Hi there,

Unlike RNA polymerase, DNA polymerase is unable to initiate nucleotide polymerisation from a single stranded DNA matrix. The role of the primer is to provide a 3' OH extremity for elongation and by annealing to the complementary single stranded DNA the primer provides the matrix to copy to the DNA polymerase.

First very important point is that DNA polymerase requires a free 3' hydroxyl group to add its nucleotide.  Its active site has been identified which is not capable of initiating polymerization by itself.

Where as, the active site of RNA polymerase is capable of initiating polymerization at a promoter sequence by bonding between two nucleotides. This difference is attributed to the geometry and nature of their respective active sites.

And

Second point is that DNA Polymerase has got the proof reading mechanism known as exonucleolytic proofreading unlike the RNA polymerase. As the errors in making RNA are not passed on to the next generation, RNA polymerase enzymes do not need efficient exonucleolytic proofreading.The defective RNA molecule that is produced has no long-term significance.

read this article hope it will help

https://www.ncbi.nlm.nih.gov/books/NBK22374/

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