A cell makes a copy of its DNA before mitosis occurs so there is a set of DNA for the daughter cell after mitosis has occurred. Because each cell needs its own set of DNA, there must be two sets of DNA present in a cell before it divides into two.
Mitosis is the process of cell division that creates a new cell identical to the original. Somatic cells, such as muscles, hair and skin, undergo mitosis regularly in humans and other organisms. This is an important type of cell division needed to facilitate the repair of damaged cells, growth and replacement of old cells with new ones.
When a new cell is created, it must have the same library of genetic information all other cells in the body have access to. Because all the material in the new cell must come from the first cell, the original cell must make a copy of its DNA before completing the process of mitosis. These two sets of DNA only exist for as long as it takes the cell to undergo mitosis, which can be anywhere from 30 to 90 minutes in certain human cells. When the cell division is complete, both of the cells have a single identical copy of DNA.