Transcription

In genetics, the process of copying DNA to mRNA by an enzyme called [RNA polymerase]?. Transcription is the first step of protein biosynthesis.

Bacterial transcription

A (simple) model for a bacterial gene to be transcribed looks like this :

   upstream        ~17 bp       The gene to transcribe     downstream
 5'----------|-35|---------|-10|----------------------|T|------------3'
 3'----------|-35|---------|-10|----------------------|T|------------5'
                                |
                                |--------------------->
                                         mRNA

where the region between -35 and -10 base pairs is called promoter, and |T| stands for terminator. The DNA between promoter and terminator is copied to mRNA, which is then translated into protein.

Promoters can differ in strength, that is, how attractive they are for RNAP. The more similar they are to a [consensus sequence]?, the stronger they are. The "ideal" promoter in [E. coli]? looks like this:

 5'----TTGACA---|17 bp|----TATAAT---|7bp|---|[[purine]]s|----3'

Initiation

The RNA polymerase (or RNAP) holoenzyme? consists of a core, made of four subunits (ααββ'), and the σ-factor?. The followings steps occur upon initiation:

1. The RNAP recognizes the pomoter region of the gene and binds to the DNA at that specific location. At this stage, the DNA is still double-stranded and called closed complex.
2. The DNA is unwound and becomes single-stranded at the initiation site (the -10 promoter region). This is called open complex.
3. The DNA is melted (the strands are locally separated), the σ-factor leaves the holoenzyme, and the transcription process begins. This is the elongation phase.

Elongation

The RNAP runs along the DNA, synthesizing mRNA in the process. In bacteria, the nascending mRNA is processed right away by ribosomes.

Termination

The elongation stops if:

• The terminator is reached. The terminator is usually a palindromic DNA sequence that forms a hairpin.
• A [ρ factor]? (a protein) binds and runs along the mRNA towards the RNAP. When ρ-factor reaches the RNAP, is causes RNAP to dissociate from the DNA, terminating transcription.
• The RNAP comes along a region with repetitious base pairs (for example, TTTTTT). This will terminate transcription.