Notes on the Cell Cycle

Introduction to the Cell Cycle

• The cell cycle is a series of phases and steps a cell goes through to replicate itself, resulting in one cell becoming two.
• It is crucial for cellular replication and control of cell growth.
• Future discussions will cover regulation including proto-oncogenes, tumor suppressor genes, and DNA repair genes.

What is a Cell?

• A cell is the basic unit of all living things classified by three main components:
  • Cell Membrane: A phospholipid bilayer surrounding the cell.
  • Nucleus: Houses genetic material (DNA in the form of chromatin).
  • Cytoplasm: The fluid within the cell.

Stages of the Cell Cycle

Interphase

Interphase consists of three phases:

1. G1 Phase (Gap 1):

   • Preparations for DNA replication begin here.
   • Key activities:
     • Increase number of organelles.
     • Synthesize proteins and enzymes required for DNA replication.
     • Repair DNA damage (e.g., thymidine dimers).
   • Most cells spend considerable time in G1 phase.

2. S Phase (Synthesis):

   • The primary function is DNA replication.
   • New double-stranded DNA is synthesized (semi-conservative model).
   • DNA polymerases (Type 1 and Type 3) are crucial for dependable replication.
   • At the end of this phase, the cell has 92 chromosomes (46 pairs).

3. G2 Phase (Gap 2):

   • Focused on cell growth and preparation for mitosis.
   • Ensures enough cytoplasm and organelles are present for two cells.
   • Completes checks for DNA replication errors.

M Phase (Mitosis)

• Mitosis is separated into several stages:

1. Prophase:

   • Chromatin condenses into visible chromosomes.
   • Nuclear envelope begins to dissolve.
   • Microtubule organization centers (centrioles) form.

2. Metaphase:

   • Chromosomes align along the metaphase plate.
   • Microtubules connect to kinetochores on chromosomes for separation.

3. Anaphase:

   • Sister chromatids are pulled to opposite poles of the cell by motor proteins (dynein and kinesin).
   • Cohesin proteins connecting chromatids are cleaved for separation.

4. Telophase:

   • Nuclear envelope reforms around separated chromosomes.
   • Chromosomes begin to de-condense back into chromatin.

5. Cytokinesis:

   • The actual physical separation of the cytoplasm; a cleavage furrow forms, leading to two distinct daughter cells.

Types of Cells Based on Replication Behavior

• Labile Cells: Constantly replicate (e.g., skin cells, GI tract cells, hematopoietic stem cells).
• Stable Cells: Can replicate in response to a strong stimulus (e.g., liver cells, kidney tubules).
• Permanent Cells: Do not replicate (e.g., neurons, cardiac muscle).

Cell Cycle Checkpoints

1. G1/S Checkpoint: Ensures cell conditions are right for DNA synthesis.
2. G2/M Checkpoint: Verifies that DNA has been replicated correctly before mitosis.
3. M Checkpoint: Confirms that all chromosomes are aligned properly before separation.

Aging and Telomeres

• Telomeres shorten with repeated cell divisions which can lead to cellular senescence (the state where cells no longer divide).

Conclusion

• The cell cycle is essential for growth and maintenance of living organisms. Understanding its phases aids in comprehending processes like tissue growth and cancer progression.

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References:

• Future videos will cover detailed regulation, tumor suppressor genes, and DNA repair mechanisms.