Transcription (genetics)
In genetics, transcription is the process of copying DNA to RNA by an enzyme called RNA polymerase (RNAP). Transcription to yield an mRNA is the first step of protein biosynthesis.
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2 Eukaryotic transcription 3 Measuring transcription 4 History |
Bacterial transcription
A (simple) model for a bacterial gene to be transcribed looks like this :
upstream promoter The gene to transcribe downstream
5'----------|-35|---------|-10|----------------------|T|------------3'
3'----------|-35|---------|-10|----------------------|T|------------5'
|
|--------------------->
mRNAwhere 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|---|purines|----3'
Initiation
The followings steps occur upon initiation:- The RNAP recognizes the promoter 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.
- Promoter binding is a two step process. Binding is much tighter above 15°C
- The DNA is unwound and becomes single-stranded at the initiation site (the -10 promoter region). This is called open complex.
- The DNA is melted (the strands are locally separated), the σ-factor leaves the holoenzyme, and elongation begins.
By the late 1960s several papers that came out of the Harvard University Biological Laboratories established the basic mechanics of gene expression in bacteria.
See also:
Regulation
Selective transcription is mainly responsible for the differential protein synthesis among various types of cells in the same organism.Elongation
The RNAP runs along the DNA, synthesizing mRNA in the process. In bacteria, the nascending mRNA is processed right away by ribosomes.Termination
Two termination mechanisms are well known.Intrinsic termination
Intrinsic termination involves terminator sequences within the RNA as it is being made that signal the RNA polymerase to stop. The terminator sequence is usually a palindromic DNA sequence that forms a hairpin.Rho-dependent termination
Rho-dependent termination uses a termination factor called ρ factor to stop RNA sysnthesis at specific sites. This protein binds and runs along the mRNA towards the RNAP. When ρ-factor reaches the RNAP, it causes RNAP to dissociate from the DNA, terminating transcription.Other termination mechanisms
Transcription will terminate if the RNAP comes across a region with repetitious base pairs (for example, TTTTTT).Eukaryotic transcription
Gene expression in eukaryotes is largely controlled by transcription via transcription factors. As eukaryotes are much more complex than prokaryotes, and have their genetic material stored in the nucleus, the transcription mechanisms are more complicated here. For example, eukaryotes have three RNA polymerases, in contrast to prokaryotes, which only have one.
Also, eukrayotic RNAPs need specific accessory proteins to become active. The C-terminus of all RNAPs is highly conserved and contains the actual transctiptional mechanism.Initiation
The core promoter of eukaryotic genes stretches from position -45 to 0. Additionally, there can be an upstream control element present at the -180 to -107 region, which can amplify the RNAP binding by a factor of up to 100. This UCE usually contains a TATA box, a highly conserved DNA sequence that reads
A similar sequence, thus not that highly conserved, is found in the INR element (initiator element, part of the complex core promoter).Elongation
Termination
A major difference between prokaryotic and eukaryotic transcription is that the latter have splicing of the primary transcript, modifying the mRNA created during transcription.Measuring transcription
Transcription can be measured in a variety of ways:
History
RNA synthesis by RNA polymerase had been established in vitro by several laboratories by 1965, however the RNA synthesized by these enzymes had properties that suggested the existence of an additional factor needed to terminate trascription correctly.