Which clone orientation produces 1.7 kb and 4.1 kb fragments on EcoRI+HindIII digestion?
Insert orientation determines relative positions of internal restriction sites to vector backbone sites. In one orientation, EcoRI and HindIII may be close together, producing one very small fragment that may be overlooked and one almost full-length band resembling empty vector. Flipping insert swaps distances, placing EcoRI site distant from HindIII site, generating 1.7 kb and 4.1 kb fragments that sum to total recombinant size of 5.8 kb. Observing these distinct sizes allows unambiguous discrimination between forward and reverse clones, critical for ensuring correct promoter-driven expressio
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.