Cellular Readiness
Before a cell can divide, it prepares during interphase. This preparatory period is not considered a part of mitosis itself, but it is a prerequisite for successful cell division. During interphase, the cell undergoes significant growth and duplicates its organelles.
The most important event during interphase is the replication of the cell’s entire DNA. Each chromosome duplicates, forming two identical sister chromatids that remain attached. This activity ensures all necessary components and genetic material are ready for division.
Chromosomes Condense
The first true stage of mitosis, called prophase, marks the beginning of visible changes within the cell’s nucleus. During this phase, the loosely organized genetic material, known as chromatin, begins to coil and condense. This condensation process makes the chromosomes progressively shorter and thicker, becoming discernible under a microscope as distinct structures. Each chromosome at this point consists of two identical sister chromatids joined together.
Concurrently, the nucleolus, a structure within the nucleus involved in ribosome production, typically disappears. The nuclear envelope, which encloses the genetic material, also begins to break down into small vesicles. Outside the nucleus, the mitotic spindle starts to form; this structure is composed of microtubules that emerge from centrosomes, which begin to move towards opposite ends of the cell, establishing the poles for the upcoming division.
Chromosomes Align
Following the condensation of chromosomes, the cell enters metaphase, a crucial stage where chromosomes achieve a precise arrangement. During metaphase, the condensed chromosomes migrate to the cell’s center. They align along an imaginary plane known as the metaphase plate, which is positioned equidistant from the two spindle poles.
Each sister chromatid within a chromosome is attached to a spindle fiber originating from opposite poles. These attachments occur at a specific region on the chromosome called the centromere. This precise alignment and attachment are essential for ensuring that each new daughter cell receives an identical and complete set of chromosomes.
Sister Chromatids Separate
The next stage, anaphase, is characterized by the dramatic separation of the sister chromatids. At the beginning of anaphase, the centromeres that have been holding the sister chromatids together suddenly divide. Once separated, each chromatid is now considered an individual chromosome.
These newly independent chromosomes are then rapidly pulled towards opposite poles of the cell. This movement is facilitated by the shortening of the spindle fibers, which act like ropes pulling the chromosomes apart. The precise and simultaneous separation of sister chromatids ensures that an equal distribution of genetic material occurs, leading to genetically identical sets of chromosomes at each pole.
Cell Division Completes
The final stages of mitosis involve telophase and cytokinesis, which together complete the process of cell division. In telophase, the separated chromosomes arrive at the opposite poles of the cell and begin to decondense, returning to their more extended, chromatin form. A new nuclear envelope forms around each set of chromosomes at the poles, effectively creating two new nuclei within the single parent cell.
Simultaneously, the nucleoli reappear within these newly formed nuclei, and the mitotic spindle disassembles. Overlapping with telophase, cytokinesis is the physical division of the cytoplasm. In animal cells, a cleavage furrow forms as a contractile ring of actin and myosin filaments pinches the cell membrane inward, eventually splitting the cell into two distinct daughter cells. Plant cells, with their rigid cell walls, form a cell plate in the middle, which grows outward to create a new cell wall that separates the two daughter cells.