UNIT_4_AP_BIO_Teacher__Lesson_5_
Lesson Overview
Aim: Understanding the regulation of the cell cycle through checkpoints and regulatory proteins.
Cell Cycle Phases
Interphase
G1 Phase: Growth phase before DNA synthesis begins.
S Phase: DNA replication occurs here.
G2 Phase: Preparation for cell division.
Checkpoints
G1/S Checkpoint: Determines if conditions are right for DNA synthesis.
G2/S Checkpoint: Ensures DNA is replicated and undamaged before mitosis.
M Checkpoint: Confirms chromosomes are properly attached to the mitotic spindle before proceeding to anaphase.
Regulation of the Cell Cycle
Learning Objectives
Describe checkpoints' roles in regulating the cell cycle.
Explain effects of disruptions in the cell cycle on organisms.
Think-Pair-Share Activity
Discussion Question: What could happen if checkpoints or regulatory proteins were absent during the cell cycle?
Internal Cell Cycle Regulators
Cyclins
Function: Proteins whose concentrations vary at different stages of the cell cycle.
Synthesized and degraded at specific times.
Cyclin-Dependent Kinases (CDKs)
Function: Enzymes with constant concentration throughout the cell cycle.
Active only when bound to a specific cyclin.
Cyclin-CDK Complexes
Active CDK complexes regulate cell cycle events by phosphorylating target proteins.
Different cyclin levels during various stages ensure correct processes.
Progression Through the Cell Cycle
Mechanism of Action:
Growth factors signal the production of cyclins.
Cyclins accumulate and activate CDKs.
CDKs initiate signal transduction to progress through check points.
Phase-specific cyclins are degraded to prevent errors.
External Cell Cycle Regulators
Types
Growth Factors: Stimulate cell growth and activate CDKs to promote cell cycle progression.
Contact Inhibition: Cells stop dividing when they make contact with surrounding cells.
Anchorage Dependence: Cells require attachment to a substrate for division to occur.
Checkpoints Overview
Importance of Checkpoints
Cells must pass through 3 major checkpoints to progress in the cell cycle.
Checkpoints assess damage and environment conditions.
Healthy cells complete the cycle while damaged cells may induce cell cycle arrest.
Major Checkpoints
G1 Checkpoint
Function: Most critical checkpoint that checks for cell size, growth signals, and DNA damage.
Go Signal: Cells continue to S phase.
Stop Signal: Cells enter G0 phase (non-dividing state).
G0 Phase
Some cells, like muscle and nerve cells, may remain in G0 permanently, while others can re-enter the cycle if needed.
G2 Checkpoint
Function: Ensures DNA replication is complete and checks for DNA damage before mitosis.
Go Signal: Cells proceed to mitosis.
Stop Signal: Cells attempt to repair damage or undergo apoptosis if irreparable.
M Checkpoint
Function: Ensures proper attachment of microtubules to chromosomes.
Go Signal: Cells continue to anaphase.
Stop Signal: Cells pause for completing spindle attachment.
Outcomes of Checkpoint Assessment
Repairable Mutations
If the DNA is repairable, cells can restore function and produce identical daughter cells.
Irreparable Mutations
If damage is severe, cells undergo apoptosis, preventing potential diseases like cancer.
Comparison: Necrosis vs. Apoptosis
Distinction important for understanding cell death mechanisms.
Review Questions
Signal Transduction Pathways
Question Example: Impact of mutation preventing ligand binding on cell division implications.
Answer Choices: Varying impacts on signal transduction that affect cellular proliferation.
Cyclin B and CDK1 Interaction
Discussion: Role of cyclin B in controlling mitotic entry and effects of toxins preventing its degradation during the cell cycle.