How does RNA polymerase II work?

RNA polymerase II transcribes protein-coding genes into messenger RNA (mRNA) in eukaryotes.

RNA polymerase II (RNAP II) is a 12-subunit enzyme that’s only present in the nucleus of eukaryotes. It influences gene expression directly through its production of pre-mRNA transcripts. After RNAP II releases the pre-mRNA transcripts from within the nucleus, the transcripts go through multiple biochemical modifications that prepare them for translation.

RNAP II also synthesizes micro RNA (miRNA) molecules, which are non-coding transcripts responsible for regulating gene expression and the activity of mRNAs after transcription. 

RNA polymerase II cannot work on its own to support accurate initiation. Instead, it creates a large multiprotein - DNA complex with initiation factors [transcription factor (TF) IIB, TFIID, TFIIE, TFIIF, and TFIIH] that assemble on promoter DNA for the purpose.