In mitosis, the chromosomes duplicate once and divide once. The chromosomes line up end to end, so that when they are pulled apart each cell receives a full set of genetic material.
In meiosis, the chromosomes duplicate once but divide twice. This reduces the amount of genetic material in each cell by half. When the cells divide the first time, chromosomes line up in pairs instead of end to end. This ensures that each cell receives half the genetic material.
