RNA polymerase binding is helped by proteins known as transcription factors. The transcription factors bind to the promoter region, which is a region in the DNA that is upstream from the gene and regulates the gene activity. They specifically recognize a stretch of DNA known as the TATA box. The assembly of RNA polymerase with the transcription factors binding to the promoter is known as the transcription initiation unit.
When RNA polymerase binds to the promoter region, it pries two strands of the DNA apart. The RNA polymerase only binds to one of the strands of the DNA and synthesizes mRNA in the 5' to 3' direction. The DNA strand that is used to synthesize the mRNA is known as the template strand, and the stretch of DNA that is transcribed is known as the transcription unit.
After the RNA polymerase has finished synthesizing the mRNA, the mRNA leaves the DNA and is either modified or is used to make protein.