Practice question

Actinomycin D chromopeptide antibiotic composed of phenoxazone ring with two cyclic pentapeptide lactones from Streptomyces antibioticus binds DNA via intercalation. Phenoxazone inserts between GC base pairs preferably dGpdC steps unwinding helix about 26 degrees, while lactone peptides occupy minor groove bonding to guanine amino groups stabilizing complex. Intercalation acts as roadblock preventing RNA polymerase II and I progression blocking promoter clearance and elongation. Low doses mainly inhibit transcription, higher doses impede replication. Used experimentally to halt transcription measuring mRNA half-lives and clinically as anticancer agent targeting rapidly dividing cells.