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Cell Cycle Control and Checkpoints

Dec 12, 2025

Overview

  • Lecture covers the cell cycle fundamentals and molecular control mechanisms.
  • Context: part of a series (lectures 10–12) on how cell numbers are controlled; upcoming topics include programmed cell death and cancer.
  • Real clinical examples (pancreatic cancer, leukemia, retinoblastoma) are used to connect cell-cycle concepts to disease.

Cell Cycle Structure

  • Main phases: G1 → S → G2 → M (cycle is circular; any phase can be a starting point).
  • S phase = DNA synthesis/replication.
  • M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).
  • Common misconceptions:
    • DNA replication occurs in S phase, not during mitosis.
    • M phase includes both mitosis and cytokinesis, not just mitosis.

Checkpoints And Their Roles

  • Checkpoints ensure conditions are suitable for progression (green light = proceed, red light = stop and wait).
  • Major checkpoints:
    • Start (G1 → S): is environment favorable to enter S phase?
    • G2 → M (G2M): is all DNA replicated and ready for mitosis?
    • Metaphase → Anaphase: are chromosomes attached to the spindle?
  • Checkpoints are mediated by cyclin–CDK complexes controlling phosphorylation of target proteins.

Cyclins And CDKs: Basics And Nomenclature

  • CDKs are kinases that phosphorylate target proteins to drive cycle progression.
  • CDKs require cyclins (regulatory subunits) to be active.
  • Naming: stage + "CDK" (e.g., S CDK, M CDK) refers to the cyclin–CDK complex, not the CDK alone.
  • Cyclin dynamics:
    • Cyclins are synthesized and degraded at specific cell-cycle stages.
    • Different cyclins are used for different stages; cyclins are never reused for other stages.
    • Some organisms (e.g., budding yeast) use the same CDK with different cyclins across stages; vertebrates can also do this.

Mechanisms That Regulate Cyclin–CDK Activity

  • Cyclin synthesis/degradation:
    • Polyubiquitination of cyclins targets them for proteasomal degradation.
    • APC/C with CDC20 acts as an E3 ubiquitin ligase for M cyclin degradation.
    • Degradation of cyclin inactivates the corresponding CDK (red light).
  • CDK activation by phosphorylation:
    • Cyclin binding exposes the T-loop of CDK (partial activation).
    • CDK-activating kinase (CAK) phosphorylates the T-loop → full activation.
    • The activating phosphate on CDK is not transferred to substrates (ATP is used for substrate phosphorylation).
  • Reversible phosphorylation by phosphatases:
    • Protein phosphatases (e.g., PP2A) remove phosphates placed by cyclin–CDKs, opposing CDK activity.
    • PP2A and CDKs often act on the same targets, creating regulatory balance.
  • CDK inhibitor proteins (CKIs):
    • CKIs (e.g., p27, p21) bind cyclin–CDK complexes and block activity.
    • CKIs can block ATP binding and distort the active site, producing inactivation (red light).
  • Inhibitory phosphorylation:
    • Wee1 kinase adds an inhibitory phosphate at a distinct site from the activating phosphate → CDK inactivation.
    • Cdc25 phosphatase removes the inhibitory phosphate → CDK reactivation.
    • CDC25 must be phosphorylated (activated) to remove Wee1's inhibitory phosphate.

Positive Feedback And Switch-Like Behavior

  • Active M CDK creates positive feedback loops to ensure an all-or-none decision to enter M phase:
    • Active M CDK phosphorylates and activates CDC25 → more removal of inhibitory phosphate → more active M CDK.
    • Active M CDK phosphorylates and inhibits Wee1 (and PP2A) → less addition or removal of inhibitory signals → more active M CDK.
  • These feedback loops create a sharp transition (commitment) to mitosis rather than a gradual response.

Examples Of Targets Of Specific Cyclin–CDK Complexes

  • S CDK phosphorylates proteins required for DNA replication, e.g., DNA helicases (promotes origin firing, strand separation).
  • M CDK phosphorylates proteins involved in mitosis, e.g., nuclear lamins (nuclear envelope breakdown) and microtubule-associated proteins (mitotic spindle assembly).

Summary Table: Cyclin–CDK Patterns (vertebrates vs. budding yeast)

| Component | Budding Yeast | Vertebrates | | CDK Usage | Often a single CDK reused across stages | Can reuse the same CDK, but multiple CDKs exist | | Cyclin Usage | Different cyclins for each stage; cyclins change to alter targets | Different cyclins for each stage; cyclins change to alter targets | | Cyclin Reuse | Cyclins are stage-specific and not reused | Cyclins are stage-specific and not reused |

Molecular Inputs That Alter Cyclin–CDK Activity

  • External or internal signals influence checkpoints and cyclin–CDK activity:
    • Growth factors (mitogens) → promote cyclin–CDK activity (green light).
    • Favorable extracellular environment → green light.
    • DNA damage or unreplicated DNA → inhibit cyclin–CDK activity (red light).
    • Chromosome unattached to spindle → inhibit progression to anaphase (red light).
  • Use logical reasoning to predict whether an input gives a red or green light; exact molecular placements are descriptive, not required to memorize.

Cancer Connections And Clinical Examples

  • Pancreatic cancer:
    • CT scan example: tumor in pancreas head; poor prognosis (≈25% 1-year survival, ≈5% 5-year survival).
    • Frequently mutated genes: RAS (activates MAPKs → increased mitosis), Myc, and p53 (covered in later lecture).
  • Leukemia:
    • Worldwide cause of ~300,000 deaths per year; uncontrolled white blood cell proliferation.
    • BCL2 commonly mutated (anti-apoptotic); BCL2 discussed in next lecture.
  • The "Four Horsemen" of cancer (common critical genes): RAS, Myc, p53, BCL2.
  • Retinoblastoma (eye cancer) example:
    • E2F is a transcription factor required for expression of S-phase genes (including cyclins for S phase).
    • RB protein (retinoblastoma protein) inhibits E2F (keeps cell in red light).
    • Phosphorylation of RB inactivates it → E2F active → S-phase gene expression.
    • RB loss/mutation → constitutive E2F activity → uncontrolled S-phase entry → tumor formation (retinoblastoma).
    • Both too little RB (cancer risk) and too much RB (blocks necessary cell proliferation) are harmful — balance is essential.

Key Terms And Definitions

  • Cyclin: regulatory protein whose levels cycle; required for CDK activation.
  • CDK (Cyclin-Dependent Kinase): kinase that phosphorylates substrate proteins to drive cell-cycle events.
  • CAK (CDK-Activating Kinase): phosphorylates the CDK T-loop to achieve full activity.
  • CKI (CDK Inhibitor): protein (e.g., p27, p21) that binds cyclin–CDK complexes and inhibits activity.
  • Wee1 kinase: adds inhibitory phosphate to CDK → inactivation.
  • Cdc25 phosphatase: removes inhibitory phosphate from CDK → activation.
  • APC/C (Anaphase-Promoting Complex/Cyclosome): E3 ubiquitin ligase that targets cyclins (e.g., M cyclin) for degradation with CDC20.
  • PP2A: protein phosphatase that opposes CDK phosphorylation on shared targets.
  • E2F: transcription factor promoting S-phase gene expression.
  • RB (Retinoblastoma protein): tumor suppressor that inhibits E2F.

Action Items / Next Steps

  • Read assigned materials for lectures 10–12 focusing on cell-cycle control and cancer links.
  • For next class (long session): prepare for elaboration on programmed cell death and detailed cancer mechanisms (Myc, p53, BCL2).
  • Practice conceptual questions on:
    • Timing of DNA replication vs. mitosis.
    • Mechanisms of CDK activation/inhibition (cyclin binding, CAK, Wee1, Cdc25, CKIs, ubiquitin-mediated cyclin degradation).
    • Logical effects of molecular inputs (growth factors, DNA damage) on the cell cycle.

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