Practice question

Transcription-coupled nucleotide excision repair is subpathway accelerating removal of bulky helix-distorting lesions that block elongating RNA polymerase. When RNA polymerase II stalls at cyclobutane pyrimidine dimer or 6-4 photoproduct located on transcribed template strand, stalled complex is recognized by CSB and CSA ubiquitin ligase recruiting TFIIH and core excision factors. Repair focused exclusively on template strand because non-transcribed strand lesions do not impede polymerase translocation. This strand-specific prioritization ensures rapidly transcribed essential genes restored preferentially, explaining clinical features of Cockayne syndrome defective in this pathway versus global genome repair.