prophase
Prophase is the first and longest phase of mitosis, see Figure below . During prophase, the chromatin DNA coils up into visible chromosomes, each made up of two sister chromatids held together by the centromere. Also during this phase, the nucleolus disappears, and the spindle begins to form from the centrioles. Most eukaryotic cells contain structures known as centrosomes, consisting of a pair of centrioles. During prophase, the centrioles begin to move to opposite ends, or poles, of the cell. As the centrioles migrate, the fiber-like spindle begins to elongate between the centrioles. The spindle is a thin, cage-like structure made out of microtubules. In plant cells, the spindle forms without centrioles. The spindle plays an essential role moving chromosomes and in the separation of sister chromatids.