Mitosis is the fundamental process by which a single cell divides into two genetically identical daughter cells. This division is the mechanism behind growth, tissue repair, and the replacement of old or damaged cells. The process ensures that new cells receive an exact, complete set of genetic material from the parent cell. Although division occurs as a continuous cycle, scientists break it down into distinct stages for better understanding.
The Preparatory State
The cell spends the majority of its life in interphase, the preparatory stage before active division begins. Although sometimes called a “resting phase,” the cell is highly active, performing normal functions and growing significantly. During this time, the cell copies all internal structures, including the specialized centers that organize division.
The most important event in interphase is the replication of the cell’s DNA, which occurs in the S (Synthesis) phase. This replication results in two identical copies of the genetic instructions, which will be separated later in mitosis. This complete duplication is necessary for daughter cells to receive the full complement of DNA.
Beginning the Division Process
The transition to active division begins with prophase, the first stage of mitosis. The long, thread-like DNA material, called chromatin, begins to coil and condense tightly, forming visible, compact structures. These condensed structures are the chromosomes, each consisting of two identical copies, referred to as sister chromatids, joined together.
Simultaneously, the nuclear envelope surrounding the DNA starts to break down and dissolve. Outside the nucleus, the mitotic spindle, a framework of protein fibers called microtubules, begins to assemble. The structures organizing these fibers move toward opposite sides of the cell, establishing the poles for division.
Alignment and Separation
Following the breakdown of the nuclear envelope, the chromosomes are moved into position for the split. This movement culminates in metaphase, where all the paired chromosomes line up precisely along the cell’s center. This central line is called the metaphase plate, and this alignment ensures each new cell receives an equal share of genetic material.
Specialized spindle fibers attach to the center of each chromosome, forming connections from both poles of the cell. Fibers from one pole attach to one sister chromatid, and fibers from the opposite pole attach to the other. This creates tension that holds the chromosome steady at the center. This arrangement acts as an internal checkpoint, delaying the process until every chromosome is correctly positioned.
The separation of the genetic material occurs in anaphase, a rapid stage where the sister chromatids are pulled apart. The protein holding the sister chromatids together dissolves, and the chromatids instantly become individual chromosomes. The attached spindle fibers shorten, reeling the chromosomes toward the opposite ends of the cell. This synchronized movement ensures an identical set of chromosomes moves to each pole.
Finalizing the Split
Once the separated chromosomes arrive at the opposite poles, telophase, the final stage of nuclear division, begins. This phase reverses the events of prophase to establish two new, distinct nuclei. A new nuclear envelope forms around each complete set of chromosomes at the two poles.
The tightly coiled chromosomes begin to relax and uncoil, returning to their extended, thread-like chromatin form. Although mitosis (the division of the nucleus) is complete, the cell still needs to physically separate. This final step is called cytokinesis, the division of the cell’s cytoplasm and organelles.
In animal cells, cytokinesis involves the formation of a cleavage furrow, a contracting ring that pinches the cell membrane inward until the cell splits. In plant cells, a cell plate forms down the middle to build a new cell wall, separating the two daughter cells. Cytokinesis yields two new cells, each an exact copy of the original parent cell.