Practice question

Triplex-forming oligonucleotides polypurine tracts 15-30 nucleotides bind major groove duplex DNA without unwinding helix forming Hoogsteen or reverse Hoogsteen hydrogen bonds with purine strand Watson-Crick duplex generating three-stranded structure requiring homopurine-homopyrimidine mirror repeats where one strand purine-rich. Formation stabilized N3 protonated cytosine acidic pH binding G-C base pair or using 5-methylcytosine or pseudoisocytosine allowing neutral pH triple helix. When targeted promoter enhancer oncogene c-myc P2 promoter 27 bp purine tract, triple helix sterically occludes binding transcription factors Sp1 recognizing GC boxes, NF-kB, TATA-binding protein required preinitiation complex assembly TFIIB RNA polymerase II reducing transcription initiation frequency measured nuclear run-on assay. Additionally triple helix impedes helicase unwinding origin causing replication stall. Unlike antisense targeting mRNA antigene approach acts genomic DNA level blocking mRNA synthesis upstream, but limited requirement lengthy purine tracts and poor cellular uptake requiring nuclear delivery agents. This mechanistic insight guides vector optimization, dosing strategies, and clinical safety monitoring essential for translational development and regulatory evaluation.