The helix has to unwind before separating. Once it's separated, then new bases can be added. Once the bases are added, the separate strands wind into helices.
In DNA replication, there are three primary steps: initiation, elongation, and termination. DNA is packed into tightly coiled structures called chromatin to fit within a cell's nucleus, which loosens prior to replication, allowing the cell replication machinery to access the DNA strands.
