Cell Cycle

Cell Cycle:

  • The cell cycle is a series of events that take place in a cell leading to its division and duplication.
  • It consists of interphase (G1, S, G2) and mitosis (M phase).
  • Interphase is the period between cell divisions when the cell grows and prepares for division.
  • Mitosis is the actual process of cell division, resulting in two genetically identical daughter cells.

 

Cell Cycle

 

Interphase:

G1 Phase (Gap 1):

  1. Cell Growth:
    • The cell increases in size and carries out its normal functions.
  2. Cellular Functions:
    • Organelles are duplicated, and the cell performs its specialized functions.
  3. Checkpoint (G1 Checkpoint):
    • Determines if conditions are favorable for cell division.
    • Checks cell size, nutrient availability, and DNA integrity.
  4. Decision Point:
    • Cells may exit the cell cycle and enter a non-dividing state (G0) if conditions are not favorable.
  5. Regulatory Proteins:
    • Cyclins and cyclin-dependent kinases (CDKs) play a crucial role in G1 phase regulation.

S Phase (Synthesis):

  1. DNA Replication:
    • DNA synthesis occurs, resulting in the duplication of the cell's genetic material.
    • Each chromosome is replicated, forming two sister chromatids.
  2. Centrosome Duplication:
    • The centrosome, a cellular structure involved in spindle formation, is duplicated.
  3. Checkpoint (G2 Checkpoint):
    • Checks for completeness and accuracy of DNA replication.
    • Ensures the cell is ready for mitosis.
  4. Regulatory Proteins:
    • Various enzymes and proteins are involved in coordinating DNA replication.

G2 Phase (Gap 2):

  1. Continued Cell Growth:
    • The cell continues to grow and synthesize proteins.
    • Preparation for cell division.
  2. Checkpoint (G2 Checkpoint):
    • Ensures DNA is undamaged and fully replicated before entering mitosis.
    • If DNA damage is detected, cell cycle progression may be halted for repairs.
  3. Microtubule Formation:
    • The microtubule-organizing center (MTOC) helps in the formation of the mitotic spindle.
  4. Regulatory Proteins:
    • Cyclins and CDKs, along with other regulatory proteins, control progression through G2.

 

Interphase: Stages, Cell cycle, Diagram, Video

 

G0 Phase( Quiescent Stage):

The G0 phase (G zero phase) of the cell cycle is a resting or quiescent phase where cells exit from the active cell cycle and temporarily stop dividing. Cells in the G0 phase are not actively preparing to divide and are not progressing through the typical phases of the cell cycle, such as G1, S, G2, and mitosis. Instead, cells in G0 phase are in a state of arrested growth, and they may remain in this phase for an extended period or indefinitely.

Not all cells go through the G0 phase, and the decision to enter G0 phase is often influenced by external signals and internal factors. Cells may enter the G0 phase in response to specific signals, lack of nutrients, or environmental conditions. For example, cells in the G0 phase might be differentiated, specialized cells that perform specific functions in tissues and organs.

Cells in G0 phase can potentially re-enter the cell cycle and start dividing again if they receive the appropriate signals. However, some cells, such as certain mature nerve cells and muscle cells, may stay in G0 phase permanently and not re-enter the cell cycle.

 

Mitosis (M Phase):

  1. Prophase:

    • Chromosomes condense and become visible.
    • Spindle fibers form, and the nuclear envelope breaks down.
  2. Metaphase:

    • Chromosomes align at the metaphase plate.
  3. Anaphase:

    • Sister chromatids separate and move to opposite poles.
  4. Telophase:

    • Chromatids arrive at poles, and nuclear envelopes reform.
    • Results in two identical daughter nuclei.

Cytokinesis:

  • Division of the cytoplasm and organelles.
  • In animal cells, a cleavage furrow forms.
  • In plant cells, a cell plate develops into a new cell wall.

 

Regulatory Checkpoints:

  1. G1 Checkpoint:

    • Ensures cell size, nutrients, and DNA integrity before DNA synthesis.
  2. G2 Checkpoint:

    • Verifies DNA replication and checks for DNA damage before mitosis.
  3. Mitotic Checkpoint (Spindle Checkpoint):

    • Ensures proper chromosome attachment to spindle fibers before anaphase.

Regulatory Proteins:

  • Cyclins and CDKs:
    • Regulate the cell cycle by activating key proteins.
  • Tumor Suppressor Genes (e.g., p53):
    • Monitor DNA integrity, preventing cell cycle progression with damaged DNA.
  • Oncogenes:
    • Mutated genes promoting excessive cell division, are associated with cancer.