pre-mRNA

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The precursor form of the eukaryotic mRNA is referred to as pre-mRNA (to English pre-mRNA , short for English precursor messenger ribonucleic acid ), mostly synonymous with hnRNA ( English heterogeneous nuclear ribonucleic acid ), or also as primary transcript .

The pre-mRNA is the first RNA form that is created as a direct result of transcription in the course of gene expression of the genetic information ( DNA ) in proteins in eukaryotes, but not in prokaryotes ( bacteria and archaea ). After the complete processing of "primary transcript" this messenger is ribonucleic acid then "mature mRNA" or simply called mRNA. This is then available for translation within the framework of protein biosynthesis (protein synthesis) .

The hnRNA occurs as an intermediate product in the first translation step, the transcription: not all DNA sections that are in a gene actually code for proteins, only the sections known as exons . During transcription, the enzyme RNA polymerase II now copies the entire area of ​​the DNA, including all introns , i.e. non-protein-coding sections. The product is called hnRNA. Only the RNA sequence resulting from the splicing , in which the introns are cut out, is called (mature) mRNA.

Eukaryotes versus prokaryotes

In eukaryotes, the pre-mRNA is modified by adding a cap structure (at the 5 'end), a poly (A) tail and by splicing the introns in the cell nucleus . In some cases, this already happens during the transcription. The fully processed RNA is called (mature) mRNA. This is transported into the cytoplasm , where translation begins.

In prokaryotes there is no processing because the prokaryote DNA is free in the cytoplasm, in most cases has no introns, and thus the mRNA is directly present after transcription, which is usually still translated during transcription. This then also enables gene control mechanisms such as B. the trp operon .

See also

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