Cell Division Lecture Notes

Introduction to Cell Division

• Definition: Process where a parent cell divides into two or more daughter cells.
• Human Cell Composition:
  • Contains 46 chromosomes:
    • 22 pairs of homologous chromosomes
    • 2 sex chromosomes
• During division:
  • Contents including organelles and cytoskeleton double.
  • Chromosomes duplicate into two sister chromatids.
  • Results in two identical daughter cells, each with 46 chromosomes.

Cell Cycle Overview

• Ordered sequence of events preparing a cell for division.
• Typical human cell cycle time: 24 hours, minimum 12 hours.
• Four Phases:
  • G1 (Gap 1) Phase
  • S (DNA Synthesis) Phase
  • G2 (Gap 2) Phase
  • M (Mitosis) Phase

Phases of the Cell Cycle

Interphase

• Consists of G1, S, and G2 phases.
• Excludes G0 phase.

G1 Phase

• Duration: Several hours to months, follows mitosis.
• Critical for:
  • Synthesis of RNA, proteins, and organelles.
  • Protein synthesis for mitosis (e.g., mitotic spindle).
• Checkpoints:
  • G1 Checkpoint (Restriction Checkpoint): Restricts entry into S phase without growth signals.
  • Loss of G1 checkpoint (e.g., p53 malfunction) leads to unregulated division.

S Phase

• Duration: Approximately 8 hours.
• DNA replication results in sister chromatids.
• Histones duplicate.
• Mismatch Repair takes place.
• Once initiated, must be completed.

G2 Phase

• Duration: 2 to 5 hours.
• Preparation for division by reorganizing organelles and cytoskeleton.
• G2 Checkpoint: Ensures DNA replication completeness and checks for DNA damage.
  • Mitosis promoting factor: cyclin B/cdk1 complex.

M Phase

• DNA Condensation: Sister chromatids become visible.
• Cytokinesis: Division into two cells.
• Mitotic Stages:
  1. Prophase: Formation of mitotic spindle.
  2. Metaphase: Alignment of sister chromatids.
  • Clinical Link: Vincristine prevents microtubule attachment.
  3. Anaphase: Separation of sister chromatids.
  4. Telophase: Physical division of cell, chromosome decondensation.
  • Clinical Link: Spindle poisons arrest mitosis.

G0 Phase

• Resting Phase: Cell exits cycle and becomes quiescent.
• Reentry into cycle requires external stimuli (growth factors).
• Cell Type Dependence:
  • Neurons: Spend most lifespan in G0.
  • Intestinal lining cells: Rarely enter G0.

Additional Notes

• Cell Cycle Control: Checkpoints and regulatory proteins ensure proper division.
• Clinical Implications: Understanding of cell cycle aids in cancer treatment (e.g., targeting checkpoints, spindle formation).