Mitosis is the process by which a single cell divides its duplicated genetic material to produce two genetically identical daughter cells. This type of cell division is used by somatic cells—all body cells other than reproductive cells—for growth and repair. Mitosis is organized into distinct stages, and prophase is the first and longest, serving as the cell’s systematic preparation for chromosome separation. During prophase, the cell organizes and compacts its long strands of DNA while building the specialized machinery required to move these structures, ensuring each new cell receives a complete genome.
Chromatin Condensation and Sister Chromatid Formation
The most significant event of prophase is the reorganization of the cell’s genetic material. Before mitosis, the DNA exists as a diffuse network called chromatin, spread throughout the nucleus. During prophase, this dispersed chromatin undergoes coiling and folding to transform into compact, rod-shaped structures known as chromosomes.
This compaction process is driven by protein complexes, such as condensin II, which introduce supercoils into the DNA structure. This tight packaging is necessary to prevent the DNA from being tangled or broken as it is moved across the cell later in mitosis. As the chromosomes condense, they become visible under a light microscope as distinct units.
Because the DNA was duplicated during the preceding S phase, each fully condensed chromosome consists of two identical copies, referred to as sister chromatids. These two DNA molecules are held tightly together along their length by the protein complex cohesin. They are joined at a constricted region called the centromere, resulting in the characteristic X-shape of a mitotic chromosome. This pairing prepares the genome for precise segregation into the two future daughter cells.
Centrosome Separation and Spindle Fiber Assembly
While the chromosomes condense within the nucleus, mechanical restructuring occurs in the cytoplasm. Animal cells contain a pair of microtubule-organizing centers called centrosomes, duplicated earlier in the cell cycle. During prophase, these two centrosomes begin to move away from each other, migrating toward opposite poles of the cell.
This separation is facilitated by motor proteins that push the centrosomes apart along the newly forming microtubules. As the centrosomes move, they nucleate a dense array of microtubules around themselves, forming structures known as asters. Microtubules from the two separating asters begin to overlap in the central region of the cell, forming the nascent mitotic spindle apparatus.
The forming spindle is composed of different classes of microtubules. Astral microtubules radiate outward toward the cell membrane, helping to anchor and position the spindle within the cell. Polar microtubules extend toward the center, where they interdigitate with fibers from the opposite pole, establishing the bipolar framework. This early spindle assembly is the framework responsible for accurately capturing and pulling the chromosomes apart in later stages.
Defining the End of Prophase
The transition from prophase to prometaphase is marked by the breakdown of the nuclear envelope. This double membrane structure separates the nucleus, where the chromosomes are held, from the cytoplasm, where the mitotic spindle is forming. As prophase ends, regulatory proteins trigger the phosphorylation of the nuclear lamins, structural proteins that support the nuclear envelope.
This phosphorylation causes the nuclear lamina to disassemble, leading to the fragmentation of the nuclear membrane into small vesicles. The dissolution of the nuclear envelope is necessary because it allows the mitotic spindle microtubules to gain access to the condensed chromosomes. Once the barrier is removed, spindle fibers can attach to specialized protein structures called kinetochores, which form on the centromere of each sister chromatid. When the nuclear envelope is fully fragmented, prophase is complete, and the cell enters prometaphase, where chromosomes begin moving toward the cell’s equator.