The big idea: Transcription begins when transcription factors bind to the promoter, and RNA polymerase then binds there too.
The points to remember
- The promoter is a DNA sequence just before (upstream of) a gene.
- The promoter is not transcribed: it is a binding site, not part of the code for the polypeptide.
- Proteins called transcription factors bind to the promoter.
- RNA polymerase binds to the transcription factors and the promoter.
- RNA polymerase then separates the strands and starts transcribing at the beginning of the gene.
Remember it as: Factors first, polymerase second, then off along the gene.
Real example: the TATA box. Many human genes have a short promoter sequence rich in T and A, about 25 to 30 bases before the start of the gene. A transcription factor binds this TATA box and helps place RNA polymerase in the right spot.
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Why the promoter matters: Whether transcription factors bind the promoter decides whether a gene is switched on.
Promoter
- Binding site for transcription factors and RNA polymerase
- Not transcribed
- Controls whether the gene is read
Gene (coding sequence)
- Transcribed into mRNA
- Codes for the polypeptide
- Controls what is made
A change in the promoter: If the promoter is changed so transcription factors cannot bind, the gene may not be transcribed at all, even though the gene itself is normal.
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How it is asked: Inside a longer transcription answer (RNA polymerase binds the promoter), or a short Outline of how transcription starts.
IB-style questionOutline[3 marks]
Outline how transcription of a gene is initiated.
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