Why does a primer need to be supplied before DNA template can be copied?This is involving DNA Replication.
Initiation of DNA synthesis requires an RNA primer. DNA polymerases cannot initiate DNA chains de novo. (RNA polymerase, in contrast, can initiate chains de novo during trascription). A pre-existing polynucleotide chain (the primer) is required to which polynucleotides are added. The primer RNA is a short polynucleotide chain with a 5' triphosphate end and a 3'-OH end. The RNA primer is synthesized by the DNA template close to the origin of replication. This synthesis is probably catalysed by a modified RNA polymerase which may contain a starting factor. In E. coli RNA polymerase is required just before replication of DNA, but protein synthesis is not.