function of RNA polymerases

Both RNA and DNA polymerases can add nucleotides to an existing strand, extending its length.  However, there is a major difference between the two classes of enzymes:  RNA polymerases can initiate a new strand but DNA polymerases cannot. Therefore, during DNA replication, an oligonucleotide (called primer) should first be synthesized by a different enzyme.

The chemical reaction catalyzed by RNA polymerases is shown in Figure 4-B-2.  The nucleotides used to extend a growing RNA chain are ribonucleoside triphosphates (NTPs).  Two phosphate groups are released as pyrophosphate (PP_i) during the reaction.  Strand growth is always in the 5' to 3' direction.  The first nucleotide at the 5' end retains its triphosphate group (Figure 4-B-3).

Figure 4-B-2.  The chemical reaction catalyzed by RNA polymerases.

Figure 4-B-3.  Simplified presentation for the chain elongation.  The vertical line represents the pentose and the slanting line denotes the phosphodiester bond.  Bases are designated as N₁, N₂, etc.