Cell Cycle Overview

Importance of the Cell Cycle

• Series of phases and steps for cell replication (interphase and mitosis).
• Essential for cell replication and controlling cell growth.
• Regulation involves proto-oncogenes, tumor suppressor genes, and DNA repair enzymes and genes.

Components of a Cell

• Cell Membrane: Phospholipid bilayer surrounding the cell.
• Nucleus: Contains genetic material (chromatin - DNA + histone proteins).
• Cytoplasm: The fluid inside the cell.

The Phases of the Cell Cycle

1. G1 Phase (Gap 1):

   • Cell grows and produces new organelles.
   • Synthesis of proteins and enzymes for DNA replication.
   • DNA repair (thymine dimers) to prevent replication errors.
   • Most cells stay in this phase (variable duration: hours to years).

2. S Phase (Synthesis):

   • DNA replication occurs, doubling the genetic material (46 to 92 chromosomes).
   • Maintained by DNA polymerases (high fidelity).
   • Duration is typically constant (about 6 hours).

3. G2 Phase (Gap 2):

   • Further cell growth and preparation for mitosis.
   • Increase in cytoplasm and organelles to ensure equal division.
   • Lasts approximately 2 hours.

4. Interphase: Combined stages of G1, S, and G2 phases.

Checkpoints

• G1/S Checkpoint: Ensures DNA is undamaged before replication.
• G2/M Checkpoint: Ensures no errors in DNA replication before entering mitosis.
• M Checkpoint: Ensures chromosomes are aligned properly before separation.

Mitosis (M Phase)

1. Prophase:
   • Chromatin condenses into chromosomes.
   • Nuclear envelope dissolves via cyclin-dependent kinases.
   • Microtubule Organization Centers (centrioles) appear.
2. Metaphase:
   • Nuclear envelope is dissolved.
   • Chromosomes align at the metaphase plate.
   • Microtubules connect to chromosomes at the kinetochore.
3. Anaphase:
   • Chromosomes are pulled to opposite poles of the cell.
   • Motor proteins (dynein, kinesin) facilitate movement.
4. Telophase:
   • Nuclear envelope reforms.
   • Chromatin becomes loose.
   • Cell begins to form two separate cells (cleavage furrow).
5. Cytokinesis:
   • Equal distribution of cytoplasm.
   • Formation of two identical diploid cells (2n, 46 chromosomes each).

Types of Cells Regarding the Cell Cycle

Labile Cells (Proliferative): Constantly going through the cell cycle.

• Examples: Skin epithelium, GI tract, urinary tract, hematopoietic stem cells.

Stable Cells: Replicate when stimulated.

• Examples: Hepatocytes, kidney tubule epithelium, alveolar cells of the lungs.

Permanent Cells: Do not re-enter the cell cycle.

• Examples: Neurons, skeletal muscle, cardiac muscle.

Additional Considerations

• Aging and Telomeres: With age, telomeres shorten, making cells unable to re-enter the cycle (cell senescence).
• G0 (Quiescent) Phase: Cells rest and may re-enter the cycle if stimulated (stable cells).