G1 and G2 stand for 'gaps'. This refers to the fact that nothing very obvious is occurring in the nucleus of the cells during these stages. The cells are actually very active. They are growing and preparing to divide.
S stands for synthesis. This is the phase of the cell cycle in which the DNA is copied or replicated.
M stands for mitosis. This is the stage of the cell cycle in which the cell actually divides into two daughter cells.
To help you visualize the process, the same animated illustration of the process is shown below and at the end of this topic.
Many cancer drugs act by blocking one or more stages of the cell cycle. In order to better understand the defects found in cancer cells and the mechanisms of action of those anti-cancer drugs designed to block cell division, we will examine the cell cycle in more detail.