This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, 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 chromosomal DNA insert in plasmid vectors, D. To detect host DNA in situ
C: To identify desired chromosomal DNA insert in plasmid vectors
Answer: C: To identify desired chromosomal DNA insert in plasmid vectors 
Explanation: Screening for selecting the cells with vector DNA containing the desired insertional DNA sequence can be done by :


Antibiotic resistance markers:- Allow only cells in which the vector has been transfected to grow.
Blue-white screening:- It is based on lac operon. The hydrolysis of X-gal by β-galactosidase cause blue colour in colonies & indicates that colonies contain the unligated vector. Whereas, white colonies indicate insertion of foreign DNA and loss of cells ability to hydrolyse marker.
PCR, restriction fragment analysis and DNA sequencing:- Confirmatory methods.