Final answer:
In blue-white screening, blue colonies represent cells with a functional lacZ gene, indicating non-recombinant plasmids. Degraded foreign DNA used in the cloning experiment would result in only blue colonies being present.
Step-by-step explanation:
In blue-white screening, blue colonies represent cells that contain non-recombinant plasmids. This means that these cells have a plasmid with an intact lacZ gene, which encodes the enzyme beta-galactosidase. When exposed to a chromogenic substrate, such as X-gal, beta-galactosidase cleaves the substrate and produces a blue product, indicating that no foreign DNA has been inserted into the plasmid's polylinker site, and therefore, these plasmids are not recombinant. On the other hand, white colonies indicate the presence of recombinant plasmids, as the insertion of foreign DNA disrupts the lacZ gene, preventing production of the enzyme and thus, the blue coloration. If your lab partner left foreign genomic DNA on the lab bench and it degraded, but you still used it in your molecular cloning experiment, you would expect to see blue colonies only.
As no functional foreign DNA would have been inserted into the plasmids.In blue-white screening, blue colonies represent cells without plasmids. This is because blue colonies have a functional beta-galactosidase enzyme, indicating that the lacZ gene is uninterrupted and there is no foreign DNA inserted into the plasmid vector. On the other hand, white colonies represent cells with recombinant plasmids containing a new insert. These colonies lack a functional beta-galactosidase enzyme due to the insertion of foreign DNA, disrupting the lacZ gene.