Mitosis is the fundamental process of cell division that allows a single parent cell to divide into two genetically identical daughter cells. This duplication enables multicellular organisms to grow and replace damaged or old cells. The primary purpose is ensuring each new cell receives a complete copy of the genetic blueprint. This mechanism requires the cell’s DNA to change its physical state for precise handling and separation during division.
The Structure of DNA Before Visibility
Before the active division phase, the cell spends the majority of its life in Interphase. During this preparatory stage, the genetic material exists as chromatin, a complex of DNA wrapped around specialized proteins called histones. Chromatin is unstructured, resembling tangled, fine threads dispersed throughout the nucleus.
In this diffuse, unwound state, the DNA is used for processes like gene expression. However, it is not compact enough to be seen as distinct, individual chromosomes under a standard light microscope. Although the DNA has already duplicated during the S phase of Interphase, the copies remain intertwined within the loose chromatin structure. This non-visible, thread-like conformation must be transformed before the cell can safely divide the genetic material.
Prophase: The Phase of Chromosome Condensation
The moment chromosomes become visible marks the beginning of Prophase, the first stage of mitosis. Visibility is achieved through condensation, where the diffuse chromatin fibers coil and fold upon themselves. This compaction is driven by specialized protein complexes, such as condensins, which supercoil the DNA.
This extreme coiling reduces the length of the DNA strands significantly, transforming the tangled material into discrete, compact, rod-shaped structures. These newly visible structures are the mitotic chromosomes, each appearing as an ‘X’ shape. The ‘X’ form signifies duplicated DNA, with each arm being an identical copy, known as a sister chromatid, joined at the centromere.
Condensation continues throughout Prophase, making the chromosomes progressively thicker and more distinct. By the end of this stage, the nuclear envelope dissolves, allowing the compact chromosomes to be accessed by the cell’s division machinery. This transition organizes the genetic information into manageable packets for separation.
The Movement of Visible Chromosomes
Once the chromosomes are fully visible, they begin to move and organize during Metaphase. Microtubules from the forming spindle apparatus attach to the centromere of each chromosome, pulling them into a single, straight line at the cell’s center, known as the metaphase plate. At this point, the chromosomes are at their most condensed and most clearly distinguishable under a microscope.
Anaphase involves the separation of the sister chromatids. Specialized proteins holding the chromatids together are cleaved, allowing the newly separated chromatids—now considered individual chromosomes—to be pulled toward opposite poles of the cell. This movement ensures an identical set of genetic material travels to each daughter cell.
Finally, during Telophase, the separated chromosomes arrive at their poles, and the process of visibility is reversed. New nuclear envelopes form around each cluster of DNA, and the chromosomes begin to decondense. As the DNA unwinds back into its loose, thread-like chromatin form, the distinct, rod-shaped chromosomes disappear from view, returning to the non-visible state of Interphase in the two new daughter cells.