What is gene regulation
Gene regulation regulates gene expression in the organism, i.e. controls which gene is expressed (read) when, where and to what extent. In multicellular organisms in particular, it is important that the development is precisely coordinated, that is, that all gene-regulated functions work ’hand in hand’ so as not to cause chaos.
There are several opportunities for gene regulation:
An important possibility of influencing is found in the decision as to which gene is to be transcribed, i.e. at the initiation of transcription. Here, the promoter controls whether the gene is read or not. To activate a gene, a protein complex consisting of several transcription factors and RNA polymerase II attaches to the core promoter. It is known as the pre-initiation complex. If, instead, a repressor binds to the promoter, the gene becomes inactive and therefore not read. The special transcription factors also bind to enhancers or silencers on the gene, which either increases or decreases transcription.
A second possibility for influencing is the initiation of translation. Here, too, a pre-initiation complex is formed that interacts with the small subunit of the ribosome. Regulatory action can also be taken here, for example by modifying the pre-initiation complex (e.g. by phosphorylation, i.e. by clamping on a phosphate residue).
Another important regulatory mechanism is the synthesis of so-called antisense RNA (aRNA). This is read from the non-coding strand (template strand) of the DNA. It attaches itself complementarily to the mRNA and thus prevents translation. The antisense RNA is always transcribed when a second promoter is activated at the 3'-end of a gene, which then triggers the transcription of the non-coding strand of the DNA.
The most important possibility for regulation, however, is the amount and shelf life of the mRNA. Depending on whether you have to react quickly to a situation, the RNA is short-lived; with longer processes it can also be stable for hours and accordingly be read more often. The stability of the mRNA depends on the length of the poly (A) tail and on special base sequences (AUUUA) that are repeated differently. The more often these occur, the faster the mRNA is broken down.
Regulatory factors can also intervene in the processing of the pre-mRNA, for example when introns are spliced out or when the poly (A) tail or the cap is attached, and when it is transported into the cytoplasm.
Shortly after the start of transcription, a molecule is bound to the beginning of the RNA produced, which usually consists of a converted guanosine nucleotide. It has an important function in stabilizing the resulting RNA strand, which after its completion has to be transported from the cell nucleus into the cytoplasm. This structure is also known as the 5'-cap structure.
Tailing also has a similar function, in which a polyadenyl tail or poly (A) tail, which contains many adenine nucleotides, is attached to the 3'-end of a resulting RNA strand. Like capping, this serves the stability of the RNA and prevents its premature degradation in the cytoplasm. The poly (A) tail probably also helps with the start of translation.
Here, after the transcription, the non-coded areas (introns) are cut out of the RNA strand. What remains are the exons which, together with the capped and polyadenylated RNA ends, form the matured mRNA. This is then transported from the cell nucleus into the cytoplasm. Depending on requirements, sometimes only certain introns are spliced out of the pre-mRNA, the other introns are masked so as not to trigger premature degradation. This is known as alternative splicing. In this way, different proteins can be synthesized from one gene.
Splicing takes place in the so-called spliceosome, a protein complex that binds to the pre-mRNA in the cell nucleus. If the complex does not bind or if the introns are not removed, the RNA is broken down in the nucleus.
Capping tailing and splicing are considered post-transcriptional modifications because they take place after transcription (so-called processing).
See also: genome, gene expression, gene drift, gene transfer, genotype, genetic engineering, cell nucleus.
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