Changes in the code
Every time one of your cells divides into two, it must copy the DNA code in its nucleus, one copy for each of the new cells. You have around 6000 million chemical 'letters' - base pairs - of DNA code in nearly every cell. Although they are not always copied perfectly, your cells correct almost all mistakes immediately. A few go undetected, but most of these changes, known as mutations, have no effect because they occur in the DNA that is not in a gene - non-coding DNA, sometimes called 'junk' DNA.