1. G0 phase is viewed to be a phase where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle . G0 is sometimes referred to as a "post-mitotic" state, since cells in G0 are in a non-dividing phase outside of the cell cycle. Therefore should not be a regular part of cell cycle.
2. If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted, and the cell attempts to either complete DNA replication or repair the damaged DNA. This self-destruction mechanism ensures that damaged DNA is not passed on to daughter cell and is important in preventing cancer.