Overview
Initiation
Transcription begins with the RNA polymerase binding the DNA promoter. Several transcription factors associate with the RNA Polymerase. At this point the DNA is "melted" and transcription is allowed to begin. After several abortive transcripts, where only about five nucleotide mRNA strands are synthesized, the polymerase enters the elogation strand, where the mRNA transcript is synthesized.
Elongation
The polymerase runs along the DNA strand in a 5' to 3' direction, synthesizing an mRNA strand that is complementary to the template strand of DNA. This process occurs at about fifty nucleotides per second. The enzyme maintains a transcription bubble throughout the elongation process, reannealing upstream DNA and unwinding downstream DNA at the same rate.
Termination
Termination is quite variable between prokaryotic cells and eukaryotic cells. All cells recognize termination signals on the DNA strand, followed by the dissociation of the polymerase and associated factors from the DNA strand. After termination is complete the mRNA strand may be immediately translated into protein, or modified by adding a cap to the 5' end or polyadenylation of the 3' end. In eukaryotes splicing occurs where introns are removed from the mRNA transcript, and the exons are joined and translated.