Presented in the digital fluorescence microscopy image above is a single rat kangaroo (PtK2) kidney cell in the early stages of prophase. The chromatin is stained with a blue fluorescent probe (DAPI), while the microtubule network (mitotic spindle) is stained green (Alexa Fluor 488) and cellular mitochondria are stained with a red dye (MitoTracker Red CMXRos). Each duplicated chromosome contains two identical sister chromatids joined together along their length, with a constricted region occurring at a specific DNA sequence known as the centromere. The chromatin in the centromere exhibits a somewhat higher degree of condensation than the rest of the chromosome. Centromeres are necessary for proper segregation of the sister chromatids during later stages of mitosis.