One of the defining characteristics of 'life' is the ability to reproduce.
Cells reproduce using a process called mitosis .
During mitosis, the cells undergo duplication and division, in other words, they produce copies of themselves. This duplication process is a normal function of cells and allows for growth and the replacement of worn or damaged cells. Some cells, such as the epithelial or skin cells, reproduce frequently, whilst others, such as mature nerve cells reproduce rarely if ever. Although cellular reproduction is a normal process, sometimes the process becomes uncontrolled and in such cases tumours and cancers develop, we shall see later that certain chemicals and radiation can promote such behavior. Much of this activity is invisible to the conventional light microscope and it is only during the mitosis stage itself that the frantic cellular activity becomes apparent.