The Cell Cycle

The Cell Cycle Overview

  • The cell cycle refers to the process by which a cell duplicates its contents and divides into two daughter cells. \n- It consists of several phases that ensure the faithful duplication and division of both nuclear and cytoplasmic contents.

Phases of the Cell Cycle

  • G1 Phase: First gap phase where the cell grows, assessing size and environment; if conditions are favorable, it progresses to the next phase.
  • S Phase: DNA synthesis phase where DNA is replicated.
  • G2 Phase: Second gap phase where the cell prepares for mitosis, continuing to grow and producing necessary proteins.
  • M Phase: Mitosis, where cell division occurs.

Key Transitions and Checkpoints

  • G1 Checkpoint (Restriction Point):
    • Checks cell size, growth factors, nutrients, and DNA integrity to determine if the cell can transition to the S phase.
  • G2-M Transition:
    • Transition from G2 to M phase influenced by cell size, DNA replication success, and the presence of DNA damage.
  • Metaphase-Anaphase Transition:
    • Ensures that all chromosomes are correctly attached to the spindle fibers before proceeding to anaphase.

Cell Cycle Regulation

  • Cell Cycle Control System:
    • A critical regulatory system consisting of two main protein types:
    1. Cyclin-dependent Kinases (Cdk): Always present, but active only when bound to cyclins.
    2. Cyclins: Present only at certain times in the cell cycle, they regulate progression through the cycle.

Cyclins and Cdks

  • G1 Cyclin and Mitotic Cyclin:
    • G1 cyclins drive the cell past the G1 checkpoint, while mitotic cyclins promote progression through mitosis.
  • Cyclin-Cdk Complexes:
    • When cyclins bind to Cdks, they activate the kinase function which phosphorylates target proteins, triggering necessary cell cycle events.

Checkpoints in Detail

  • G1 Checkpoint:
    • Monitors cell size and conditions; successful progression results in G1 cyclin production and formation of an active G1 Cdk-cyclin complex that activates the machinery for DNA replication.
  • G2 Checkpoint:
    • Ensures DNA replication is complete and checks for DNA damage; if conditions are satisfied, mitotic cyclin increases, forming the M-phase Promoting Factor (MPF) that catalyzes the transition to mitosis.
  • Metaphase Checkpoint:
    • Confirms all chromosomes are attached to the spindle apparatus and ensures the fidelity of chromosome segregation.

Mitosis Phases

  1. Prophase: Chromosomes condense, and the nuclear envelope breaks down.
  2. Prometaphase: Mitotic spindle forms, and chromosomes attach to spindle fibers.
  3. Metaphase: Chromosomes align at the metaphase plate.
  4. Anaphase: Chromatids are pulled apart to opposite poles of the cell.
  5. Telophase: Nuclear envelope reassembles, and chromosomes relax.

Cytokinesis

  • Occurs during the end of anaphase and telophase, involving the contraction of microfilaments resulting in cell membrane cleavage into two separate daughter cells.

Meiosis Overview

  • Meiosis consists of two rounds of division: Meiosis I and Meiosis II.
  • In Meiosis I, homologous chromosomes separate; in Meiosis II, sister chromatids separate.
  • This process results in four haploid gametes with half the genetic material of the original cell.

DNA Damage Checkpoint

  • Operates at all phases, uses the p53 protein to halt progression if significant DNA damage is detected, facilitating repair or apoptosis if damage is irreparable.