The first phase of mitosis, prophase , is marked by the condensation of chromosomes. During interphase, the DNA is replicated resulting in a chromosome that is now comprised of two genetically identical strands of DNA (the identical strands of DNA are connected at a location called a centromere and are referred to as "sister chromatids ”). These replicated chromosomes remain relatively stretched out until the start of prophase. It is difficult to visualize individual chromosomes prior to prophase using standard microscopy. This changes once the cell progresses into G2, and by the end of prophase - the chromosomes are completely condensed and visible under a light microscope.
In this complex process of chromosome condensation, each replicated elongated chromosome becomes supercoiled and, as a result, becomes considerably shorter and more tightly packed (the replicated chromosome shown in the image above). Each replicated chromosome is only a few microns long, and by the end of prophase appears as two replicated, identical chromatids attached at the centromere (a condensed area found on all eukaryotic chromosomes). It is important to consider the significance of chromosome condensation; a highly compacted chromosome is easier to move than a stretched-out one. Moreover, chromosomes will physically be moved around in the cell later. If the DNA were stretched out, it would be subject to physical shearing. In the condensed form, it is less prone to physical damage.