Question

Why is it important that the nitrogenous bases (A, T, G and C) are weakly bonded together by hydrogen bonds?

Answer 1

When the cell divides, it "unzips" the DNA to copy it. Therefore, the bonds must be weak. However if there were no bonds, the DNA would simply fall apart.

Answer 2

During cell division, the DNA must be replicated, since the daughter cells need a complete set of DNA. To create TWO DNA, it must replicate. To replicate, DNA unzips, the bases pair up with a different base, and the backbone bonds.