Question

If you forgot to add DNA ligase to the ligation mixture in the lab, what could you expect to observe on the AMP-X-gal plate?

a. White colonies only

b. No bacterial growth

c. Blue and white colonies

d. Blue colonies only

Answer 1

Answer- d. Blue colonies only

This is because this procedure is blue-white screen. In this, recombinant bacteria, with the DNA portion of interest inserted in it using vector, are detected along non-recombinant bacteria, using different coloured colonies.

The cells transformed with recombinant DNA form white colonies, while cells with non-recombinant plasmids form blue colonies.

Now, to be able to take up the recombinant DNA, it needs to be ligated into the host genome; in absence of ligase, this won't happen and all the cells will have non-recombinant plasmids only. Hence there will be blue colonies only.