Mitosis is part of the cell cycle. Cycle implying no end. Between fertilization and the blastula stage this is true. Ultimately though, most cells "rest" at "G0", often times for the rest of their lives. This animation concentrates of the roles of the chromosomes and the centrosome (black). The "G" phases are growth phases. "S" is when DNA synthesis occurs. The nuclear envelope is pictured as translucent white when it is present. The rest of the cell and its components have been left out for clarity.