Transcription is the process of producing single-stranded RNA from a double-stranded DNA template. The enzyme that catalyzes this reaction is RNA Polymerase. Transcription takes place in both eukaryotes and prokaryotes and is performed in three stages. The three stages are: initiation, elongation, and termination.

Initiation

At the initiation stage, the RNA polymerase recognizes and binds to the DNA molecule. The recognition-site on the DNA molecule is the promoter region. Various elements on the DNA template that are upstream or downstream of the promoter region serve to help the RNA polymerase effectively bind to the correct region. The RNA polymerase proceeds to unwind a short stretch of DNA in order to create a template strand for which the RNA polymerase encodes complementary base pairs from. After events either by transcription factors in eukaryotic transcription or by subunits of the RNA polymerase in prokaryotic transcription, the RNA polymerase proceeds to enter into the elongation stage.

Elongation

At the elongation stage, the RNA molecule is synthesized by the addition of specific ribonucleoside triphosphate molecules to the pre-existing RNA molecule. The addition specificity of these ribonucleotide molecules is determined by the template strand. These molecules are added through a reaction, where a 3' hydroxyl group from the terminal nucleotide of the existing RNA molecule cleaves the 5' triphosphate of the next ribonucleoside triphosphate into pyrophosphate and a ribonucleoside monophosphate and creates a phosphodiester bond between the two ribonucleotides. This reaction proceeds at about a rate of 50 nucleotides per second in a locally denatured bubble of double-stranded DNA called the transcription bubble, until an entire RNA molecule has been produced by a single RNA polymerase.

Termination

Finally, the RNA polymerase will recognize termination signals that will signify where the transcript ends. There are various ways that termination takes place, but all of these processes involve the dissociation of RNA polymerase from the DNA template. After the RNA polymerase has dissociated from the DNA template, a new, fully-synthesized RNA molecule has been produced. This molecule may be proceed to be translated into a protein, or it may function as a tRNA, rRNA, small nuclear RNA, or small nucleolar RNA molecule. In eukaryotic transcription, the RNA molecules go on to be post-transcriptionally modified by the addition of a 5' end cap and a 3' end poly-A tail. Furthermore, the introns in RNA molecules in eukaryotes get spliced out, where only the translated exons are present.

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