Practice question

Ultraviolet radiation at 254-280 nm causes adjacent pyrimidines on same strand to undergo photochemical cyclization forming cyclobutane pyrimidine dimers and 6-4 photoproducts, covalently linking C5-C6 double bonds and distorting double helix by kinking backbone about 30 degrees. Thymine-thymine dimers are most frequent due to sequence abundance, blocking transcription and replication leading to mutations if translesion polymerases bypass incorrectly. Nucleotide excision repair employs UvrABC in bacteria and XPC-Rad23 in humans to excise damaged oligonucleotide segment. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.