Messenger RNA

As mRNA ( messenger RNA English ), also known as messenger RNA, the RNA transcript of a gene associated with a sub- portion of the DNA is referred to. The mRNA is synthesized in the transcription of the enzyme RNA polymerase.

Transcription

The transcription occurs in the cytoplasm in prokaryotes, eukaryotes in the nucleus ( nucleoplasm ). The process is similar in prokaryotes (living without a nucleus ) and eukaryotes ( organisms with nuclei ), but can begin in prokaryotes immediately the translation ( the actual process of the synthesis of proteins ). Ribosomes attach themselves to the mRNA before transcription is complete. The mRNA of eucaryotes is however usually processed before the end of transcription, and then removed from the cell nucleus.

In contrast to prokaryotes, eukaryotes have only one type of the enzyme RNA polymerase, which takes over the synthesis of RNA. In contrast, eukaryotes have three RNA polymerases:

Another major difference between prokaryotic and eukaryotic mRNA is that prokaryotic mRNA can be polycistronic eukaryotic mRNA, however, is always monocistronic. This enables prokaryotes to have only a single mRNA transcript of the information of several ( on the DNA behind the other ) genes. One such area is called jointly transcribed operon. In this way it is possible that already in prokaryotes or the first proteins are fertigtranslatiert during mRNA synthesis.

Eukaryotic pre- mRNA processing

In eukaryotes, mRNA, first in a preliminary stage must ( pre-mRNA or hnRNA ( ENGL. Heteronucleic RNA or pre-mRNA ) ) is present, are first processed and then pass through the pores of the cell nucleus into the cytoplasm, to there by the ribosomes to be detected, the start of protein synthesis:

• The 5 'end (this end was first synthesized in the transcription ) gets a 5'- cap structure ( from the English cap = cap ). This cap consists of a modified form of the guanosine, the 7- methylguanosine. The cap structure has at least two important functions: First, it acts on the transport into the cytosol, as a signal for the small ribosomal subunit binds to the mRNA, and, secondly, it protects the mRNA prior to hydrolyzing enzymes.
• The 3 'end, which was finally transcribed in transcription, undergoes polyadenylation. In this process, a poly ( A) tail of 30 to 200 adenine nucleotides is attached to protect the mRNA from enzymatic degradation. In addition it facilitates the export of the mRNA from the nucleus to the cytoplasm.
• By splicing introns which do not contain coding information is removed and the remaining exons are joined. This process takes place in the spliceosome: A complex of snRNPs (English Small Nuclear ribonucleoprotein ), consisting of the snRNAs U1, U2, U4, U5 and U6 and about 50 proteins, other splicing factors and hnRNA. By alternative splicing may result in the same pre-mRNA, different mRNAs that result in the translation of different proteins.

At this point also engage various regulatory processes of the cell. Via antisense RNA and RNA interference, mRNA can be reduced and thus, the translation can be prevented. Still takes place in some cases, so-called RNA Edition, i.e. the nucleotides in the mRNA to be changed. One example is the mRNA of apolipoprotein B, which is edited in some tissues but not in others. Edition formed by an earlier stop codon ( another function ) results in the translation to a shorter protein.

Translation and degradation

The process of translation (Latin for " transfer ," engl. "Translation " ) takes place on the ribosomes that are free in the cytoplasm or attached to the endoplasmic reticulum.

Each ribosome binding to the mRNA, synthesized according to the sequence of a protein of the amino acids available in the cell. This process produces a single protein molecule normally. Then the ribosome from the mRNA triggers. In general, several ribosomes attach to a mRNA and synthesize the encoded protein in it. Later, the mRNA by the enzyme RNase ( ribonuclease ) is broken down into its component parts, which can then be used by the cell again to build new RNA molecules. The degradation process takes place in the so-called P-bodies, specific cell structures in the cytoplasm. These structures have recently discovered a further form of expression regulation, at the mRNA and - instead of being removed - can be stored for re- translation.