The process of transcription can be broken down into three major stages: initiation, elongation, and termination.  Transcription begins at the initiation stage with the binding of RNA polymerase to a promoter region in the DNA.  The promoter region is a sequence of DNA that helps to direct the binding of the RNA polymerase and align it to the appropriate place at the beginning of a gene.  The exact mechanism of this process differs in eukaryotes and prokaryotes, and will be discussed in detail later.  In general, 5 transcription factors and a mediator complex must associate with RNA polymerase II to begin transcription in eukaryotes.  Once RNA polymerase has bound to the appropriate promoter region, it must unwind a section of the DNA double helix to gain access to a single stranded template.  The DNA enters the polymerase and moves to an active site to begin the addition of nucleotides by base pair matching.  The formation of the transcription factors, RNA polymerase II, mediator, and DNA forms the initiation-competent complex ready to begin the elongation of an RNA transcript.

The elongation step of RNA synthesis consists of the formation of an mRNA strand using one of the two DNA strands (sense strand) as a template.  During the elongation process, the unwound DNA forms a “transcription bubble” that moves along the DNA strand with the RNA polymerase.  As the DNA is unwound, RNA polymerase is able to add ribonucleotides complementary to the DNA sense strand to form an RNA-DNA hybrid of about 8 base pairs in length.  The growing RNA chain is formed as RNA polymerase catalyzes the formation of phosphodiester bonds between the ribonucleotides.  This process continues along the DNA strand with the transcription bubble moving about 50 nucleotides per second.  The DNA strand is then rewound as RNA polymerase leaves it behind.

Elongation continues until the RNA polymerase reaches a terminator region within the DNA that signals the end of the gene that is being transcribed.  The termination signal is contained within the newly synthesized strand of RNA.  There are multiple possible termination sites for an RNA strand.  At this point, RNA polymerase will dissociate from the DNA strand and the mRNA will be free to move on to translation of its protein product.