DNA coping is not perfectly accurate. During reproduction, no bio-chemical reaction is absolutely reliable so the process of copying the DNA will have some variations each time, As a result, the DNA copies generated will be similar but may not be identical to the original. Thus the surviving new born cells are similar to but subtly different from each other. This inbuilt tendency for variation during reproduction is the basic for evolution. DNA remains constant The DNA in the cell nucleus is the information source for making proteins.
The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions - to build copies of their DNA. This creates two copies of the DNA in a reproducing cell, and they will need to be separated from each other. DNA copying occurs accompanied by the creation of an additional cellular apparatus. The DNA copies then separate, each having its own cellular apparatus for maintaining life processes.