Most cells only measure a few thousandths of a millimeter in diameter. Each human being starts life as a single cell, a fertilized egg, which divides into more cells during the embryonic development. In the course of this duplication, cells begin to differentiate into muscle cells, skin cells, nerve cells, and so on.
Cells continue to divide and differentiate throughout a person's life.
Most human cells reproduce through the process called mitosis. In mitosis, a cell divides and forms into two identical daughter cells. Each daughter cell then doubles in size and becomes capable of dividing. During the period between divisions, the cell grows and carries on its normal activities, which include a duplication of its chromosomes, a process which precedes mitosis. As these enzymes appear in a cell, their chemical reactions cause the cell to become specialized.