DNA polymerases add nucleotides to the 3′ end of a polynucleotide chain. ... To initiate this reaction, DNA polymerases require a primer with a free 3′-hydroxyl group already base-paired to the template. They cannot start from scratch by adding nucleotides to a free single-stranded DNA template.
How is DNA replicated? Replication occurs in three major steps: the opening of the double helix and separation of the DNA strands, the priming of the template strand, and the assembly of the new DNA segment. During separation, the two strands of the DNA double helix uncoil at a specific location called the origin.