When the genetic code is copied mistakes are often made which cause sudden random changes to the code. These changes are called mutations. The genes of all life forms undergo mutations from time to time.
Proteins are important.
Some proteins, known as enzymes, regulate biochemical reactions while others, known as structural proteins, make up the structure of living things. Even a small change such as one amino acid can alter the structure, and therefore the function, of a protein. For an enzyme protein this could mean that an essential chemical reaction would now be unable to take place. The frequency of mutation depends partly on how often the organism reproduces or its cells divides. Most mutations will be harmful or even lethal to the organism, but a few may be beneficial.