Cell Cycle

The Cell Cycle

  • All living organisms have cells that come from pre-existing cells, a concept dating back over three billion years.

  • Continuous cell growth and division are essential for survival; halting cell division could lead to death within days.

Key Events in the Cell Cycle

  • Precise timing of certain events is crucial to ensure proper cell division, maintaining the integrity of the organism.

Stages of Mitosis

  • Correct order of mitosis stages:

    • Prophase

    • Prometaphase

    • Metaphase

    • Anaphase

    • Telophase

Overview of the Cell Cycle

  • Cell Division is vital for organism development.

  • Key Concepts:

    • Different stages of the cell cycle: S phase and M phase are critical.

    • Cell grows more during interphase than it divides.

Eukaryotic Cell Cycle Phases

  • The cell cycle consists of four primary phases:

    1. G1 Phase (Gap 1)

      • Growth and protein synthesis.

    2. S Phase

      • DNA synthesis and replication of chromosomes into sister chromatids.

    3. G2 Phase (Gap 2)

      • Preparation for mitosis, ensuring all components are ready.

    4. M Phase

      • Includes mitosis and cytokinesis.

Cell Cycle Phases Detailed

  • S Phase:

    • DNA is replicated, forming sister chromatids and centrosome replication in animal cells.

  • M Phase:

    • Involves mitosis and cytokinesis, where duplicated chromosomes are separated into daughter cells.

G0 Phase

  • Cells may enter the G0 phase if conditions are unsuitable for division. In G0, cells neither divide nor prepare to divide.

Cell-Cycle Control System

  • Critical mechanisms ensure cells only proceed to the next phase under ideal conditions:

    • Checkpoints: Monitor events like DNA replication and cell integrity.

    • Checkpoints are regulated by Cyclin-dependent kinases (CDKs) and cyclins.

Cyclins and CDKs

  • CDKs are constant in level but require cyclins for activation:

    • Cyclins control the timing of the cell cycle by their synthesis and degradation throughout the cycle.

  • Failure in checkpoint leads to delays and potential errors in the division process.

CDK Activating and Inhibition Mechanisms

  • Phosphorylation: Activates or inhibits CDKs, depending on the site of modification.

    • Cdk-activating kinase (CAK) activates by phosphorylation.

    • Wee1 kinase adds inhibitory phosphates, preventing entry into mitosis.

    • Cdc25 phosphatase counteracts inhibitory phosphorylation, facilitating entry to mitosis.

Anaphase-Promoting Complex / Cyclosome (APC/C)

  • APC/C triggers degradation of cyclins and proteins like securin, initiating the anaphase.

  • APC/C is crucial for the metaphase-to-anaphase transition, allowing sister chromatid separation.

Cytokinesis

  • Involves the division of cytoplasm and formation of cleavage furrow through a contractile ring made of actin and myosin.

  • RhoA GTPase plays a pivotal role in regulating the contraction leading to cell division.

Regulation of Cell Division and Growth

  • Mitogens: Stimulate cell division by activating G1/S-CDK activity.

  • Growth Factors: Promote synthesis of proteins and increase cell mass.

  • Survival Factors: Prevent apoptosis, enhancing cell longevity and proliferation.

Coordination of Growth and Division

  • Cell growth is coordinated with division to ensure cells do not shrink; signals determine this coordination.

  • Pathways controlling translation and protein synthesis contribute to overall growth and cellular activities.

Summary of Cell Division Mechanisms

  • Cell division is stimulated by mitogens.

  • Growth is supported by factors that promote cellular mass increase.

  • Survival factors preserve cell integrity by inhibiting death signals.

Practice Questions

  • What mechanisms regulate entry into S phase?

  • Describe the phases of the cell cycle, including subphases of mitosis and cytokinesis.

  • Define cyclins and cyclin-dependent kinases and their roles in the cell cycle.