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:

  1. Growth factors signal the production of cyclins.

  2. Cyclins accumulate and activate CDKs.

  3. CDKs initiate signal transduction to progress through check points.

  4. 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.