Step-by-step explanation:
The information stored in the DNA is essential for life. If a cell dies the body must replace that cell. The only way to replace the cells is to first copy the information that the cell contained. There is a complex system of proteins and enzymes that unravel the DNA double helix so that the DNA can be copied.
If a single cell dies it can be replaced through mitosis. The two daughter cells are identical to the original cell whose DNA was copied. This system works well with single cell and simple organisms.