Exam Preparation Advice

  • Advise against bringing dinner to the exam.
    • Suggest small snacks are acceptable.
    • Warn that junk food can be distracting or inappropriate.

Review Session Overview

  • Scheduled for today and Wednesday, following the end of last class.
  • Focus on general topics related to cell cycle signaling and regulation.

Cell Cycle Signaling Review

  • Key Components:
    • Cyclin-dependent kinases (CDK)
    • G1 cyclin
    • Rb protein (Retinoblastoma protein)
    • E2F transcription factor

CDK Activation and Function

  • The CDK itself does not change; it induces…
    • The transcription of G1 cyclin.
    • Binds to G1 CDK, leading to activation.
  • Upon activation, CDK phosphorylates (Rb).
  • Phosphorylation of (Rb) leads to activation of E2F.
    • E2F is responsible for:
    • Transcription of G1 factors and S cyclins.
    • Activation of G1/S CDK and S CDK to progress through Start and into S phase.
  • S Phase Function:
    • S CDK facilitates DNA replication.

G2 Phase Activities

  • In G2, most DNA has been replicated.
  • Synthesis of M cyclin begins during this phase.

M Cyclin and CDK Regulation

  • Although M cyclin and CDK are present, M CDK levels remain low due to:
    • An inhibitory phosphate.
    • CDC25 removes the inhibitory phosphate for activation.
  • Inhibition of CDC25:
    • Prevented by DNA damage through CHECK1 and CHECK2, thus blocking entry into M phase if damage is detected.

Mitosis Overview

  • Centrosomes:
    • Centrosomes double in number.
    • Isotonic framework begins to form.
Prophase Events
  • Chromosomes condense and become visible.
  • Breakdown of the nuclear envelope.
  • Microtubules start attaching to kinetochores.
Metaphase Events
  • APC (Anaphase Promoting Complex) is phosphorylated by MCDK.
  • APC triggers several events after chromosomes align:
    • CBC20 (Co-activator of APC) inhibition is relieved, enabling movement of sister chromatids.
    • This process includes both:
    • Anaphase A: Movement of chromosomes to opposite poles.
    • Anaphase B: Movement of spindle poles toward the cell cortex.

APC Functionality

  • APC degrades S cyclin, leading to reduction in S CDK activity.
  • As S CDK levels drop, CDK inhibitors (CDKN2) act upon the CDC20 complex,
    • Resulting in keeping CDK levels low until G1 phase.
  • Cdh1 keeps activity suppressed following the drop in M CDK activity.
Telophase and Cytokinesis
  • Events in telophase include:
    • Reformation of the nuclear envelope.
    • Decondensation of chromosomes.
    • Formation of a contractile ring at the cleavage furrow, starting cytokinesis but not completing it.
  • Cytokinesis is a separate process from mitosis, falling under M phase.

Questions and Interactive Session

  • Open floor for questions; clarification offered on complex processes.
  • Interactive decision on focus areas for remaining review time:
    • Reviewing previous exams.
    • Discussing slides and practice questions.