Other Mutations A number of environmental agents such as radiation (UV, X-rays, radioactive elements) and chemicals (pesticides, cigarette smoke) can cause mutations (changes) in DNA. A number of enzymes monitor the DNA and repair these changes. For example, excision repair occurs when a mutated segment of DNA is removed and replaced with a new segment. A common type of mutation caused by ultraviolet radiation occurs when two thymines become bonded to each other, forming a kink in the DNA molecule. This type of mutation, called a thymine dimer , can result in incorrect nucleotides being paired with it when the strand is replicated. To repair this mutation, an enzyme removes a segment of DNA that contains the dimer and replaces the removed nucleotides with nucleotides complimentary to the opposite strand. The new DNA is then bonded to the original strand with DNA ligase. Xeroderma pigmentosum is a genetic disease in which some repair enzymes do not function.
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