Scientists believe that most mutations are caused by mistakes in the DNA replication during reproduction or development. Mutations can also be a result of exposure to things in the environment. These are called Some mutagens occur ly. Ultraviolet radiation from the sun, for example, can damage DNA. It does so by changing the chemical structure of nucleotides. Many organisms have evolved ways to repair the damage by removing and replacing altered nucleotides. Too much exposure to ultraviolet radiation, however, is one of the leading causes of cancer. This is particularly true of skin cancer in humans. other forms of radiations include cosmic rays, x-rays, and the emissions of radioactive materials. They also produce mutagenic effects.