This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
In molecular cloning, Blue-white screening is used for 
Select and explain. 
A. To screen for recombinant vectors, B. To detect gene mutations, C. To identify desired chromosal DNA inse in plasmid vectors, D. To detect host DNA in situ
C: To identify desired chromosal DNA inse in plasmid vectors
Answer: C: To identify desired chromosal DNA inse in plasmid vectors 
Explanation: The blue-white screen is a screening technique that allows for the rapid and convenient detection of recombinant bacteria in vector-based molecular cloning experiments. DNA of interest is ligated into a vector.The vector is then inseed into a competent host cellble for transformation, which are then grown in the presence of X-gal.Cells transformed with vectors containing recombinant DNA will produce white colonies; cells transformed with non-recombinant plasmids (i.e. only the vector) grow into blue colonies.This method of screening is usually performed using a suitable bacterial strain, but other organisms such as yeast may also be used.DM VASUDEVAN 7th edition pg no 626 627