Lecture Notes: The Cell Cycle

Overview

• Average human height: ~2 meters
• Cell size: ~100 micrometers (1/1,000,000 of human height)
• Comparison: Human beings grow and reproduce; cells grow and divide (cell division)

The Cell Cycle

• Purpose: Describes the lifespan and division process of a cell
• Analogy: Similar to seasons in a year (growth and rest phases)

Main Phases

1. Interphase (like fall/winter)

   • Period of cell growth, not division
   • Cells spend most of their time here
   • Exception: Cancer cells (divide more than they grow)

2. Mitosis (like spring/summer)

   • Active cell division
   • Abbreviated as "M"

Interphase Subphases

1. G1 Phase (Growth Phase 1)

   • Longest phase
   • Production of extra organelles (ribosomes, proteins)
   • Prepares for eventual cell division

2. S Phase (Synthesis Phase)

   • DNA replication occurs
   • Chromosomes duplicate from 23 pairs to 46 pairs

3. G0 Phase (G not/Quiescent Phase)

   • Cells exit the cycle
   • No more cell division (example: neurons in the brain)
   • End point for these cells, do not re-enter the cycle

4. G2 Phase (Growth Phase 2)

   • Further preparation for mitosis
   • Production of microtubules (for pulling chromatids apart)

Mitosis

• Final phase in the cell cycle
• Cell divides into two daughter cells
• Each daughter cell then enters G1 phase again (cycle restarts)

Summary

• The cell cycle is cyclical and continuous except for cells entering G0 phase
• Mitosis and Interphase are the two overarching stages
• Cell cycle ensures growth, division, and replication of cells