Problem 1 Why is it important that a cloning vector should h

Problem 1: Why is it important that a cloning vector should have a number of unique restriction enzyme recognition sites?  
Problem 2: You have a restriction fragment generated by digestion with PvuII which you want to clone into the multiple cloning site in pUC18. What restriction enzyme would you cut the vector with? Justify your answer.

Solution

If a fragment is generated by PvuII, then it will have blunt ends and technically, SmaI present in the pUC18 vector can be used to cut the vector and mediate the cloning (as SmaI also generates blunt ends).

But a blunt end ligation is not efficient and the insert might also be cloned in the opposite orientation; that is why people prefer bidirecetional cloning using two restriction enzymes that generate staggered ends. In this case, the fragment