The replication and transcription of genetic information requires the stepwise extension of DNA or RNA primers in sequence-specific, templatedirected reactions (Scheme1). Template- directed primer extension also underlies PCR,[1] dideoxy sequencing,[2] and attempts to recreate life from inanimate materials.[3] Primer extension presents a formidable chemical challenge. Four different, richly functionalized monomers (the mononucleotides) have to ...