Discussion
Blue white screening is done to :
Answer: B
Blue-white screening is done to check the presence of a cloned insert in a plasmid.
Vectors such as pUC carry the lacZ-alpha fragment with the cloning sites inside it. If the plasmid has no insert, the alpha fragment complements the host's defective beta-galactosidase and colonies turn blue on X-gal. If foreign DNA has been inserted, lacZ-alpha is disrupted, no active enzyme forms, and the colonies stay white.
The presence of the plasmid itself is tested by antibiotic selection (only cells carrying the ampicillin resistance gene grow), which is done before the colour screen. The screen says nothing about whether the insert is expressed or how many copies of the plasmid a cell carries.
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