Chapter 9, Problem 20P

Your undergraduate research advisor has assigned you a task: Insert an EcoRI-digested fragment of frog DNA into the vector shown in Problem 19. Your advisor suggests that after you digest your plasmid with EcoRI, you should treat the plasmid with the enzyme alkaline phosphatase. This enzyme removes phosphate groups that may be located at the 5′ ends of DNA strands. You will then add the fragment of frog DNA to the vector and join the two together with the enzyme DNA ligase.

You don’t quite follow your advisor’s reasoning, so you set up two ligations, one with plasmid that was treated with alkaline phosphatase and the other without such treatment. Otherwise, the ligation mixtures are identical. After the ligation reactions are completed, you transform E. coli with a small aliquot (portion) of each ligation and spread the cells on petri plates containing both ampicillin and X-gal. The next day, you observe 100 white colonies and one blue colony on the plate transformed with alkaline-phosphatase-treated plasmids, and 100 blue colonies and one white colony on the plate transformed with plasmids that had not been treated with alkaline phosphatase.

a.	Explain the results seen on the two plates.
b.	Why was your research advisor’s suggestion a good one?
c.	Why would you normally treat plasmid vectors with alkaline phosphatase, but not the DNA fragments you want to add to the vector