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This unzipping takes place in both directions from the replication origins, creating a replication bubble. The replication is therefore said to be bi-directional. Once the two strands are separated a small piece of RNA, called an RNA primer, is attached to the DNA by an enzyme called DNA primase. These primers are the beginnings of all new DNA chains since the enzyme responsible for the copying of the DNA, DNA polymerase, can not start from scratch.
DNA polymerases cannot initiate DNA synthesis - they can only add on to an existing strand of DNA or RNA. The existing strand is said to prime DNA synthesis. RNA polymerases, however, can begin copying a parent strand with no primer - just as they do in transcription of genes. During DNA replication, RNA primers are produced by RNA polymerases in order to provide a starting point for DNA polymerases to extend from.