Mitosis is the process by which a cell divides its nucleus to create two genetically identical daughter cells. Metaphase represents the third stage of this process. During interphase, the cell’s genetic material is duplicated, resulting in replicated chromosomes. Each replicated chromosome consists of two identical DNA molecules, known as sister chromatids, joined together at the centromere. Metaphase is when these sister chromatids are positioned in preparation for separation.
Attachment and Tension Generation
The foundation of metaphase involves the precise engagement of the mitotic spindle. This spindle is composed of microtubules, which connect to the sister chromatids at specialized protein complexes called kinetochores, located on the centromere. A proper connection, termed bi-orientation, occurs when the two kinetochores of a single replicated chromosome attach to microtubules emanating from opposite poles of the cell. The microtubules from one pole pull one chromatid, and those from the opposite pole pull the other. This tug-of-war generates mechanical tension, which signals that the attachment is correct and ready for the next stage.
Alignment at the Metaphase Plate
The continuous push-and-pull forces generated by the spindle microtubules actively move the replicated chromosomes. The sister chromatids are repeatedly shifted back and forth until they settle into a single, straight line across the center of the cell. This imaginary plane at the cell’s equator is known as the metaphase plate. Positioning all replicated chromosomes precisely on the metaphase plate ensures that when the sister chromatids finally separate, each new daughter cell will receive one complete and identical copy of every chromosome.
The Spindle Assembly Checkpoint
Before the cell commits to dividing the genetic material, the Spindle Assembly Checkpoint (SAC) is activated. The SAC acts as a quality control system, pausing mitosis until it confirms that every kinetochore is correctly attached to the spindle and experiencing appropriate tension. If any kinetochore remains unattached or is attached incorrectly, the SAC remains active. An active checkpoint generates a biochemical signal that prevents the cell from proceeding to the next stage. Key proteins like Mad2 and BubR1 are responsible for this regulation, working together to inhibit the Anaphase-Promoting Complex/Cyclosome (APC/C), the enzyme complex that triggers chromatid separation.
Preparing for Separation
Once all sister chromatids are properly aligned and the SAC confirms tension, the inhibitory signal is silenced. The cell is cleared to initiate separation, marking the end of metaphase. This transition is swift and irreversible, driven by the activation of the APC/C. The activated APC/C targets and degrades securin, an inhibitory protein that keeps the protease separase inactive. Separase then cleaves the cohesin complex, the molecular glue holding the sister chromatids together. This allows the joined sister chromatids to be pulled apart toward opposite poles by the spindle microtubules, beginning the anaphase stage.