It takes only a few hours to copy the 6 billion bases of a single human cell. Only about one in a billion nucleotides is incorrectly paired. 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 Y-shaped regions of replicating DNA molecules where new strands are growing.