Practice question

Blunt ends generated by enzymes like SmaI or EcoRV terminate without single-stranded overhangs, presenting flush double-stranded termini with no template for polymerase extension. Sticky ends with 5' overhangs provide a recessed 3'-OH that Klenow can extend using complementary labeled dNTPs, incorporating radioactivity efficiently. Since blunt termini lack this overhang, they cannot be filled in, thus remain unlabeled under standard fill-in reactions. Labeling both ends or using Taq is not typical, and 5'-only phrasing misrepresents fill-in mechanics. Hence blunt ends are generally non-labelable by fill-in.