Prophase is the initial stage of cell division, specifically in the process called mitosis, which is how most body cells reproduce. This phase serves as the fundamental preparatory step where the cell gets its internal components ready to separate into two identical daughter cells. The goal of this stage is to organize the genetic material and the cellular machinery needed to precisely divide the replicated DNA. By the end of this stage, the cell’s internal environment is restructured to ensure successful separation in later phases.
Chromosomes Become Visible
The most striking change is the organization of the cell’s genetic material, a process known as chromatin condensation. Before prophase, the DNA exists as chromatin, a loose, tangled network dispersed within the nucleus. This structure must transform into distinct, compact structures to be moved without becoming damaged or tangled.
The long, thread-like DNA molecules begin to coil and fold tightly, shortening their overall length significantly. Specialized protein complexes, such as condensin, facilitate this coiling by bundling the DNA. This compaction makes the genetic material observable under a light microscope as individual, rod-shaped chromosomes. Each structure is already duplicated, consisting of two identical copies, called sister chromatids, joined together at the centromere.
The condensed, X-shaped chromosomes reduce the physical volume of the genetic material. Tightly packed chromosomes are easier for the cell to manipulate and move precisely to opposite ends during later separation stages. Simultaneously, the nucleolus, a structure inside the nucleus, begins to break down and disappear.
The Nuclear Barrier Disappears
As chromosomes condense, the cell simultaneously dismantles the physical structures defining the nucleus, preparing the internal space for chromosome movement. The nuclear envelope, the double membrane separating the chromosomes from the cytoplasm, starts to fragment into small vesicles. This disassembly is triggered by the phosphorylation of proteins that make up the nuclear lamina.
The breakdown of the nuclear barrier releases the condensed chromosomes directly into the cytoplasm, making them accessible to the division machinery. Outside the nucleus, the centrosomes—structures that organize the cell’s internal framework—begin migrating toward opposite sides of the cell. These centrosomes duplicated during interphase, and their separation establishes the two poles of the future mitotic spindle.
As the centrosomes move apart, they extend a network of protein filaments called microtubules, initiating the formation of the mitotic spindle. These fibers will eventually act as tracks to pull the sister chromatids apart. Motor proteins help push the centrosomes toward their final positions at the cell’s poles. This movement ensures that the spindle apparatus is properly oriented and ready to capture the chromosomes.
Setting the Stage for Chromosome Alignment
The completion of prophase marks the point where the cell has executed the necessary structural changes to begin the actual separation process. The key accomplishment is the transformation of diffuse genetic material into manageable, condensed chromosomes, now freely positioned in the cell. The mitotic spindle, anchored by the separated centrosomes, extends its microtubules toward the chromosomes. Spindle fibers attach to the kinetochore, a specialized protein structure on each chromosome’s centromere. This attachment guides the chromosomes to the cell’s mid-line for alignment.