Overview of Transcription

The details of transcription vary between eukaryotes and prokaryotes, but the overall process of making an RNA copy of DNA is similar enough that it can be summed up without brining up these differences. This generalized mechanism involves three steps: initiation, elongation, and termination. Initiation involves the binding of RNA polymerase to the gene and starts transcription. Elongation is the part of transcription where the individual nucleotide tri-phosphates bind and polymerize to form the RNA strand. Termination is the addition of the poly-Adenosine tail and the separation of the RNA transcript from the gene. Since the mechanism for prokaryotic transcription is better understood and similar, diagrams from transcription will be of this type.

Initiation:

Initiation begins with RNA polymerase binding to the promoter of the gene. The promoter is a segment of DNA that precedes the coding region of the gene. The purpose of this part of initiation is to align the RNA polymerase to the first coding nucleotide on the gene, which is designated the +1 position. After RNA polymerase is bound to the promoter, it melts the DNA that surround the +1 site, which means that the double stranded DNA is separated into an open complex. After the open complex is created, it is now possible to match individual nucleotide residues to the DNA template strand, form the first few phosphodiester bonds, move away from the promoter, and begin elongation (1).

Elongation:

During elongation of a transcript the RNA polymerase catalyze successful polymerization of nucleoside monophosphates to form the greater part of an mRNA transcript. The start of elongation is the escape from the promoter. During elongation the polymerase stabilizes because it is now both bound tightly to the start of the transcript and the DNA template strand (2).

The fundamental features of elongation complexes are conserved among all DNA-dependant polymerases. These basic features include a catalytic site, a region where the DNA template is single stranded, and a number of nucleic acid binding sites. In order for base pairing to facilitate a region of the DNA template is melted within the polymerase, called the transcription bubble. Inside this transcription bubble is the active site of RNA polymerase. At this active site the 3’-OH terminus of the transcript is aligned with the DNA template (2). This allows the base pairing of the corresponding nucleotide.

Surprisingly enough there is no exogenous nucleotide hydrolysis to drive elongation (2). Other than the formation of the bonds to polymerize the transcript, no energy is needed. This means the polymerase traverses through the DNA, keeping the transcription bubble open, with no energy expended. RNA polymerase is very efficient.

Termination:

Termination is the final part of transcription. This process is highly variable between prokaryotic and eukaryotic systems. Although some mechanisms involving termination are well understood, this process is much more complicated than originally thought (3).

In both prokaryotic and eukaryotic systems there is a sequence in the gene that initiates the termination process (3). Termination and transcription both ultimately end with the transcript and polymerase eventually disassociating from DNA.