Segments of these strings of DNA are called genes. Genes have from a few hundred pairs to many thousands of pairs of As, Ts, Cs, and Gs joined together in a precise code that is unique to each gene. 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.