The short RNA primer has a free 3' -OH group to which DNA polymerases can then add nucleoside triphosphates one at a time. A simplified diagram of the process of transcription or RNA synthesis is shown here. Note that on your CD, there is an animation called Basic Transcriptional Mechanism which will help you to visualize the polymerase movements involved. The replication of DNA requires the assembly of many different proteins into a molecular machine at the replication fork. This figure shows some of the major features of the replication fork.
Replication begins with the local unwinding of the double helix by an enzyme called a helicase.
Helicases work in combination with another enzyme called a topoisomerase which removes supercoils in the DNA that result from helicase activity. RNA polymerases called primases then produce short RNA primers using the unwound DNA strands as templates. Several features of the process of DNA synthesis guarantee that the process will be different on the two parental strands. This is due to the fact the two parental strands run in opposite directions, i.e. they are antiparallel with respect to their 3' and 5' ends. And second, DNA synthesis can proceed only in the 5' to 3' direction.