Why dna polymerase require a primer but rna polymerase don't?

[Ahmed Abdullah]

I asked this question in some other place but they had simply said that .. the DNA polymerase don't have the ability to start DNA polymerization but I already know that and it doesn't answer my question.

RNA is not much different from DNA (virtually same as far as polymerization reaction is concerned). If RNA polymerase can start without any primer ... it seems that there is not much mechanistic problem for DNA polymerase in doing so. Does it has something to do with accuracy? Like, the kind of design required to start polymerization from scratch comes at the cost of fidelity (i.e replication process become more error prone). Or there is a fundamental difference (completely different mechanism) in the way RNA polymerase and DNA polymerase work?
By the way a complementary question, why RNA polymerase has low fidelity?

[bobze]

I asked this question in some other place but they had simply said that .. the DNA polymerase don't have the ability to start DNA polymerization but I already know that and it doesn't answer my question.

RNA is not much different from DNA (virtually same as far as polymerization reaction is concerned). If RNA polymerase can start without any primer ... it seems that there is not much mechanistic problem for DNA polymerase in doing so. Does it has something to do with accuracy? Like, the kind of design required to start polymerization from scratch comes at the cost of fidelity (i.e replication process become more error prone). Or there is a fundamental difference (completely different mechanism) in the way RNA polymerase and DNA polymerase work?
By the way a complementary question, why RNA polymerase has low fidelity?

Because DNApol needs a free 3' hydroxyl group to add its nucleotide too. Its active site isn't capable of initiating polymerization itself.

The active site of RNApol is capable of initiating polymerization at a promoter sequence. The difference in the two, is in the geometry and nature of their active sites.

Edit for your complementary question; RNA polymerase has "low fidelity" because it cannot proof read like DNApol can. It lacks an exonuclease that will work for proof reading.

[Ahmed Abdullah]

Because DNApol needs a free 3' hydroxyl group to add its nucleotide too. Its active site isn't capable of initiating polymerization itself.

The active site of RNApol is capable of initiating polymerization at a promoter sequence. The difference in the two, is in the geometry and nature of their active sites.

Edit for your complementary question; RNA polymerase has "low fidelity" because it cannot proof read like DNApol can. It lacks an exonuclease that will work for proof reading.

Precisely the kind of answer I was looking for. Thank you very much . :!!)

[Ahmed Abdullah]

bobze, can you post some link where I can get the details. Thnx anyway.

[mishrashubham]

bobze, can you post some link where I can get the details. Thnx anyway.

http://en.wikipedia.org/wiki/DNA_polymerase

DNA polymerase can add free nucleotides to only the 3' end of the newly-forming strand. This results in elongation of the new strand in a 5'-3' direction. No known DNA polymerase is able to begin a new chain (de novo). DNA polymerase can add a nucleotide onto only a preexisting 3'-OH group, and, therefore, needs a primer at which it can add the first nucleotide. Primers consist of RNA and/or DNA bases. In DNA replication, the first two bases are always RNA, and are synthesized by another enzyme called primase. An enzyme known as a helicase is required to unwind DNA from a double-strand structure to a single-strand structure to facilitate replication of each strand consistent with the semiconservative model of DNA replication.
Error correction is a property of some, but not all, DNA polymerases. This process corrects mistakes in newly-synthesized DNA. When an incorrect base pair is recognized, DNA polymerase reverses its direction by one base pair of DNA. The 3'-5' exonuclease activity of the enzyme allows the incorrect base pair to be excised (this activity is known as proofreading). Following base excision, the polymerase can re-insert the correct base and replication can continue.

[Ahmed Abdullah]

Because DNApol needs a free 3' hydroxyl group to add its nucleotide too. Its active site isn't capable of initiating polymerization itself.

The active site of RNApol is capable of initiating polymerization at a promoter sequence. The difference in the two, is in the geometry and nature of their active sites.

Edit for your complementary question; RNA polymerase has "low fidelity" because it cannot proof read like DNApol can. It lacks an exonuclease that will work for proof reading.

The answer is mechanistic one, it answers why by the existing structure the DNA polymerase can't initiate DNA replication (polymerization reaction). And I appreciate the answer.

I am also curious about the evolutionary answer. Why evolution selected DNA polymerases unable of initiating dna replication? At least what is rationale for such an enzyme?