Cell Cycle Regulators - Khan Academy Lecture Notes

Introduction to Cell Cycle Regulation

• Cell cycle transitions are influenced by both external and internal cues.
• Core regulators: cyclins, cyclin-dependent kinases (Cdks), and the anaphase-promoting complex/cyclosome (APC/C).
• These regulators ensure proper execution and sequence of cell cycle events.

Cyclins

• Definition: Cyclins are proteins that regulate the cell cycle.
• Types: Four main types in eukaryotes - G1, G1/S, S, and M cyclins.
• Function: Each cyclin is associated with a specific phase of the cell cycle.
  • M cyclin: Promotes M phase events like nuclear envelope breakdown.
• Expression: Cyclin levels vary throughout the cell cycle, often peaking when needed for specific cell cycle phases.

Cyclin-dependent Kinases (Cdks)

• Function: Enzymes that, when activated by cyclins, phosphorylate target proteins to drive the cell cycle forward.
• Mechanism: Binding of cyclin activates Cdk, directing it to specific targets.
  • Example: G1/S cyclins target S phase events like DNA replication.
• Conservation: Highly conserved across species, from yeast to humans.

Maturation-promoting Factor (MPF)

• Discovery: Initially identified as an unknown factor in M phase cells.
• Composition: Consists of a Cdk bound to M cyclin.
• Function: Triggers M phase by phosphorylating proteins involved in nuclear envelope breakdown and chromosome condensation.

Anaphase-promoting Complex/Cyclosome (APC/C)

• Function: Destroys M cyclins to push the cell out of mitosis, allowing entry into G1.
• Mechanism: Tags proteins with ubiquitin, marking them for destruction by the proteasome.
  • Example: Triggers separation of sister chromatids during mitosis.

Checkpoints and Regulation

• Role of Checkpoints: Ensure cell cycle does not proceed if conditions are unfavorable.
  • Example: p53 halts the cell cycle in response to DNA damage.
• p53 Function: Stops cell division, activates DNA repair, and can trigger apoptosis if damage is irreparable.
• p53 Importance: Prevents mutations from being passed to daughter cells; frequently mutated in cancers.

References

• The article draws on various scientific sources and research studies to explain cellular processes.

These notes summarize the key points on cell cycle regulators, focusing on the roles and mechanisms of cyclins, Cdks, MPF, and APC/C within the cell cycle.