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
What will happen to DNA if salt is added to it? 
Select and explain. 
A. Increase melting point (Tm), B. Decrease Tm, C. Not affect Tm, D. Melting lead to denaturation of DNA
A: Increase melting point (Tm)
Answer: A: Increase melting point (Tm) 
Explanation: This makes the DNA less hydrophilic (less soluble in water). Ethanol has a lower dielectric constant than water so it&;s used to promote ionic bonds between the Na+ (from the salt) and the PO3- (from the DNA backbone) causing the DNA to precipitate.