Part a what is the function of rna polymerase? hints what is the function of rna polymerase? it adds nucleotides to the 5' end of the growing mrna molecule. it relies on other enzymes to unwind the double helix. it unwinds the double helix and adds nucleotides to a growing strand of rna. it proceeds slowly along the dna strand, requiring about a minute to add two nucleotides to the growing mrna molecule. all of the above.