p53, p21, Rb) governing checkpoints, mitotic stages, and the caspase cascades of apoptosis.
Cell Cycle Phases & Cyclin-CDK Regulation
The cell cycle alternates between Interphase (~90% duration) and M Phase (division).
Phases of Interphase
- G1 Phase: Cellular growth, RNA/protein synthesis, and organelle duplication (diploid: 2n, 2c, where n is ploidy and c is chromatid content).
- G0 Phase (Quiescence): Non-dividing states:
- Permanent G0: Terminally differentiated cells that cannot divide (neurons, cardiac/skeletal myocytes).
- Quiescent G0: Facultative arrest; cells re-enter G1 upon injury/mitogens (hepatocytes, memory lymphocytes).
- Labile: Continuously cycling stem cells (hematopoietic, gut crypt enterocytes).
- S Phase (Synthesis): Nuclear DNA replicates. Each chromosome forms sister chromatids tethered by cohesin rings. Centrosomes duplicate (ploidy 2n, DNA content 4c).
- G2 Phase: Growth and tubulin synthesis; replication fidelity is verified before chromosome condensation (2n, 4c).
- M Phase: Mitosis (nuclear karyokinesis) and cytokinesis (cytoplasmic division) produce two identical diploid cells (2n, 2c).
Cyclin-CDK Heterodimers
Progression requires catalytic Cyclin-Dependent Kinases (CDKs) whose levels remain constant, bound to oscillating regulatory Cyclins:
- G1 Phase: Cyclin D pairs with CDK4/6.
- G1 → S Transition: Cyclin E pairs with CDK2.
- S Phase: Cyclin A pairs with CDK2.
- G2 → M Transition: Cyclin B pairs with CDK1 (historically Maturation-Promoting Factor / MPF). Binding of cyclins pulls the inhibitory T-loop from the active site; full activation requires phosphorylation by CDK-Activating Kinase (CAK). Ubiquitin-mediated proteasomal destruction of cyclins terminates kinase activity.
Cell Cycle Checkpoints & Tumor Suppressors
G1/S Restriction Point (R-Point) & The Rb-E2F Switch
The R-point commits cells irrevocably to division independent of growth factors:
- Arrested State: In early G1, Retinoblastoma protein (Rb) is hypophosphorylated. Hypophosphorylated Rb sequesters the E2F transcription factor and recruits HDACs to silence S-phase promoters.
- Mitogenic Trigger: Mitogens stimulate Cyclin D synthesis via Ras-MAPK signaling. Cyclin D-CDK4/6 initiates Rb phosphorylation; Cyclin E-CDK2 then hyperphosphorylates Rb.
- E2F Release: Hyperphosphorylated Rb dissociates from E2F. Free E2F transcribes S-phase machinery (DNA polymerases, thymidine kinase, PCNA, Cyclin A/E), driving entry into S phase.
p53 – p21 DNA Damage Axis
- Sensors: Double-strand breaks activate ATM/Chk2; single-strand breaks activate ATR/Chk1.
- p53 Stabilization: Sensor kinases phosphorylate p53 (Ser15/Ser20), disrupting binding of the E3 ubiquitin ligase MDM2. Stabilized p53 tetramers accumulate in the nucleus.
- p21 Induction: p53 transcribes p21 (CIP1/WAF1), a universal CDK inhibitor that inactivates Cyclin D-CDK4/6 and Cyclin E-CDK2, arresting the cycle in G1. Irreparable damage prompts p53 to transcribe pro-apoptotic Bax, PUMA, and Noxa.
- Cancer: TP53 is mutated in >50% of human malignancies; germline loss causes Li-Fraumeni syndrome.
G2/M & Spindle Assembly Checkpoint (SAC)
- G2/M Checkpoint: Wee1 kinase maintains Cyclin B-CDK1 (MPF) in an inactive state via inhibitory phosphorylation (Thr14/Tyr15). Following replication verification, Cdc25 phosphatase dephosphorylates CDK1, activating MPF to phosphorylate nuclear lamins (envelope breakdown) and condensins.
- Spindle Assembly Checkpoint (SAC): Monitors metaphase spindle attachment. Unattached kinetochores recruit the Mitotic Checkpoint Complex (MCC: Mad2, BubR1, Bub3, Cdc20), sequestering Cdc20.
- Anaphase Trigger: Bipolar tension across all kinetochores disassembles MCC, releasing Cdc20 to activate the E3 ligase APC/C:
- polyubiquitinates Securin and Cyclin B for proteasomal degradation.
- Loss of Securin frees active Separase.
- Separase cleaves Cohesin rings, allowing spindle microtubules to pull sister chromatids apart.
Mitosis Phases & Cytokinesis
- Prophase: Chromatin condenses into paired sister chromatids linked at centromeres; nucleoli disappear; centrosomes migrate to opposite poles, polymerizing the mitotic spindle.
- Prometaphase: Lamin phosphorylation disassembles the nuclear envelope; trilaminar kinetochores assemble at centromeres to capture spindle microtubules.
- Metaphase: Bipolar spindle tension aligns chromosomes along the equatorial metaphase plate.
- Anaphase: Separase cleaves cohesin:
- Anaphase A: Kinetochore microtubules depolymerize at both ends, reeling daughter chromosomes poleward.
- Anaphase B: Interpolar microtubules slide past each other via kinesin-5 motors to push poles apart, while astral microtubules pull poles via cortical dynein.
- Telophase: Nuclear envelopes reassemble via lamin dephosphorylation; chromosomes decondense into diffuse euchromatin; nucleoli reappear, and the spindle disassembles.
- Cytokinesis:
- Animal Cells: An equatorial actomyosin contractile ring (F-actin and non-muscle myosin II) constricts inward, deepening the cleavage furrow to pinch daughter cells apart.
- Plant Cells: Rigid cell walls prevent furrowing; a microtubule phragmoplast directs Golgi vesicles containing pectin to fuse from the center outward, forming a cell plate.
Programmed Apoptosis vs. Pathological Necrosis
Apoptosis: Controlled Cellular Suicide
Apoptosis is an ATP-dependent, genetically regulated suicide pathway characterized by cell shrinkage, pyknosis (chromatin condensation), karyorrhexis (nuclear fragmentation), membrane blebbing, and formation of apoptotic bodies. Outer-leaflet exposure of phosphatidylserine directs macrophage phagocytosis without inducing inflammation.
- Intrinsic (Mitochondrial) Pathway: DNA damage or cellular stress induces pro-apoptotic Bax and Bak to overcome anti-apoptotic Bcl-2/Bcl-xL, forming pores in the outer mitochondrial membrane (MOMP):
- Cytochrome c releases from the intermembrane space into the cytosol.
- Cytosolic Cytochrome c binds Apaf-1 in the presence of dATP, assembling the heptameric Apoptosome.
- The apoptosome recruits and activates initiator procaspase-9.
- Active Caspase-9 cleaves and activates executioner Caspase-3 and Caspase-7.
- Extrinsic (Death Receptor) Pathway: Extracellular ligands (FasL, TNF-alpha) bind death receptors (Fas/CD95, TNFR1). Trimerization recruits adaptor FADD to form the DISC complex, activating initiator procaspase-8. Caspase-8 activates caspases-3/7 and cleaves Bid into tBid, amplifying the mitochondrial pathway.
- Execution Caspases: Cysteine-aspartic proteases cleaving after aspartic acid residues. They cleave ICAD to release active CAD endonuclease (producing diagnostic 180–200 bp internucleosomal DNA laddering), degrade nuclear lamins, and inactivate PARP.
Necrosis: Pathological Lytic Demolition
Necrosis is a passive, ATP-independent death triggered by severe trauma, ischemia, or toxins. Depletion of ATP causes Na+/K+-ATPase failure, driving cellular swelling (oncosis), organellar rupture, and membrane lysis. Release of intracellular contents and DAMPs (e.g., HMGB1) provokes severe local inflammation.
Apoptosis vs. Necrosis Comparison
| Feature | Apoptosis | Necrosis |
|---|---|---|
| Trigger | Physiological signals, DNA damage, infection | Severe acute trauma, ischemia, metabolic poisons |
| Energy Need | ATP-dependent (Active process) | ATP-independent (Passive collapse) |
| Morphology | Cell shrinkage, pyknosis, blebbing | Cell swelling (oncosis), organelle lysis |
| Membrane | Maintained; apoptotic bodies formed | Lost early; plasma membrane ruptures |
| DNA Pattern | Internucleosomal 180–200 bp laddering (CAD) | Diffuse, random non-specific smearing |
| Inflammation | Zero inflammation (phagocytic clearance) | Severe acute inflammation (DAMP release) |
A molecular biology laboratory investigates human cell lines with targeted gene knockouts. In a newly engineered cell line, the retinoblastoma protein (Rb) is rendered constitutively unphosphorylated and insensitive to kinase action. How will this modification affect cell cycle progression across the G1/S boundary?
The cell will undergo uncontrolled, rapid entry into S phase due to constitutive, continuous transcription of Cyclin E and DNA polymerase.
The cell will arrest permanently at the G1/S restriction point because unphosphorylated Rb remains tightly bound to E2F, repressing S-phase gene transcription.
The cell will bypass G1 entirely, replicating centrosomes in G0 and progressing directly into mitosis.
The cell will proceed through S phase normally but will arrest at the spindle assembly checkpoint due to lack of securin degradation.
During live-cell fluorescence imaging of human fibroblasts undergoing mitosis, a pharmacologist introduces a small molecule that binds kinetochores and prevents tension generation by spindle microtubules. At which mitotic transition will the cell arrest, and what biochemical mechanism enforces this arrest?
G2 to Prophase transition; Wee1 kinase maintains CDK1 in a permanently dephosphorylated active state.
Prophase to Metaphase transition; Mad2 stimulates the polyubiquitination of condensins to inhibit chromosome condensation.
Metaphase to Anaphase transition; the Mitotic Checkpoint Complex sequesters Cdc20, preventing APC/C from ubiquitinating securin to liberate separase.
Anaphase to Telophase transition; active separase fails to cleave cohesin, halting the depolymerization of kinetochore microtubules.
A biomedical research team exposes malignant osteosarcoma cells to high-dose ionizing radiation. Intracellular DNA damage initiates the intrinsic apoptotic cascade. Which precise sequential molecular pathway executes programmed cell death in these cells?
DNA damage stabilizes p53, which induces Bax/Bak oligomerization in the outer mitochondrial membrane; Cytochrome c is released into the cytosol, binds Apaf-1 to form the apoptosome, activates procaspase-9, and downstream Caspase-3 executes cellular cleavage.
DNA damage activates Fas death receptors on the cell surface, which recruit FADD to form the DISC complex, directly activating procaspase-3 independent of mitochondria.
DNA damage directly activates V-type H+-ATPases on lysosomes, releasing acid hydrolases into the neutral cytoplasm to induce rapid cellular swelling and oncosis.
DNA damage causes acute cellular ATP depletion, causing failure of the Na+/K+-ATPase, rapid cellular influx of sodium and water, and necrotic plasma membrane rupture.
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