Chapter 12 Part 3

Overview of the Cell Cycle and Mitosis

Understanding the mitotic spindle and the behavior of chromosomes during cell division.

The Mitotic Spindle

Definition: A structure made of microtubules that separates sister chromatids during cell division.
Major Components:

  • Kinetochore Microtubules: Attach to the kinetochores on chromosomes and are crucial for chromosome movement, ensuring proper segregation to daughter cells.
  • Non-Kinetochore Microtubules: Do not connect directly to chromosomes; they play a role in elongating the cell during mitosis, which is essential for successful cytokinesis.
  • Aster Fibers: Help anchor the spindle to the cell membrane and contribute to cell elongation by pushing against the cell cortex, ensuring stability of the spindle apparatus throughout cell division.
Kinetochore Structure

Kinetochore Proteins: Located at the centromere of sister chromatids, these proteins connect to spindle fibers. The kinetochore acts as the attachment site for microtubules, essential for chromosome movement during mitosis. They also sense tension from microtubule binding, facilitating effective movement.

Stages of Mitosis
  1. Prophase:
    • Chromosomes condense and become visible, referred to as sister chromatids post-replication.
    • Centrosomes migrate to opposite poles of the cell, allowing the mitotic spindle to begin forming between them, establishing a framework for chromosome separation.
  2. Prometaphase:
    • The nuclear envelope fragments into vesicles, allowing spindle fibers to contact chromosomes.
    • Chromosomes further condense, ensuring proper alignment. Spindle fibers continue to develop, attaching to kinetochores.
  3. Metaphase:
    • Chromosomes align at the metaphase plate, equidistant from both poles of the spindle. This alignment ensures each daughter cell receives an identical set of chromosomes.
  4. Anaphase:
    • Sister chromatids are pulled apart towards opposite poles. Kinetochore microtubules shorten while non-kinetochore microtubules lengthen, assisting in cell elongation and ensuring equal distribution of chromosomes. The cohesion between sister chromatids is broken, allowing independent movement.
  5. Telophase:
    • Chromosomes decondense, and the nuclear envelope reforms around each set of chromosomes at the poles, preparing for the final stage of cell division.
    • The spindle apparatus disassembles as it’s no longer needed, and the cell prepares for cytokinesis.
    • Cytokinesis: In animal cells, a cleavage furrow forms, pinching the cell into two daughter cells. In plant cells, a cell plate forms, developing into a new cell wall through vesicle fusion at the cell's center.
Kinetochore Dynamics

Microtubule shortening occurs at the kinetochores as sister chromatids are pulled apart, ensuring efficient separation.
Motor Proteins: Facilitate movement along microtubules and aid in the depolymerization process, playing a crucial role in chromatid movement dynamics. These proteins are essential for energy-dependent transport mechanisms enabling chromosome segregation.

Experimental Evidence

Fluorescent labeling and bleaching experiments demonstrate that shortening occurs at the kinetochore ends of microtubules connected to sister chromatids, confirming their role in separation and enhancing understanding of mitosis.

Cleavage Furrow vs. Cell Plate
  • Animal Cells: Utilize a cleavage furrow, mediated by actin and myosin, to physically split the cell. This contraction of the cytoskeleton is crucial for cytokinesis.
  • Plant Cells: Form a cell plate via vesicle fusion, creating a new cell wall between daughter cells, maintaining their structural integrity.
Onion Root Tip Experiment

Onion root tips are commonly used in laboratories to observe mitosis due to their rapid cell division.
Key phases visible include prophase, prometaphase, metaphase, anaphase, and telophase. These observations effectively illustrate the characteristic features of each stage of mitosis, providing a clear and practical method for students to visualize the complex processes involved in cell division.