What enzyme breaks hydrogen bonds in DNA replication?

Hydrogen bonds in DNA replication are broken down by the helicase class of enzymes. This causes the double-stranded DNA molecule to unzip into two single strands, allowing each strand to be replicated. 

A DNA molecule is made up of two long strands of nucleotides held together by hydrogen bonds between complementary nitrogenous bases, resulting in a structure that resembles a coiled ladder. In order to facilitate DNA replication, the two strands of the molecule must be peeled apart. This can only be done by breaking the hydrogen bonds that hold together the nitrogenous bases. When DNA helicase unwinds, it acts as a wedge between the two DNA strands. As it keeps unwinding further, helicase, powered by ATP, keeps breaking apart the hydrogen bonds separating the two DNA strands in a manner similar to a zip opening. This causes the two strands of DNA to separate at the beginning of DNA replication.