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Understanding the Cell Cycle Stages

May 11, 2025

Cell Cycle Lecture Notes

Introduction

  • The cell cycle is crucial for cell replication and growth control.
  • Two main components: Interphase and Mitosis.
  • Future topics include regulation of the cell cycle, proto-oncogenes, tumor suppressor genes, and DNA repair enzymes.

What is a Cell?

  • Basic unit of all living things.
  • Three main components:
    • Cell Membrane: Phospholipid bilayer surrounding the cell.
    • Nucleus: Houses genetic material (DNA in chromatin form).
    • Cytoplasm: Gel-like fluid inside the cell.

Interphase

Overview

  • Prepares the cell for mitosis by duplicating DNA and organelles.
  • Consists of three phases: G1, S, and G2.

G1 Phase (Gap 1)

  • Prepares for DNA replication.
  • Activities:
    • Increase in organelles (e.g., ribosomes, mitochondria).
    • Synthesize proteins and enzymes.
    • Repair DNA for thymine dimers.
  • Most cells exist in G1.
  • Types of cells:
    • Labile (Proliferative) Cells: Constantly replicating (e.g., skin, GI tract).
    • Stable Cells: Can replicate with strong stimulus (e.g., liver, kidney tubules).
    • Permanent Cells: Do not replicate (e.g., neurons, skeletal muscle, cardiac muscle).

S Phase (Synthesis)

  • DNA replication occurs.
  • Main Functions:
    • Forms a replication bubble to duplicate DNA.
    • Maintained by DNA polymerases, ensuring minimal errors.
  • Lasts about 6 hours.

G1/S Checkpoint

  • Ensures DNA is undamaged before replication.

G2 Phase (Gap 2)

  • Cell growth and preparation for mitosis.
  • Main Function:
    • Increases cytoplasm and cell size.

Mitosis

Overview

  • Division of the cell into two identical cells.
  • Consists of four phases: Prophase, Metaphase, Anaphase, Telophase, followed by Cytokinesis.

Prophase

  • Condensation of chromatin into chromosomes.
  • Key Events:
    • Dissolution of the nuclear envelope.
    • Appearance of centrioles and formation of the microtubule organization center.

Metaphase

  • Chromosomes align on the metaphase plate.
  • Key Events:
    • Microtubules attach to the kinetochore of chromosomes.

Anaphase

  • Separation of sister chromatids.
  • Key Events:
    • Motor proteins (dynein and kinesin) facilitate movement of chromatids to opposite poles.

Telophase

  • Reformation of the nuclear envelope.
  • Key Events:
    • Chromatin decondenses.
    • Formation of cleavage furrow marks the start of cytokinesis.

Cytokinesis

  • Final division of the cytoplasm to form two separate cells.

Post-mitosis

  • Cells may re-enter the cell cycle or enter a quiescent phase (G0).
  • Factors Influencing Post-mitosis:
    • Stimuli can reintroduce stable cells into the cycle.
    • Permanent cells typically remain non-replicative.

Aging and Cell Cycle

  • Telomeres shorten with each replication, leading to cellular senescence.

Checkpoints in Cell Cycle

  • G1/S Checkpoint: Ensures readiness for DNA replication.
  • G2/M Checkpoint: Ensures DNA replication was error-free.
  • M Checkpoint: Ensures proper chromosome alignment before separation.

These notes summarize the key phases and processes involved in the cell cycle, providing a comprehensive overview for understanding how cells replicate and divide.

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