The DNA double helix is unzipped by the enzyme *HELICASE*. The enzyme breaks the hydrogen bond between the bases thereby causing the strands of the double helicase to unzip. The action of the helicase enzyme uses energy from ATP. The unzipping of the double helix does not occur in a swift move. One end of the DNA molecule with 5'-3' is opened up showing the antiparallel single strands. The end that is opened is termed INITIATION SITE of replication. The opening look like someone standing with legs wide open. The wide opening is called REPLICATION FORK. The single strands of the fork are usually unstable but their stability is maintained by the DNA-binding-proteins. Having opened the double helix, the replication would then continue in the 5'-3' direction of one of the two antiparallel single strands.
