Lecture Notes: The Cell Cycle

Introduction

• Importance of the Cell Cycle
  • Essential for cell replication, turning one cell into two.
  • Controls cell growth.
  • Future discussions to include cell cycle regulation involving proto-oncogenes, tumor suppressor genes, and DNA repair enzymes.

Basics of a Cell

• Definition of a Cell
  • Basic unit of all living things.
  • Eukaryotic cells consist of:
    • Cell Membrane: Phospholipid bilayer.
    • Nucleus: Contains genetic material (DNA as chromatin).
    • Cytoplasm.

Overview of the Cell Cycle

• Purpose: Duplicate DNA to create two identical cells.
• Phases: Interphase (G1, S, G2), Mitosis (Prophase, Metaphase, Anaphase, Telophase).

Interphase

• G1 Phase (Gap 1)

  • Cell grows, organelle production, protein/enzyme synthesis.
  • Repair DNA damage before replication (e.g., thymidine dimers).
  • Most cells spend varying time in G1 (labile, stable, and permanent cells).

• S Phase (Synthesis)

  • DNA replication (semi-conservative model).
  • DNA polymerases ensure minimal mistakes.
  • Results in a cell going from 2N to 4N (46 to 92 chromosomes).

• G2 Phase (Gap 2)

  • Cell growth to ensure equal division.
  • Prepare for Mitosis.

• Interphase Overview

  • Includes G1, S, G2 phases.
  • G1-S Checkpoint ensures readiness for DNA replication.

Mitosis (M Phase)

• Stages of Mitosis
  • Prophase: Chromatin condenses into chromosomes, nuclear envelope dissolves, centrioles form microtubules.
  • Metaphase: Chromosomes align on metaphase plate, microtubules attach to kinetochores.
  • Anaphase: Sister chromatids separate and are pulled to opposite poles.
  • Telophase: Chromosomes decondense, nuclear envelopes reform, cytoplasm divides (cytokinesis).

Types of Cells Related to the Cell Cycle

• Labile (Proliferative) Cells

  • Constantly in cell cycle (e.g., skin, GI tract, hematopoietic stem cells).

• Stable Cells

  • Enter cell cycle upon stimulus (e.g., liver hepatocytes, renal tubules, alveolar cells).

• Permanent Cells

  • Do not re-enter cell cycle (e.g., neurons, cardiac and skeletal muscle).

Regulation and Checkpoints

• Checkpoints
  • G1-S Checkpoint: DNA integrity before replication.
  • G2-M Checkpoint: Ensure DNA replication was error-free.
  • M Checkpoint: Ensure chromosomes are aligned properly before separation.

Additional Concepts

• Senescence: Cells cease to divide, related to telomere shortening.
• Quiescent Phase (G0): Resting phase for stable cells that can re-enter cycle if needed.

Summary

• Cell cycle includes key regulatory phases ensuring proper cell replication.
• Cells vary in their cell cycle activity based on type and stimulus.
• Importance of checkpoints to maintain genetic integrity and function.

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• The cell cycle is crucial for understanding cell growth, division, and regulation.
• The lecture details the phases and important checkpoints for maintaining cell function and integrity.