Mitosis is a fundamental process of cell division where a single parent cell divides to produce two genetically identical daughter cells. This process is essential for growth, tissue repair, and the replacement of worn-out cells. Mitosis ensures that each new cell receives a complete and identical set of chromosomes, maintaining genetic stability. Prophase serves as the initial and often the longest stage.
Chromosome Condensation
During the interphase preceding mitosis, the cell’s genetic material exists as diffuse, thread-like structures called chromatin. As a cell transitions into prophase, this chromatin becomes progressively more compact and organized. This compaction allows the long DNA molecules to condense into distinct, rod-shaped structures known as chromosomes, which become visible under a light microscope.
Each chromosome at this stage consists of two identical copies, sister chromatids, produced during the DNA replication phase of interphase. These sister chromatids are held together tightly at a constricted region called the centromere. Condensation is facilitated by specialized protein complexes called condensins. This extensive compaction prevents the long DNA strands from becoming tangled or damaged during cell division, ensuring their accurate segregation into daughter cells.
Spindle Formation
As chromosomes condense, another key event in prophase involves the formation of the mitotic spindle, a structure of microtubules that guides chromosome movement. In animal cells, this process begins with the centrosomes, which duplicated during interphase and move away from each other. These centrosomes migrate towards opposite poles of the cell, establishing the two poles of the future spindle apparatus.
Microtubules extend from these separating centrosomes, forming a radial array called asters. These microtubules elongate and interact, forming the complete mitotic spindle. While the spindle begins to assemble in prophase, its primary function is to attach to the condensed chromosomes and pull the sister chromatids apart to opposite ends of the cell during subsequent stages of mitosis.
Nucleolus Disappearance
A notable change within the nucleus during prophase is the gradual disappearance of the nucleolus. The nucleolus is a dense structure involved in the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes. Its disappearance signifies a temporary halt in ribosome synthesis as the cell directs resources towards cell division.
This cessation of ribosome synthesis and the disassembly of the nucleolus are linked to ongoing chromosome condensation, as highly condensed DNA cannot be transcribed into RNA. The nucleolus shrinks and disperses, releasing its components into the nucleoplasm. The nuclear envelope remains largely intact during prophase, though it begins to break down towards the end of this stage, marking the transition to prometaphase.