Requires the cooperation of over a dozen enzymes and other proteins. In many organisms DNA replication begins at special sites called origins of replication. Specific proteins required to initiate replication bind to each origin. The DNA double helix opens at the origin and replication forks spread in both directions away from the central initiation point creating a replication bubble. Bacterial or viral DNA molecules have only one replication origin.
Eukaryotic chromosomes have hundreds or thousands of replication origins.
The many replication bubbles formed by this process, eventually merge forming two continuous DNA molecules. The X-shaped regions of replicating DNA molecules where new strands are growing. Single-strand binding proteins are proteins which keep the separated strands apart. Strand separation severely stresses the DNA molecule. Another protein, topoisomerase, is required to relieve the stress and untangle any kinks or snarls.