Prophase marks the initial stage of cell division. This phase prepares the cell by organizing its genetic material for precise distribution into new daughter cells. During prophase, significant cellular reorganization takes place, setting the foundation for accurate chromosome segregation.
Chromosome Condensation
Before prophase, the cell’s genetic material exists as diffuse chromatin within the nucleus. During prophase, this chromatin compacts into discrete, rod-like structures known as chromosomes. This condensation makes the long, thin strands much shorter and thicker, preventing tangling and damage during subsequent division stages.
Each replicated chromosome consists of two identical sister chromatids, tightly joined at a constricted region called the centromere. The precise and organized condensation ensures chromosomes are manageable units, ready for efficient separation.
Nuclear Envelope and Nucleolus Changes
As chromosome condensation progresses, the nuclear envelope, the double membrane surrounding the nucleus, begins to break down. This breakdown effectively eliminates the physical barrier between the nucleus and the cytoplasm, allowing cellular machinery to access the condensed chromosomes.
Simultaneously, the nucleolus, a dense structure within the nucleus, also disappears. This dissolution clears the central region of the cell, making way for the formation of the mitotic spindle. Both the nuclear envelope breakdown and the nucleolus disappearance are coordinated events that prepare the internal cellular environment for the attachment and movement of chromosomes.
Mitotic Spindle Assembly
While the chromosomes condense and the nuclear structures dissolve, the cell also initiates the assembly of the mitotic spindle, a dynamic structure composed of microtubules. In animal cells, this process begins with the duplication of the centrosome, which serves as the primary microtubule-organizing center. These two newly formed centrosomes then begin to migrate away from each other, moving towards opposite poles of the cell. This movement establishes the two focal points from which the spindle will extend.
As the centrosomes move apart, microtubules, which are hollow protein filaments, polymerize and extend from each centrosome. These growing microtubules form the framework of the mitotic spindle, radiating outwards like spokes from a wheel. Some of these microtubules will grow towards the center of the cell, eventually overlapping with microtubules from the opposite pole, contributing to the structural integrity of the spindle. The continuous polymerization and depolymerization of these microtubules allow the spindle to dynamically adjust its structure.
Other microtubules, known as kinetochore microtubules, will specifically attach to the kinetochores, which are protein complexes located at the centromere of each sister chromatid. This attachment is a precise process that ensures each chromosome is properly oriented and connected to the spindle from both poles. The formation of this intricate network of microtubules is essential because it will serve as the cellular machinery responsible for pulling the sister chromatids apart and distributing them equally into the two future daughter cells. The completion of mitotic spindle assembly effectively sets the stage for the next phases of cell division, where chromosomes will be aligned and then separated.