The process that enables this to occur is called mitosis. Mitosis is occurring in your body right now. It has been since you were just a fertilized egg. It is how you have gone from just this one cell to the billions of cells that you are now.
The human sperm has 23 chromosomes. The human egg (ova) has 23 chromosomes. When the egg is fertilised these chromosomes pair up. The fertilised egg therefore has 23 pairs of chromosomes. Every human cell (apart from the sperm and ova) have 23 pairs of chromosomes.
If when the fertilised egg underwent its first division, it merely split its contents there’d be only 23 chromosomes in each daughter cell.
Next division there’d be 11 ½ chromosomes in each cell!!! These cells would no longer be human.
Mitosis allows the cells to maintain the correct number of chromosomes with each division.
During the early stage of mitosis the chromosomes thicken and copy themselves. This is called replication.
This process of replication results in two identical sister chromatids.