The function of RNA polymerase II is to synthesize mRNA using a DNA strand as a template, explains UC Davis BioWiki. The process, by which an mRNA strand is constructed from a DNA strand, is called transcription, and the product is called a primary transcript, notes North Dakota State University.
To begin transcription, RNA polymerase II binds to a segment on a gene called a promoter, explains North Dakota State University. The promoter consists of nucleotide sequences, which are TATAAT and TTGACA in eukaryotes and TATA and CAAT in prokaryotes. T stands for a thymine nucleotide, A for adenine, G for guanine and C for cytosine, notes Pearson Prentice Hall. In bacterial cells, a molecule called a sigma factor must be present for RNA polymerase II to be able to identify the promoter. In eukaryotic cells, transcription factors help RNA polymerase II locate the promoter, explains North Dakota State University.
Although the most commonly referenced form of RNA polymerase is RNA polymerase II, there are also two lesser-known types, notes UC Davis BioWiki. The first is RNA polymerase I, which is located in a cell's nucleolus and is involved in the synthesis of rRNA. The second is RNA polymerase III, which is found in the nucleoplasm and deals with the synthesis of tRNA and certain snRNAs.