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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.
Function of the RNA primer: DNA polymerases need a double-stranded DNA region to which they can attach in order to begin copying the rest of the DNA strand. In order to provide this double-stranded attachment site, RNA primers are added by primase, an RNA polymerase which does not require such an attachment site itself. When DNA replication starts, one primer is needed at the start of the leading strand.