Everything about Enhancer Genetics totally explained
In
genetics, an
enhancer is a short region of
DNA that can be bound with
proteins (namely, the
trans-acting factors, much like a set of
transcription factors) to enhance
transcription levels of
genes (hence the name) in a
gene cluster. Furthermore, an enhancer is a
cis-acting DNA sequence(s) which can increase transcription of genes. An enhancer doesn't need to be particularly close to the genes it acts on, and need not be located on the same
chromosome. In
eukaryotic cells the structure of the
chromatin complex of DNA is folded in a way that functionally mimics the supercoiled state characteristic of
prokaryotic DNA, so that although the DNA is far from the genes in nucleotides, it's geometrically close to the
promoter and gene. This allows it to interact with the
general transcription factors and
RNA polymerase II.
An enhancer may be located
upstream or downstream of the gene that it regulates. However, an enhancer doesn't need to be located near to the
transcription initiation site to affect the transcription of a gene, as some have been found to bind several hundred thousand
base pairs upstream or downstream of the start site. Enhancers don't act on the promoter region itself, but are bound by activator proteins. These activator proteins interact with the
mediator complex, which recruits polymerase II and the general transcription factors which then begin transcribing the genes. Enhancers can also be found within
introns. An enhancer's orientation may even be reversed without affecting its function. Furthermore, an enhancer may be excised and inserted elsewhere in the chromosome, and still affect gene transcription. That is the reason that intron
polymorphisms are checked though they're not transcribed and
translated.
Currently, there are two different theories on the information processing that occurs on enhancers:
- Enhanceosomes - rely on highly cooperative, coordinated action and can be disabled by single point mutations that move or remove the binding sites of individual proteins
- Flexible billboards - less integrative, multiple proteins independently regulate gene expression and their sum is read in by the basal transcriptional machinery
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