DNA is a double-helix formed from two complementary strands that are attached by hydrogen bonds connecting the nitrogenous base pairs in the middle.
The rules for base-pairing in DNA are as follows: adenine pairs to thymine and cytosine pairs to guanine. If there are 200 adenines in the DNA, it follows there must also be 200 thymines. Each adenine and thymine are held together by a hydrogen bond. Therefore, there must be 200 hydrogen bonds between them.
If there are 500 guanines, there are also 500 cytosines because they are complementary and form base pairs. Again, there must be 500 hydrogen bonds holding each guanine to each cytosine.
The total amount of hydrogen bonds in this molecule of DNA is 700. And, when DNA replicates, these weak bonds are broken, the double helix unwinds and separates, and either half can be used as a template for replication.
I have included a link showing the placement of hydrogen bonds in the DNA double helix.