Practice question

Cloned inserts can ligate in forward or reverse direction relative to promoter and selection markers. Because restriction sites inside insert are asymmetrically positioned, distance to flanking vector sites changes upon inversion while total plasmid size remains identical. This does not affect circular length but alters sizes of junction fragments produced by double digestion using one backbone and one insert enzyme. Appearance of alternative fragment sizes differing from predicted forward map indicates flipping. Detecting orientation-specific fragment shift is essential to ensure sense transcription, correct reading frame for fusion tags, and functional protein expression in expression vectors.