Video Lesson_Session 2_Architecture of Cell Cycle Control

Phases of the Cell Cycle

  • Interphase

    • G1 Phase (Gap 1)

    • S Phase (Synthesis)

      • DNA replication crucial for genomic stability to prevent diseases.

    • G2 Phase (Gap 2)

  • M Phase (Mitosis)

    • Mitotic phase where cell division occurs.

  • Cytokinesis

Cell Cycle Checkpoints

  • Important checkpoints to assess cell readiness:

    • G1 to S Checkpoint: Determines if conditions are favorable for DNA replication.

    • G2 to M Checkpoint: Checks if DNA is replicated correctly before mitosis.

    • M Checkpoint: Ensures all chromosomes are attached to the spindle before separation.

Variation in Cell Cycle Duration by Organism

  • Human Cells

    • Approximately 18 hours for one complete cell cycle.

    • Mitosis and cytokinesis are relatively short phases compared to interphase.

  • Xenopus Embryos (Frog)

    • Only around 30 minutes for completion of cell cycle; rapid divisions.

  • Drosophila (Fruit Flies)

    • Similar short duration for cell division as Xenopus embryos.

Cell Cycle Control System

  • Role of Cyclins and Cyclin-Dependent Kinases (CDKs)

    • CDKs must bind to cyclins to become activated.

    • CDKs catalyze the transfer of phosphate groups (phosphorylation) from ATP to target proteins, altering their activity.

  • Phosphorylation

    • Changes protein shape and function; can activate or inactivate proteins.

    • Essential for progression through checkpoints.

Key Proteins in Cell Cycle Regulation

  • Retinoblastoma Protein (Rb)

    • Tumor suppressor that inhibits progression at G1 to S checkpoint.

    • Must be phosphorylated by CDKs to become inactive and allow progression through the cell cycle.

  • p21 Protein

    • Inhibits the cell cycle in response to DNA damage, acting as a tumor suppressor by binding to CDKs, preventing their activity.

Cancer and Cell Cycle Control

  • Disruption in regulation leads to cancer.

  • Tumor Suppressor Genes:

    • Including Rb and p53, block cell division. Defects can lead to uncontrolled growth.

  • Breast Cancer Example:

    • Abnormal expression of cyclins leads to overactive CDK activity, pushing the cell through checkpoints inappropriately.

Mechanism of Tumor Suppressor Regulation in Cancer Cells

  • Mutations can prevent transcription factors from accessing tumor suppressor genes, leading to uncontrolled proliferation.

  • Normal cells use checkpoints to prevent division in response to DNA damage, while cancer cells bypass these controls.

Importance of Cell Cycle Checkpoints

  • Ensure genomic stability by preventing damaged cells from dividing.

  • Critical in distinguishing when a cell can proceed to the next phase based on its condition.

  • Understanding cell cycle mechanisms is vital for grasping diseases such as cancer.

Future Topics

  • Upcoming discussions will follow on mitosis and meiosis, focusing on the specific events in each phase.