The strands separate in a certain region. Bases for the new strand line up on the parental strand according to the rules of Watson-Crick base pairing. While replication is conceptually simple, it is mechanistically complex. DNA replication is discontinuous . The need to extend DNA chains of opposite polarity within the same replication fork means one chain grows in the opposite direction of the other.
Replication is far more accurate than any other enzyme-catalyzed process.
The 3' hydroxyl group at the end of a DNA chain reacts with the 5' alpha phosphate of an incoming dNTP substrate. DNA polymerase only adds dNTP's to the 3' end of an existing DNA chain because each successive nucleotide residue is added to the 3' end of the nucleic acid. The DNA template can be single or double stranded but is essential. A primer chain with a free 3' hydroxyl is also required . This means that the reaction is unidirectional 5' to 3' .