Transcription (genetics)

As transcription (from late Latin transcriptio " transfer " to Latin transcribere " to / overwrite" ) is referred to in genetics, the synthesis of RNA using a DNA as a template. The resulting RNA can be largely divided into three groups: mRNA (messenger RNA) ( for protein ) and tRNA ( transfer RNA ) and rRNA (ribosomal RNA). Transcription, as well as the translation of, a substantial part of the gene expression process.

In the transcription of a gene is transcribed and amplified as a RNA molecule that is a specific DNA segment serves as a template (templates ) for RNA synthesis of a new strand. ( - T - G - C A) in the nucleic bases of RNA (A - U - G - C) In this process, the nucleic bases of DNA to be rewritten. Instead of thymine comes uracil and ribose instead of deoxyribose is in the RNA before.

The process of transcription runs basically the same in eukaryotes and prokaryotes. There are differences in the control and in the subsequent modification. In prokaryotes, the control by an operator, while if the regulation of a eukaryotic enhancer or silencer can be controlled, which is in each case connected downstream of the promoter in front of or. Further, in prokaryotes, the transcription is carried out in the cytoplasm of the cell, wherein the eukaryotic cell nucleus ( caryoplasm ). In eukaryotes, the pre-mRNA is also not processed during or after synthesis before it is transported out of the nucleus into the cytoplasm. Takes place in the cytoplasm after transcription, the ribosome translation of the mRNA into a protein.

Steps of transcription

Synthesis of tRNA and rRNA

Transfer RNA (tRNA ) and ribosomal RNA (rRNA ) can be synthesized by two different RNA polymerases in DNA, both operate according to a principle other than that of the RNA polymerase II. They resemble those of prokaryotes, in which the same RNA polymerase is active as a catalyst. In eukaryotes, the synthesis of the tRNA, the 5S rRNA and the 7SL RNA by RNA polymerase III, the synthesis of the rRNA and partially also the sn- RNA ( small nuclear RNA) by the RNA polymerase I, the synthesis of m-RNA by the RNA polymerase II

Termination of transcription

In eukaryotic cells, the RNA polymerase gene does not detect the end of its own, it needs to auxiliary factors which interact with the polymerase. These protein complexes are see polyadenylation site (5'- AAUAAA -3 '), cut and initiate RNA polyadenylation, RNA polymerase while continuing to work. A model for the termination of transcription is that still continues growing, useless RNA end is degraded by an exonuclease ( Rat1 ), and indeed faster than it is extended by the polymerase. Achieved the exonuclease the transcription site, the polymerase from the DNA triggers the transcription is finally finished ( Torpedo model of transcriptional termination ). Moreover, seem more protein complexes (eg: TREX) to be important for termination.

Reverse transcription

So-called retroviruses have a genome composed entirely of RNA; among them, the translation of a DNA intermediate takes place from which the mRNA is read. This translation of the RNA genome into DNA, the reverse transcription is also useful for the RNA virus, if it is to smuggle a copy of its genome into the DNA of the host.

For the synthesis of viral DNA from RNA retroviruses known as possess the enzyme reverse transcriptase. By means of an integrase (viral or cellular ), the viral DNA will be incorporated into the cellular genome as a provirus and is then available.

Archaeal transcription

The genes of the Archaea possess promoter in a TATA box consensus sequence called. At the promoter bind the two initiation factors of archaea, TBP and TFB. This in turn binds to a polymerase, which is orthologous to eukaryotic RNA polymerase II, and consists of twelve subunits.

Bacterial transcription

In contrast to the eukaryotes, bacteria have only a RNA polymerase. The core or minimum enzyme is ( 2 × α, β, β ') of the four subunits that catalyzes the transcription, but is not able to be initiated. The core of the enzyme interacts with the loose sigma subunit and the holo- enzyme is formed (2 × α, β, β ', σ ( sigma ) ), which can perform the initiation (Sigma allows sliding along the DNA and finding the Pribnow box of the promoter ). It binds to the promoter of the non- template strand and triggers the hydrogen bonds between the base pairs, it has a Helicasefunktion what is the most important function of this polymerase.

Function of the α - subunit is the one by the amino-terminal domain due to the maintenance and stability of the structure, on the other by the carboxy-terminal domain to bind to the promoter and interaction with transcription regulatory elements.

The β - and β' - subunits interact and provide for the binding to the DNA template and a growing RNA chain.

The σ ( sigma ) subunit recognizes transcription start points. There are several sigma subunits to recognize different sets of genes. The most widespread is the σ70 subunit.

The ω ( omega ) subunit serves to stabilize the structure and maintenance and is not mandatory.