Practice question

Negative selection or insertional inactivation screening uses a reporter gene whose function is lost when foreign DNA integrates within it, allowing differentiation of recombinants from parental vectors. LacZ encoding β-galactosidase serves this purpose efficiently; intact lacZ cleaves X-gal producing blue colonies while insertion disrupts α-peptide coding, yielding white colonies. Origin of replication maintains plasmid copy number, antibiotic resistance gene AmpR provides positive selection, and T7 promoter drives transcription. Among these, only lacZ provides a scorable loss-of-function phenotype suitable as reporter for insertional disruption screening.