Transcription is the process of producing RNA from DNA; RNA polymerases carry this process out.  The synthesis of RNA from DNA is performed in three main steps: Initiation, Elongation, and Termination.  RNA polymerases are responsible for many of the main functions in transcription.  These may include: identification of promoters, helicase activity, selection of ribonucleoside triphosphate, formation of phosphodiester bond, detection of termination sequence, and interactions with transcription factors.

Initiation:

Transcription factors recognize and bind to promoters that are upstream of open reading frame.  Promoters are sequences on the DNA that are recognized by the transcription factors.  They have a specific sequence called a consensus sequence and the consensus sequence in prokaryotes is different than in eukaryotes.  After RNA polymerase binds to the DNA strand it binds other transcription factors.  The RNA polymerase then unwinds about 15-17 base pairs creating the open promoter complex.  Then only a few base pairs are transcribed.  After this certain transcription factors dissociate from the RNA polymerase to begin elongation.

Elongation:

The transcription bubble consists of about 17 base pairs of unwound DNA.  Elongation proceeds at a rate of 50 nucleotides per second.  As it proceeds the DNA is unwound in front, a nucleotide is added to the RNA strand to create a RNA/DNA hybrid.  The length of the hybrid strand is approximately eight base pairs long.  The DNA/RNA hybrid is then separated once it moves through the transcription bubble and the DNA is then rewound into the normal double helical structure.

Termination:

In prokaryotes termination can proceed in two different ways; rho independent termination and rho dependent termination.  However, termination in eukaryotes is highly variable.  Eukaryotes rely on post transcriptional modifications to properly modify the RNA.