The RNA - a multitalent in the cell

For a long time, RNA was only regarded as a messenger molecule, a mobile copy of DNA. Although this is necessary to produce proteins according to the gene sequence defined by the DNA, it also leads to a rather boring existence in the cell. Today, our view of RNA has completely changed: Countless research findings have led to the fact that RNA is now attributed a central role in a large number of important cellular processes. Of course, the messenger role of RNA remains important. However, RNA molecules take on a whole host of other vital functions in the cell. They are found in different lengths and structures. They act as messengers, as blueprints, as scaffolds for building complex structures, as interpreters and as biocatalysts for chemical reactions. In many viruses, RNA even fulfills the function that we normally attribute to DNA, i.e. RNA carries the viral genome. RNA is such a multi-talent because of its enormous flexibility and specific chemical properties. And nature, with its wealth of ingenuity, has taken advantage of this and devised RNA molecules for a multitude of biological tasks.

We can also make use of the properties of this multi-talent; as a tool for research or as a therapeutic drug in medicine. One example of its application as a tool is the "gene scissor" CRISPR, which allows targeted modification of sequences in the genome. RNA drugs are still in their infancy, but - as we, and many others, think - they have a great future ahead of them. One example is the recently approved RNA drug Spinraza®, which is used to treat a rare type of muscular atrophy called spinal muscular dystrophy.

For more than half a century it has been known that cells produce messenger RNAs (mRNAs) as mobile copies of specific DNA sequences - the genes. These mRNAs serve the cells as blueprints for the production of proteins. In order to be able to read this blueprint, the cells use a highly complicated molecular machine called a "ribosome". Ribosomes also consist of special RNA molecules and numerous proteins (we will discuss this in the next section). Ribosomes function as "molecular protein factories" by reading the nucleic acid sequence of the mRNAs and translating it into the language of the proteins. In this translation, a special type of RNA (the so-called transfer RNAs, tRNAs for short) acts as an interpreter. tRNAs understand both, the language of ribonucleic acids and that of proteins. They have a very characteristic structure. On one hand, tRNAs read the genetic language very precisely on the messenger RNA, and on the other hand they find the appropriate protein building blocks (these building blocks are called amino acids). You can read more about how this complicated process works here: Protein synthesis

RNAs not only play an important role as messenger molecules, but also as construction plans and interpreters for the retrieval and translation of all the information stored in our genes on the genome. By the way, messenger RNAs make up only about 5% of the total RNA of a cell. About 15% of all RNAs in a cell are tRNAs.

RNAs can catalyze chemical reactions - this was first observed in the early 1980s and was a real revolution at the time. It was assumed that catalytic reactions in the cell could only be carried out by certain proteins, the enzymes. RNAs that catalyze chemical reactions are called "ribozymes". This term derives from a combination of the two words ribonucleic acid and enzyme. To this day, many ribozymes have been identified, and we know, that without them, many processes within the cell would come to a complete standstill. One example for a ribozyme is the above-mentioned Ribosome. Ribosomes consist of a combination of proteins and ribosomal RNA (rRNA). Interestingly, it is mainly the rRNAs that carry out the catalytic tasks. For example, the so-called "28S rRNA" of the Ribosome catalyzes the assembly of the individual protein building blocks (i.e. the amino acids).

Both the genetic material of a worm and that of a human being have about the same number of genes for storing blueprints for proteins. Researchers estimate that there are more than 20,000 genes. However, the total genetic material - i.e. the total amount of DNA sequence on the chromosomes per cell nucleus - is about 33 times larger in humans than in worms! Today we know that the majority of the human genome (about 98%!) does not contain any information for protein production. Or, to put it another way: in principle, only about 2% of the genetic material is needed to store the blueprints for the proteins. So why do we still have such a long DNA in every cell? Why does nature take the trouble to pass on so much (and at first glance "useless") DNA from generation to generation? Copying a 2-metre-long DNA molecule every time a cell divides is a considerable effort and one can assume that this is probably not done without good reason over thousands of years! Well, according to various studies, the proportion of so-called non-(protein-)coding DNA increases with the complexity of the organism, for example from a simple yeast cell (little non-coding DNA) to the above-mentioned worm and to humans and other mammals (much non-coding DNA). We know today that only a small part of human DNA is transcribed into messenger RNAs to be translated into proteins as described above. However, a large part of the remaining DNA is also transcribed into RNAs without being translated into proteins. The functions of many of these so-called "non-coding RNAs" (ncRNAs) remain a mystery to this day. The diversity of ncRNAs seems almost limitless, both in terms of their lengths and structures, as well as their functions. Thus, ncRNAs can be very short - sometimes just a few nucleotides - but often comprise several thousand nucleotides. They can occur as single strands, double strands and in many other complex structures. Within the cell, there are those ncRNAs that remain in the nucleus after their production, while others are specifically transported into the cytoplasm or to other organelles. Examples of ncRNAs with known functions are the rRNAs and tRNAs already mentioned. Another important family of ncRNAs are the so-called "small ncRNAs". These are a whole group of RNA molecules, all of which are about 20-30 nucleotides long and which, among other things, have an essential role in controlling the stability of mRNAs. Research on ncRNAs is currently being carried out very actively by many laboratories worldwide. This has led to the discovery of countless other so-called "long ncRNAs" (lncRNAs) in recent years. While the function of most lncRNAs has not yet been deciphered, there are individual cases where a role in embryonic development or in the development of cancer could be deciphered.

In all cells known to us, genetic information is permanently stored on the DNA. With viruses, this can be different. Viruses are not living beings in the classical sense because they are not independent cells themselves but always depend on a host cell for their reproduction. Nevertheless, viruses also possess a blueprint in the form of nucleic acids, which is necessary for their reproduction in the host cell. Interestingly, the majority of all viruses use RNA as data storage. These viruses are called RNA viruses. We encounter thousands and thousands of RNA viruses in our everyday lives. Diseases such as influenza, measles, hepatitis C, Ebola, rabies and polio are all caused and transmitted by RNA viruses. Find out more about RNA viruses.