The enzyme that separates the two strands of nucleotides in DNA is helicase. Enzymes are protein molecules that speed up chemical reactions.
When DNA replicates, helicase binds to a specific site called the origin of replication. Once the strands separate, another enzyme called DNA polymerase controls the replication process. As replication progresses, helicase continues to separate the strands of DNA.
The helicase needs energy to break the hydrogen bonds between the strands, so it uses the stored energy in adenosine triphosphate, more commonly known as ATP. After the strands have been separated, a protein is used to stabilize each single strand.