Basics of transcriptional control
Transcriptional control is used in both prokaryotes and eukaryotes to regulate gene expression.
Transcription is often controlled by altering the potential of RNA polymerase to bind to a promoter sequence. If it cannot bind, the gene will not be transcribed.
This modulation is mostly controlled by transcription factors. These are proteins that bind to sequences of DNA called control elements.
Transcription factors can both induce and inhibit gene expression.
These proteins are not transcription factors as they do not bind directly to the DNA.
In prokaryotes, control of transcription is by far the most important level of gene expression regulation. There is very little control in the subsequent stages of protein synthesis.
Control of transcription is simpler in prokaryotes than in eukaryotes and for that reason is much better understood.
In prokaryotes, groups of genes with similar functions are expressed together in an operon.
The expression of genes in an operon is controlled by a single promoter sequence.
The lac-operon is a classic model of gene expression in prokaryotes.
Expression is initiated at the promoter sequence (where RNA polymerase binds). The operator region controls whether RNA polymerase can bind to the promoter. Activator and repressor proteins (types of transcriptional factors) are able to bind to it.
Eukaryotes do not have operons (groups of genes expressed together). Eukaryotic genes have more distance between them than prokaryotic genes, and eukaryotes need greater control over gene expression.
Eukaryotic genes that have a similar function tend to share control elements with similar sequences of genetic code.
This means that the same set of transcription factors can bind to all the control elements for this group of genes. This means that the genes with similar functions can be expressed at the same time (a bit like an operon, but with more control).
Each gene has its own promoter sequence . The promoter regions and other control elements of similar genes have similar sequences. This allows the genes to be expressed at the same time.