RNA polymerase (RNAP) plays a key role in the transcription process in eukaryotic cells. It is the main enzyme responsible for copying a DNA sequence into an RNA sequence. RNA polymerase along with transcription factors form a transcription pre-initiation complex (PIC) by binding to the promoter site on a DNA strand. The promoter site is a region located upstream at the 5’ end of a DNA strand. This binding of the RNAP and transcription factors to the promoter site on DNA initiates the transcription process.

Unlike prokaryotic cells where a single RNA polymerase is responsible for transcription, eukaryotic cells have three types of RNA polymerases. Each of these RNAPs plays a different role in gene expression. 

• RNAP I plays a role in the synthesis of ribosomal RNA (rRNA).
• RNAP II transcribes protein-coding genes into messenger RNA (mRNA).
• RNAP III plays a role in transcribing transfer RNA (tRNA) and other small RNAs.