b)非电离辐射 Ultra Violet light(U.V) pyrimidine dimer (tt dimer is generated by covalent links between adjacent TT 共价键 UV CTTA 环b) 非电离辐射—Ultra Violet light (U.V) ---pyrimidine dimer (TT dimer )is generated by covalent links between adjacent TT ∧ U.V. …C T T A… 共价键