What role does RNA polymerase play in transcription?

The role of RNA polymerase in transcription is primarily to synthesize mRNA from the DNA template. During transcription, RNA polymerase attaches to a specific region of the DNA corresponding to a gene and unwinds the DNA strands. As it moves along the DNA, RNA polymerase builds a strand of mRNA by incorporating ribonucleotides that are complementary to one of the DNA strands. This process results in the formation of a single-stranded mRNA molecule that carries the genetic information needed for protein synthesis.

While RNA polymerase does initiate the unwinding of DNA, its main function is the actual synthesis of RNA. The assembly of amino acids into proteins is a process that occurs during translation, which follows transcription. As for the addition of a 5' cap to mRNA, that is a post-transcriptional modification that occurs after RNA polymerase has completed mRNA synthesis and is not directly part of its primary role during transcription.