During the prophase stage of cell division, chromosomes begin to condense, coil and fold, making them visible under a light microscope. When the duplicated chromosomes continue to coil, the chromosomes are shortened and thickened to a more visible state.
Initially, chromosomes appear to be very long, thin and independent structures. Shortly after DNA begins to condense, the chromosomes change into a short and fat shape. The change in appearance causes the chromosomes to be more visible under close observation. During prophase, the duplicated chromosomes are arranged in homologous pairs after crossing over. Crossing over is the exchange of chromosome parts to give rise to genetic recombination. Crossing over occurs at a site called the chiasma. A pair of homologous chromosomes can be seen as a bivalent, a tight grouping of two chromosomes, each consisting of two sister chromatids. Chromatids are joined together by a specialized structure called the centromere. The centromere is the most central and constricted region of a DNA chromosome. The nucleolus and the nuclear envelope disappear in the stage of prophase. Spindle fibers also form and move to opposite poles of the cell during this phase. Prophase is the longest phase that occurs in cell division. After the completion of prophase, the division process continues with metaphase, anaphase and telophase.