Ribonucleic acid (RNA), also known as RNA in German, is a molecule composed of the chains of several nucleotides (basic building blocks of nucleic acids). It is found in the nucleus and cytoplasm of the cells of every living organism. Furthermore, it is present in certain types of viruses. The essential function of RNA in the biological cell is the conversion of genetic information into proteins (protein biosynthesis/new formation of proteins in cells, transcription/synthesis of RNA using DNA as a template, and translation/synthesis of proteins in the cells of living organisms, which takes place at the ribosomes according to the genetic information). Unlike DNA, the structure of the form is not a double helix, but a single helix, a single strand that is circulated by itself. Each nucleotide within RNA has three components. Among them are the four nucleic bases (adenine, cytosine, guanine, and uracil), which are often abbreviated by their initial letters, as in DNA. The nucleic base uracil differs from the nucleic base thymine from DNA only by an additional methyl group. The two other components of RNA are the carbohydrate ribose and a phosphate residue. In contrast to the deoxyribose in DNA, the ribose of RNA has a hydroxyl group (functional group consisting of a water and oxygen atom) instead of a single hydrogen atom, which provides less stability for RNA. As with DNA, the nucleotides are linked together in an alternating sugar–phosphate chain by molecular bonding. RNA is synthesized by catalyzing an enzyme from RNA polymerase. A process called transcription occurs, using DNA as a template. In what is called transcription initiation, the RNA polymerase attaches itself to a DNA sequence called a promoter. The promoter is a protein located on the DNA that enables the enzyme from the RNA polymerase to cleave it. The enzyme moves along the DNA and a new, growing RNA strand is formed, to which a nucleotide is gradually added. When the enzyme reaches the terminator, i.e. the end of a DNA segment, the synthesis is terminated and the RNA polymerase becomes detached from the DNA. There are several forms of RNA that perform specific functions in a cell and play a role in protein biosynthesis (new protein formation). Among these, four commonly occurring forms of RNA are of high importance:
- The mRNA (messenger RNA) plays an essential role in protein biosynthesis in a cell (translation), transporting information of a protein from DNA to ribosomes. In this process, the amino acid sequence of the DNA must match the three nucleotides of the RNA.
- The tRNA (transfer RNA) is an RNA whose molecules of an RNA strand consist of only about 80 nucleotides. It has the task of mediating the correct amino acid sequence during translation of the corresponding mRNA sequence.
- The rRNA (ribosomal RNA) has the task of transporting amino acids to the ribosomes, an organelle important for the assembly of proteins. Within the ribosomes, it ensures translation of mRNA into so-called polypeptides (a peptide consisting of 10 to 100 amino acids). It occurs in the nucleus, cytoplasm and also in plastids (cell organelles of plants and algae).
- The miRNA (micro RNA) is a non-coding region of mRNA, only about 25 nucleotides long, found in both animals and plants. It plays an important role in the promotion (increase in expression) and inhibition (decrease in expression) of gene expression.
The first, essential research on RNA began in 1959 by Severo Ochoa and Arthur Kornberg, who recognized its synthesis by RNA polymerase. In 1989, RNA molecules were found to have catalytic activity.