Human DNA contains 3 billion of these pairs. Genes are recipes for making proteins. These proteins determine how an organism grows and what it is like. Some proteins make cells, other proteins tell the cells how to function. Your cells are making new cells all of the time.
Different genes are active in different cells at different times.
There is a special kind of cell division that happens in sexual reproduction to form the cells that are passed on to the next generation. This division, meiosis, results in cells that have half the number of chromosomes as the original cell. Each parent produces cells that contain half of his or her genetic information. When these two special cells merge together, the two half sets of chromosomes combine to produce a fertilized egg or seed. In this way an organism inherits some traits from its mother and some from its father.