Mitosis is the process of nuclear division that occurs in somatic, or non-reproductive, body cells. This mechanism ensures that when one parent cell divides, it produces two new daughter cells that are genetically identical to the original. To organize the complex sequence of events, biologists use the mnemonic PMAT, which stands for Prophase, Metaphase, Anaphase, and Telophase. These four distinct stages represent the steps a cell takes to accurately partition its duplicated genetic material.
Mitosis: Context and Purpose of Cell Division
Mitosis facilitates growth, replaces cells that are old or worn out, and repairs damaged tissues, making it a constant process throughout life. It is the division phase (M phase) of the larger cell cycle, which begins after Interphase. During Interphase, the cell grows, duplicates its organelles, and replicates its entire DNA content. This duplication ensures the cell contains two complete sets of genetic information before mitosis begins.
The purpose of the PMAT sequence is to distribute this duplicated DNA equally and precisely between the two daughter cells. Accurate division is necessary because errors can lead to cells with an incorrect number of chromosomes. By the end of mitosis, the goal is to create two cells, each containing a full, diploid set of chromosomes, ensuring genetic stability.
Prophase: Preparation for Nuclear Separation
Prophase marks the moment when the cell begins to prepare for nuclear separation. The loose, thread-like DNA material (chromatin) starts to coil and condense tightly, transforming into distinct, visible structures called chromosomes. Each chromosome at this stage is composed of two identical sister chromatids held together at a central region called the centromere.
Simultaneously, the cell begins constructing the mitotic spindle, a framework of protein fibers called microtubules that will serve as the machinery for moving the chromosomes. Centrosomes move toward opposite sides of the cell, establishing the two poles of the forming spindle. The nuclear envelope also begins to break down, allowing the spindle fibers access to the condensed chromosomes.
Metaphase: Alignment on the Equatorial Plate
Metaphase begins when the spindle fibers, extending from both poles, attach to the chromosomes. These fibers link specifically to the kinetochore, a protein structure on the centromere of each sister chromatid pair. The defining action of this stage is the precise alignment of all the duplicated chromosomes.
The attached spindle fibers exert balanced tension, pulling each chromosome until it settles along the exact center of the cell. This central plane is referred to as the metaphase plate. Before the cell can proceed, the metaphase checkpoint verifies that every chromosome is properly attached to the spindle fibers from both poles. This ensures the genetic material is perfectly positioned for equal distribution in the next stage.
Anaphase: Separation of Sister Chromatids
The transition into Anaphase is triggered by the cleavage of the cohesin proteins holding the sister chromatids together at the centromere. Once this bond is broken, the identical sister chromatids instantly separate and are considered individual chromosomes. This event effectively doubles the number of chromosomes temporarily present in the cell.
The newly separated chromosomes are rapidly pulled toward opposite poles of the cell by the shortening of the spindle fibers. This movement is powered by motor proteins along the depolymerizing microtubules. Concurrently, other microtubules lengthen, causing the entire cell to elongate and increasing the distance between the two forming sets of genetic material.
Telophase and Cytokinesis: Finalizing the Split
Telophase represents the final phase of nuclear division. Once the two complete sets of chromosomes arrive at their poles, the mitotic spindle apparatus begins to disassemble. New nuclear envelopes form around each cluster of chromosomes, creating two distinct nuclei within the single parent cell. Inside these new nuclei, the condensed chromosomes relax and decondense, returning to their loose, thread-like chromatin state.
Following Telophase, or sometimes beginning concurrently, is Cytokinesis, the physical division of the cell. In animal cells, this process involves the formation of a contractile ring (composed of actin and myosin filaments) that tightens around the cell’s midsection like a drawstring. This tightening creates an indentation known as the cleavage furrow, which deepens until it completely pinches the parent cell into two separate daughter cells. Each daughter cell now possesses its own nucleus and a full complement of organelles, ready to begin its own cell cycle.