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
DNA double helix is maintained by: 
Select and explain. 
A. Hydrogen bond, B. Vanderwaal forces, C. Disulfide linkage, D. Covalent bond
A: Hydrogen bond
Answer: A: Hydrogen bond 
Explanation: In a DNA double helix, two polynucleotide strands come together through complementary pairing of the bases, which occurs by hydrogen bonding.Each base forms hydrogen bonds readily to only one other -- A to T and C to G -- so that the identity of the base on one strand dictates what base must face it on the opposing strand.Thus the entire nucleotide sequence of each strand is complementary to that of the other, and when separated, each may act as a template with which to replicate the other.