What is the role of RNA polymerase in transcription?

Well, I want to say A, and here's why: RNA polymerase, the enzyme that catalyzes the reactions between existing DNA segments and the developing chain on the opposite side, must continue to add nucleotides in order to fully develop the mRNA to allow for transcription of the cell's DNA into proteins which will regulate action. So, it does bind to the promoter region, the area at which RNA polymerase starts gluing together nucleotides, and continues to add them until it reaches a stop codon, which ends transcription and allows the mRNA to go on it's way. It doesn't add nucleotides to the existing end of DNA, because that's the function of DNA polymerase, and DNA doesn't and can't utilize the nucleotide base Uracil, which replaces Thymine in RNA. It also doesn't end transcription, because that's again the function of the stop codon series on the DNA, not the RNA polymerase itself. The polymerase just does the heavy lifting. Hope this helps.

A.It binds to the promoter region of a gene and adds new nucleotides to the growing end of nascent mRNA.


RNA polymerase binds to the promoter region of a gene and, after locally unwinding the DNA double helix, then uses a segment of one strand of DNA as a template for the synthesis of a complementary RNA molecule.  

NOTE: The RNA molecule that results from transcription via RNA polymerase does not have to be mRNA: it can be tRNA, rRNA, miRNA, piwiRNA, etc.