Ribonucleic acid (abbreviation RNS, also RNA; English R ibo n ucleic a cid) denotes an fromGiant molecule made up of nucleic acids.
The ribonucleic acid is a copy of the one found in the cell nucleus Deoxyribonucleic acid, which in the sequence of nucleotides (genetic code) the information for the differentHas stored genes. The information contained in the ribonucleic acid is used for the synthesis of Proteins.
Ribonucleic acid, abbreviated as RNA by abbreviationfinder.org, polynucleotide whose monomer units consist of a pentose (ribose), a purine (adenine, guanine) or pyrimidine base (cytosine, uracil) and a phosphoric acid residue in a ratio of 1: 1: 1 exists.
The RNA has a lower molecular mass than the DNA (deoxyribonucleic acid). It occurs in the cell nucleus, in the ribosomes and in the cytoplasm of all living things and is responsible for the transmission of the genetic information encoded in the DNA. Only in the case of RNA viruses is the RNA (as is usually the case with DNA) the carrier of genetic information.
The RNA is synthesized as a copy of a DNA segment with the help of special enzymes, the RNA polymerases (transcription). It can be broken down by other specific enzymes, the ribonucleases (RNases). The primary structure (sequence of nucleotides) of RNA is similar in structure to that of DNA, but in the nucleotides the sugar deoxyribose has been replaced by ribose and the pyrimidine base thymine by uracil (U). RNA thus contains the purine bases adenine and guanine and the pyrimidine bases uracil and cytosine. In the case of RNA, too, the spatial arrangement of the nucleotide chain is referred to as the secondary structure; The difference to DNA is that the RNA is usually not in a double-stranded structure.
Molecular biological significance: The following main groups are distinguished according to their functions:
1) The messenger RNA (mRNA, messenger RNA) reaches the ribosomes after its synthesis and serves there as a template for protein biosynthesis. Each protein in a cell is encoded by a special mRNA. Since proteins can vary in size, the molecular mass of the mRNA also varies considerably. In prokaryotes there is a special feature insofar as a single, so-called polycistronic mRNA, can serve as a template for several proteins.
2) The ribosomal RNA (rRNA), which makes up the largest part (about 90%) of the cellular RNA, is part of the ribosomes. The subunits of the ribosomes contain four different types of RNA as structural components, which differ in size and were originally divided according to their sedimentation behavior (their sinking speed in the ultracentrifuge): In eukaryotes these are 28 S, 18 S, 5.8 S and 5 S rRNA (S = Svedberg unit of sedimentation).
3) Transfer RNA (tRNA) consists of relatively small molecules with molecular weights between 23,000 and 30,000, which corresponds to about 80 nucleotides. The tRNA serves as a carrier of the amino acids on the growing protein chain during protein biosynthesis on the ribosomes. It was possible to show that every tRNA is formed into double strands over certain stretches through complementary base pairing with the formation of loops. The anticodon, which consists of three nucleic acid bases and is complementary to the codon on the mRNA, is located in a loop of the complicated three-dimensional structure of the tRNA. Pairing the codon and anticodon ensures the correct sequence of the amino acids in the protein.
Synthesis: All three types of RNA are synthesized in eukaryotes in the nucleus and migrate from there into the cytoplasm. The primary RNA (heterogeneous nuclear RNA or hnRNA) synthesized on the DNA is often subject to various processes of maturation up to the formation of the functional RNA. The two ends of the RNA can be modified by adding additional nucleotides. In eukaryotes, a sequence of 100-200 adenine nucleotides (poly A tail) is attached to the 3 ′ end and a methylguanosine group (5 ′ cap) is attached to the opposite end. In the next step, the non-coding sequences (introns) are cut out with the help of enzymes and the coding sequences (exons) are linked to form the mature RNA. This one as splicing The process known as splicing takes place in the cell nucleus. With the help of special RNA (small nuclear RNA or snRNA) proteins catalyze the individual conversion steps. The targeted conversion of the sequence of RNA molecules (RNA editing), e.g. B. through the controlled insertion of nucleotides, is a form of post-transcriptional change. The situation is complicated by the fact that the maturation of RNA can take place differently in different tissues; the post-transcriptional changes in RNA are therefore also involved in the processes of cell differentiation. Deviations in such processes can be the cause of diseases.
Other functions: Aside from its long-known role in the implementation of genetic information, RNA can also fulfill other functions. Ribozymes are RNAs that have enzymatic activity. The first observations of this phenomenon were made on RNA molecules that cut introns out of their own molecule and thus link sections that were originally distant. The fact that enzymatic conversions can also take place without the involvement of proteins led to the hypothesis that RNA once fulfilled central functions for primitive forms of life and thus played a decisive role in the development of life.
In recent years, scientists have been studying a phenomenon called RNA interference. The reason was the observation that short, double-stranded RNA molecules can apparently shut down the activity of certain genes. This short interfering RNA (small interfering RNA or siRNA) usually consists of 21–23 nucleotides. If it gets into the cell, it causes the homologous mRNA, i.e. the same mRNA in the sequence, to be destroyed. As a result, the conversion of the mRNA into a protein and thus the realization of the genetic information does not take place. Presumably, this mechanism acts naturally as protection against foreign RNA (e.g. viral RNA). The short interfering RNA is meanwhile being used as a genetic engineering tool and is already being tested therapeutically to block pathogenic genes. Another method is also being tested, which is used to suppress the reading of genetic information, the so-called antisense technology. A nucleotide sequence that is complementary to the nucleic acid sequence of the mRNA (the antisense RNA) is introduced into the target cell, attaches to the mRNA according to the lock and key principle and thus blocks the conversion of the genetic information into the corresponding protein. The inhibition of the synthesis of disease-causing or disease-promoting proteins with the help of antisense molecules is of therapeutic interest. B. to fight cancer. In plants (experimentally realized so far on thale cress), short single-stranded RNA (also called micro-RNA) can suppress the synthesis of hormones (such as jasmonic acid); as a result, plants grow longer and age more slowly.