Mutation Ultraviolet light 。 UV radiation causes thymine dimers Exposure to ultraviolet lighcauses adjacent thymines to become cross 。Light-repair Thymine dimer linked,forming a thymine dimer and disrupting their normal base pairing. separates thymine dimers An enzyme cuts out and removes the damaged DNA. New DNA DNA polymerase fills the gap by synthesizing new DNA,using the intact strand as a template. DNA ligase seals the remaining gap by joining the old and new DNA. Figure 8.20 • UV radiation causes thymine dimers • Light-repair separates thymine dimers Mutation Figure 8.20