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Cell Cycle Overview

  • Definition: The cell cycle is a series of phases and steps that a cell undergoes to replicate itself, transforming one cell into two. It is essential for cell growth and replication.
  • Importance: The cell cycle is critical not only for cell duplication but also for the regulation of cell growth.

Major Phases of the Cell Cycle

  • The cell cycle consists of two main parts: Interphase and Mitosis.
    • Interphase: Divided into three stages.
      1. G1 Phase (Gap 1)
      2. S Phase (Synthesis)
      3. G2 Phase (Gap 2)
    • Mitosis (M Phase): Involves the actual segregation of chromosomes.

Definition of a Cell

  • Cell Structure: A cell is the basic unit of all living things and is classified based on three components:
    1. Cell Membrane: Phospholipid bilayer surrounding the cell.
    2. Nucleus: Contains genetic material (DNA) in the form of chromatin, which consists of DNA wrapped around histone proteins.
    3. Cytoplasm: The jelly-like substance filling the cell.

Interphase

  • Purpose: Preparation for cell division.

G1 Phase (Gap 1)

  • Activities:
    • Cell Growth: Increase in cell size and organelle number (ribosomes, mitochondria, etc.).
    • Preparation for DNA Replication: Synthesize proteins and enzymes needed for DNA replication.
    • Repair Mechanisms: Check for damage such as thymidine dimers in the DNA and repair them.

Duration of G1 Phase

  • Cells spend varying amounts of time in G1, which can range from hours to years depending on the type of cell.

Types of Cells Based on Cell Cycle Dynamics

  1. Labile Cells (Proliferative Cells): Cells that continuously cycle through the cell cycle (e.g., skin cells, GI tract, blood cells - hematopoietic stem cells).
  2. Stable Cells: Not continuously cycling; can re-enter the cell cycle when stimulated (e.g., liver cells, kidney tubules).
  3. Permanent Cells: Cells that do not divide after maturity (e.g., neurons, skeletal muscle cells, cardiac muscle).

S Phase (Synthesis)

  • Activities:
    • DNA Replication: DNA unwinds and forms replication bubbles, synthesizing new double-stranded DNA.
    • Enzymatic Action: DNA polymerase types 1 and 3 are involved; they replicate DNA efficiently with minimal mistakes (approx. one mistake per million base pairs).
    • Result: DNA content doubles, leading to 92 chromosomes (46 from each parent).
  • Duration: Approximately six hours in typical cells.

G2 Phase (Gap 2)

  • Activities:
    • Final Preparations for Mitosis: Ensure there are enough organelles and cytoplasm for two cells. The cell also checks for any DNA damage post-replication.
    • Size Increase: Focus on increasing cell size to ensure equal division.
  • Duration: Usually around two hours.

Mitosis (M Phase)

  • Definition: The process through which replicated chromosomes are segregated into two new nuclei.
  • Phases of Mitosis:
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase
    5. Cytokinesis (often considered part of telophase).

Prophase

  • Chromatin Condensation: Chromatin condenses into visible chromosomes.
  • Nuclear Envelope Breakdown: The nuclear envelope dissolves to allow chromosomes to separate.
  • Microtubule Formation: Centrioles form spindle fibers, connecting to chromosomes.

Metaphase

  • Chromosome Alignment: Chromosomes align along the metaphase plate.
  • Kinetochore Formation: Each chromosome has a kinetochore where microtubules attach.

Anaphase

  • Separation of Sister Chromatids: The cohesin protein holding sister chromatids together is cleaved, allowing them to move to opposite poles.
  • Motor Proteins: Dynein and kinesin facilitate the movement of chromatids along microtubules.

Telophase

  • Chromosome De-condensation: Once chromosomes reach the poles, they begin to de-condense back into chromatin.
  • Nuclear Envelope Reforming: Nuclear envelopes reform around each set of chromosomes.
  • Preparation for Cytokinesis: Actin and myosin form a cleavage furrow, preparing to split the cytoplasm.

Cytokinesis

  • Shrinking Cytoplasm: The cleavage furrow crisply divides the cytoplasm, creating two distinct cells.
  • Completion of Division: Each cell has a nucleus with 46 chromosomes (2n).

Checkpoints in Cell Cycle Regulation

  • G1/S Checkpoint: Ensures the cell is ready for DNA synthesis and the DNA is intact before replication begins.
  • G2/M Checkpoint: Verifies the integrity of the DNA after replication to ensure no errors before mitosis.
  • M Checkpoint (Spindle Checkpoint): Confirms that all chromosomes are correctly attached to the spindle apparatus before anaphase begins.

Summary

  • Review: The cell cycle involves organized phases ensuring successful cellular replication. Each phase has specific functions and checkpoints to maintain cellular integrity and function, critical for growth, repair, and reproduction of cells.