A gene is a stretch of several thousand nucleotides that codes for a particular protein. In all, there are about 100,000 genes in the human genome , coding for proteins such as enzymes which control biochemical reactions, and for proteins like collagen and keratin, which give the body form and structure. There are a total of 20 amino acids, and each one has different properties. A protein's shape and function depend entirely on which amino acids are strung together and in what order. So, any change in a gene will change the way a protein works, usually for the worse.
Cells have to make a copy of their DNA in order to divide and thus grow.
Fortunately, the enzymes that do the copying rarely make a mistake. Only once out of every billion times do they miscopy a nucleotide, putting a T in where a C should be, for instance. That's equivalent to printing one letter wrong for every 1,000 books published. Mistakes can also creep in when DNA is repaired after being damaged by radiation or toxic chemicals. Any such change in the code is called a mutation.