The Stages of Mitosis and Cell Division

Mitosis: Nuclear Division and Cell Reproduction

Mitosis is nuclear division followed by cytokinesis, producing two identical daughter cells. The process involves five main stages: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase.

While often discussed alongside mitosis, Interphase is technically not part of mitosis itself. Interphase includes the preparatory stages of the cell cycle:

• G1 (Gap 1)
• S (Synthesis)
• G2 (Gap 2)

Interphase: Preparation for Division

The cell is engaged in intense metabolic activity in preparation for mitosis (the next four phases that lead to and include nuclear division). Chromosomes are not clearly discernible in the nucleus, although a dark spot called the nucleolus may be visible. The cell contains a pair of centrioles (or microtubule organizing centers in plants) which are organized sites for the formation of microtubules.

The Phases of Mitosis

Prophase

Chromatin in the nucleus begins to condense and becomes visible under the light microscope as distinct chromosomes. The nucleolus disappears. Centrioles begin moving to opposite poles of the cell, and fibers extend from the centrosomes. These fibers cross the cell to form the mitotic spindle.

Prometaphase

The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres, creating structures called kinetochores. Microtubules attach to these kinetochores, and the chromosomes begin to move.

Metaphase

Spindle fibers align the chromosomes along the middle of the cell nucleus. This central line is referred to as the metaphase plate. This organization helps ensure that in the next phase, when chromosomes are separated, each daughter nucleus will receive a complete copy of every chromosome.

Anaphase

Chromosome pairs separate at the kinetochores and move rapidly to opposite sides of the cell. This movement is the result of a combination of factors:

• Kinetochore movement along the microtubules of the spindle.
• Physical interaction and pushing action of polar microtubules.

Telophase

The chromatids reach the opposite poles of the cell, and new membranes are formed around the daughter nuclei. The chromosomes disperse and are no longer visible under the microscope. The spindle fibers disperse, and cytokinesis, or the partitioning of the cell, may also begin during this phase.

Cytokinesis: Cell Partitioning

Cytokinesis differs between animal and plant cells:

• In animal cells: Cytokinesis occurs when a fiber ring composed of the protein actin around the center of the cell contracts, pinching the cell into two daughter cells, each with its nucleus.
• In plant cells: The rigid cell wall requires that a cell plate is synthesized between the two daughter cells, eventually forming a new cell wall separating them.