Transcription
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Transcription is the process of synthesising a strand of RNA from a template of DNA. This is performed by an enzyme called 'RNA polymerase' (abbreviated to RNAP or RNApol)
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Overview
The mechanism of transcription may be divided into three or four stages (promoter recognition is often not considered, as it occurs befor transcription begins).
Promoter recognition
A promoter sequence in the DNA binds to proteins called 'transcriptional initiation factors' (abbreviated to TFs), such as σ factor in bacteria, to which RNA polymerase binds. The affinity that the promoter has to the TFs determines how often the gene is expressed. In eukaryotes, many promoters and an enhancer sequence may also be involved.
Chain Initiation
Once bound, the RNA polymerase denatures part of the DNA double helix (approx. 11-13 base pairs), causing it to split. The RNA polymerase can then access the template strand, beginning the process of transcription.
RNA nucleotide trisphosphates then base-pair with the DNA nucleotides, assisted by the RNA polymerase.
Chain Elongation
The RNA polymerase moves relative to the DNA (it is not known whether the RNAP or the DNA is stationary in this process) in the 5' to 3' direction, at a speed of around 50-90 nucleotides/second, unwinding the DNA as it moves. As it does, new RNA nucleotides pair and are bound, with RNAP as the catalyst, to the growing RNA strand.
Chain Termination
Once RNA polymerase reaches a chain-termination sequence on the DNA, both the RNA strand and the RNA polymerase dissociate. This may require a termination factor protein, or may rely only on the termination sequence to form a 'hairpin loop' of RNA, preventing further transcription.
References
- Hartl, Daniel L. & Jones, Elizabeth W. (2005), Genetics: Analysis of genes and genomes Jones and Bartlett Publishers. 6th Edition, p403-10
External Links
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This page is part of the EvoWiki encyclopedia of genetics and molecular biology. Topics: Genetics - Transmission genetics - Molecular genetics - Population genetics - Quantitative genetics - Molecular biology - Genomics |
