Lecture Notes: The Cell Cycle

Introduction

• The cell cycle is crucial for:
  • Replicating cells: turning one cell into two.
  • Controlling cell growth.
  • Upcoming topics: regulation of the cell cycle, proto-oncogenes, tumor suppressor genes, and DNA repair enzymes.

Definition of a Cell

• Basic unit of all living things.
• Eukaryotic cells (focus on human cells) have:
  • Cell membrane: phospholipid bilayer.
  • Nucleus: houses genetic material (chromatin - DNA wrapped around histones).
  • Cytoplasm: fluid inside the cell.

DNA and Cell Cycle

• DNA in the nucleus must be duplicated for cell replication.

Phases of the Cell Cycle

Interphase:

1. G1 Phase (Gap 1):

   • Cell prepares for replication.
   • Increase in organelles.
   • Synthesis of proteins and enzymes.
   • Checks for DNA damage (e.g., thymidine dimers).
   • Most cells spend a lot of time in G1.
   • Types of cells:
     • Labile cells: constantly proliferating (e.g., skin, GI tract, red bone marrow).
     • Stable cells: replicate with strong stimulus (e.g., liver hepatocytes, kidney tubules, alveolar cells).
     • Permanent cells: do not replicate (e.g., neurons, skeletal and cardiac muscle).

2. S Phase (Synthesis):

   • DNA replication occurs.
   • Uses DNA polymerases.
   • Checks for replication errors.
   • Duration: ~6 hours.

3. G2 Phase (Gap 2):

   • Focus on cell growth.
   • Increase in cytoplasm.

Mitosis (M Phase):

• Sequence: PMAT (Prophase, Metaphase, Anaphase, Telophase)
• Cytokinesis: final separation of cytoplasm.

Prophase

• Chromatin condenses into chromosomes.
• Nuclear envelope dissolves.
• Formation of microtubule organization centers (centrioles).

Metaphase

• Chromosomes align on the metaphase plate.
• Polar microtubules connect to kinetochores.

Anaphase

• Chromatids separate and move to opposite poles.
• Motor proteins (dynein and kinesin) assist movement.

Telophase

• Chromosomes de-condense.
• Nuclear envelope reforms.
• Formation of cleavage furrow leading to cytokinesis.

Checkpoints

• G1-S Checkpoint: checks DNA integrity before synthesis.
• G2-M Checkpoint: ensures no DNA replication mistakes.
• M Checkpoint: ensures chromosomes are aligned properly.

Additional Concepts

• Quiescent (G0) Phase: cells rest and may re-enter the cycle if stimulated.
• Cell Senescence: irreversible cell cycle arrest due to telomere shortening.(ageing)

Basics of chromosomes. No. Of centromere is no. Of chromosomes Chromosome has centromere and telomere

Conclusion

• Importance of understanding cell cycle for cell replication and growth.
• Encouragement to engage with further resources and support the platform.