Final answer:
If a DNA sample produces only a 350 bp amplicon, it could indicate a successful insertion of a foreign DNA into the lacZ gene, preventing the production of the enzyme β-galactosidase and allowing the identification of bacteria with recombinant plasmids.
Step-by-step explanation:
If only a 350 bp amplicon was produced in a DNA sample, this might suggest that the lacZ gene within that sample has been interrupted by the insertion of foreign DNA. This insertion occurs because both the foreign DNA fragment and the plasmid are cut with the same restriction enzyme, the restriction site of which is strategically located within the lacZ gene region of the plasmid. When the plasmid carries the foreign DNA insert, it disrupts the lacZ gene, preventing the production of the enzyme β-galactosidase. Consequently, bacteria with the recombinant plasmid will not change the color of the substance added to the plate, which is an indication of successful cloning.
The lacZ gene thus serves as a reporter in this cloning strategy, allowing identification of bacteria that have taken up the recombinant plasmid because they survive on ampicillin-containing media and do not cause a color change, implying the lacZ gene has been disrupted by the insert.