In blue-white screening, white colonies contain:
Blue-white screening exploits insertional inactivation of lacZ alpha peptide within vector multiple cloning site. Functional beta-galactosidase cleaves X-gal chromogenic substrate producing blue colonies. When foreign DNA inserts into MCS, it disrupts lacZ alpha reading frame, preventing alpha complementation, so active enzyme is not formed. Colonies harboring recombinant plasmid therefore remain white on indicator plates containing IPTG and X-gal. Non-recombinant vectors retain intact lacZ and give blue colonies. White phenotype thus indicates successful insertion event, providing rapid visual screening without antibiotic sensitivity assays or colony PCR for primary selection.
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.