Prophase

Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin fibers become tightly coiled, condensing into discrete chromosomes. It is crucial for the reader to note that chromatin is a complex consisting of both chromosomes and specific proteins. Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesin protein complex. Chromosomes are typically visible at high magnification through a light microscope.
Also inside the nucleus, the nucleolus in the nucleus disappears from view. This is noteworthy because the cell does not need to divide the nucleolus right away. It will later reform when the nucleus divides completely.
Close to the nucleus are structures called centrosomes, consisting of a pair of centrioles, and actin, a halo of microtubule fragments, centrioles are found in most eukaryotic animal cells. The centrosome is the coordinating center for the cell's microtubules. A cell inherits a single centrosome at cell division, which is replicated by the cell with the help of the nucleus before a new mitosis begins, giving a pair of centrosomes. The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin. Molecular motor proteins then push the centrosomes along these microtubules to opposite sides of the cell. Although centrioles help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in mitosis.