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Overview of the Cell Cycle and Mitosis

Introduction to the Cell Cycle

  • The cell cycle consists of multiple stages including growth, DNA replication, and division.
  • Mitosis is just one part of this cycle and involves the division of the cell's nucleus.

Historical Context

  • In 1882, Walther Flemming observed chromosome behavior during mitosis and cytokinesis, developing dyes for this purpose.
  • Coined the terms "mitosis" and "chromatin".

Phases of the Cell Cycle

  1. Mitosis (M Phase)

    • The mitotic phase includes two primary processes: mitosis and cytokinesis.
    • Usually the shortest phase of the cell cycle.

  2. Interphase

    • Much longer than mitosis, usually accounts for about 90% of the cell cycle.
    • Interphase can be divided into three subphases:
      • G₁ phase (First Gap): The cell grows by producing proteins and organelles.
      • S phase (Synthesis): DNA replication occurs, chromosomes are duplicated.
      • G₂ phase (Second Gap): Further growth and preparation for mitosis.
    • The durations of these phases can vary by cell type. For instance, in a human cell:
      • M phase: <1 hour
      • S phase: 10-12 hours (approximately half the cycle)
      • G₁ and G₂ phases vary; G₁ can last 5-6 hours, G₂ typically lasts 4-6 hours.

Mitosis Stages

Mitosis is broken down into five sequential stages:

  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase
  • Cytokinesis typically overlaps with the final stages of mitosis.

The Mitotic Spindle

  • Functions in the proper segregation of chromosomes during mitosis.
  • Composed of microtubules and associated proteins, begins to form during prophase.
  • Assembles at the centrosome, which organizes microtubules throughout the cell cycle.
  • Involves two essential components:
    • Centrosomes: Organize microtubule structures, containing pairs of centrioles (not essential for cell division).
    • Microtubule Dynamics: They can elongate (polymerize) by adding tubulin subunits or shorten (depolymerize) by losing them.

Kinetochore and Chromosome Movement

  • Each sister chromatid has a structure called the kinetochore at its centromere.
  • Kinetochore microtubules attach to kinetochores. Their number can vary:
    • Yeast: 1 microtubule per kinetochore
    • Mammals: Up to 40 microtubules per kinetochore.
  • Chromosomes move toward the poles in a tug-of-war motion due to microtubule attachment. They first move in one direction until captured by microtubules from the opposite pole.

Experiments on Kinetochore Functionality

  • An experiment by Gary Borisy at the University of Wisconsin examined kinetochore microtubules during anaphase:
    • Labeled microtubules with fluorescent dye.
    • Findings indicated that during anaphase, the movement of chromosomes is linked to the shortening of microtubules at the kinetochore ends rather than at the spindle pole ends.
  • This suggests a primary mechanism of chromosome movement during anaphase is by "walking" along microtubules while they depolymerize on the kinetochore end.

Anaphase

  • The shortest stage of mitosis, characterized by rapid movement:
    • Begins with the cleavage of cohesins holding sister chromatids.
    • Chromatids transition into full chromosomes and move towards opposite ends of the cell.
    • Nonkinetochore microtubules play a role in cell elongation during anaphase.

Telophase

  • Resulting daughter nuclei form as nuclear envelopes reassemble from fragments of the parent cell's nuclear envelope.
  • Chromosomes de-condense, and spindle microtubules disassemble.

Cytokinesis

  1. In Animal Cells

    • Begins with the formation of a cleavage furrow, an indentation in the cell surface near the metaphase plate.
    • A contractile ring of actin filaments and myosin forms, leading to cell division.
    • The cleavage furrow pinches the cell into two daughter cells.

  2. In Plant Cells

    • Involves the formation of a cell plate from vesicles that fuse at the metaphase plate, ultimately leading to a new cell wall.