The mitosis phase of the cell cycle is a biological process through which a single cell divides into two identical daughter cells. This process ensures that genetic material is equally distributed between the new cells. Mitosis is essential for how living organisms grow, repair tissues, and, in some cases, reproduce.
The Purpose of Mitosis
Cells undergo mitosis for several important biological reasons. One primary function is growth, as multicellular organisms develop from a single fertilized egg through repeated mitotic divisions, increasing their cell count. Mitosis also plays a significant role in the repair and replacement of damaged or old cells, such as those lining the digestive tract or skin cells, ensuring tissue integrity and regeneration. For many single-celled organisms, mitosis serves as a method of asexual reproduction, allowing them to create genetically identical offspring and expand their populations.
Prophase
The initial stage of mitosis, prophase, marks the beginning of structural changes within the cell. During prophase, the diffuse genetic material, known as chromatin, undergoes a condensation process, coiling tightly to form compact, visible chromosomes. Each of these condensed chromosomes consists of two identical copies, called sister chromatids, which are joined together at a constricted region called the centromere. Simultaneously, the nucleolus, a dense structure within the nucleus, begins to disappear.
As the chromosomes condense, the mitotic spindle, a structure composed of microtubules, starts to assemble. These microtubules extend from centrosomes, which migrate towards opposite poles of the cell. Towards the end of prophase, the nuclear envelope begins to break down. This allows the spindle microtubules to access the condensed chromosomes.
Metaphase
Following prophase, the cell enters metaphase, characterized by the precise alignment of chromosomes. During metaphase, all the duplicated chromosomes are meticulously positioned along the cell’s equatorial plane. This central line is often referred to as the metaphase plate.
The accurate alignment is facilitated by kinetochore microtubules, which are specialized spindle fibers that attach to the kinetochore. The kinetochore is a protein structure located at the centromere of each sister chromatid. Microtubules from opposing poles of the cell attach to the kinetochores of sister chromatids, exerting balanced tension that holds the chromosomes in their central position.
Anaphase
Anaphase is a stage of mitosis where the sister chromatids, which were aligned at the metaphase plate, finally separate. The protein “glue” holding the sister chromatids together at their centromeres is broken down, allowing them to detach from each other. Once separated, each chromatid is now considered an individual chromosome.
These newly individualized chromosomes are then actively pulled towards opposite poles of the cell. This movement is primarily driven by the shortening of the kinetochore microtubules, which effectively reel in the chromosomes. Simultaneously, the cell itself begins to elongate, as non-kinetochore microtubules lengthen and push the poles further apart. This coordinated separation and movement ensures that each forming daughter nucleus will receive a complete and equal set of genetic information.
Telophase and Cytokinesis
Telophase marks the final stage of nuclear division, where the events of prophase are essentially reversed, and the cell prepares for physical division. As the separated chromosomes arrive at the opposite poles of the cell, they begin to decondense, uncoiling back into their less compact, thread-like chromatin form. Concurrently, new nuclear envelopes reform around each complete set of chromosomes, establishing two distinct nuclei within the elongating cell. The nucleoli also reappear within these newly formed nuclei, and the mitotic spindle disassembles.
Following the completion of nuclear division in telophase, cytokinesis occurs, which is the division of the cytoplasm and its contents. In animal cells, cytokinesis involves the formation of a cleavage furrow, a pinching inward of the cell membrane near the metaphase plate. This furrow deepens as a contractile ring of actin and myosin filaments tightens, eventually dividing the parent cell into two separate daughter cells. In plant cells, a cell plate forms in the middle of the cell, growing outwards from the center to create a new cell wall that separates the two daughter cells. The culmination of telophase and cytokinesis results in two genetically identical daughter cells, each with its own nucleus and cytoplasm.