5. The lesion most commonly induced by UV radiation is ______.
A. Chromosome breaks
B. Transition
C. Transversion
D. Thymine dimer
The lesion most commonly induced by UV radiation is thymine dimer (option D). UV light, particularly UVB, triggers covalent bonds between adjacent pyrimidine bases like thymine, forming cyclobutane pyrimidine dimers (CPDs), with thymine dimers being the most frequent due to their prevalence in DNA sequences.
Option Analysis
-
A. Chromosome breaks: These result mainly from ionizing radiation, not UV, which primarily causes localized base modifications rather than large-scale strand breaks. UV-induced damage focuses on pyrimidine linkages without fracturing chromosomes directly.
-
B. Transition: This mutation type (purine-to-purine or pyrimidine-to-pyrimidine, e.g., C-to-T) arises indirectly from UV dimers via replication errors or deamination, but it is not the primary lesion formed.
-
C. Transversion: Involving purine-to-pyrimidine switches (e.g., G-to-T), these occur less commonly from UV through oxidative stress but do not represent the initial, most frequent DNA lesion.
-
D. Thymine dimer: Correct, as UV directly induces these intrastrand crosslinks at 5′-TT sites, distorting DNA helix and blocking replication until repaired by nucleotide excision repair.
Mechanism Insight
UV radiation absorbs into DNA, exciting electrons in thymine bases to form [2+2] cycloaddition products, creating stable thymine dimers. These lesions, if unrepaired, lead to mutations during translesion synthesis, contributing to skin cancer risk. Cells employ photolyases or excision repair to counteract this damage.
