Insertion

In blue-white screening, the cloning site is commonly positioned within the lacZ coding region of a suitable plasmid. If a foreign DNA insert is ligated into that site, it can interrupt lacZ, a process called insertional inactivation. In bacteria supplied with the appropriate substrate, cells with an intact lacZ function generally form blue colonies, whereas cells in which the gene is disrupted by an insert generally form white colonies. Thus option A is correct. The result is not an absolutely universal statement about every system because expression conditions and host genotype matter, but disruption of lacZ is the intended screening principle. The other options describe no biological consequence of insertion.

Directional cloning uses two different, incompatible ends on the vector and insert. Each end can pair only with its complementary partner, so the insert is strongly guided into one orientation rather than being able to enter randomly in either direction. This is especially useful when a gene must be expressed from a promoter in the correct 5′-to-3′ direction. It does not guarantee that every colony is correct, because incomplete digestion, recombination, or technical errors can still occur; verification by colony PCR, restriction analysis, or sequencing remains important. Option A is therefore the best outcome. DNA does not become protein during cloning, enzymes are not the intended product, and the gel remains a useful analytical tool.