RNA viruses

What are viruses and how are they constructed?

Viruses are tiny, infectious particles that come in a variety of forms. The smallest viruses have a diameter of only 20 nanometers (a nanometer is one billionth of a meter). You can hardly imagine this tiny size: If you wanted to fill a pinhead with viruses, there would be room for many thousands of billions of virus particles inside! Viruses consist of not much more than their genetic material - i.e. the viral genes - and a protein envelope (also called a "capsid") that encloses and protects the genetic material.

A virus is classified according to the shape of its genetic material. The genetic material can either be in the form of DNA, as in humans, or - and this is unique for viruses - also in the form of RNA. The genetic material of viruses is much smaller than that of humans: there are viruses that have just four genes (for comparison: humans have about 20,000). Other viruses can have several hundred genes. Because viruses are so minimally endowed, they cannot reproduce themselves. To reproduce, they need either a human, an animal or a plant host cell.

The variety of existing viruses is great, but not every virus is dangerous for humans. Each virus is specialized for a certain type of cell that it can infect. This is called "host specificity". The host specificity of viruses is determined by interactions between protein components on the virus surface and specific proteins (called receptors) on the host cell surface. You can think of this as a lock-and-key principle. A cold virus usually only has the right key to penetrate the cells of the upper respiratory tract, whereas an HI virus (HIV) exclusively attacks a specific type of white blood cell. Some viruses are not quite so specific, for example the swine flu virus can infect both domestic pigs and humans. Another example is the dangerous rabies virus. It can infect rodents, dogs and humans, among others.

Completely without their own metabolism and protein synthesis - why can viruses still reproduce?

Once the virus has docked onto a host cell, the virus genome gains access to the host cell - a process that can vary depending on the type of virus. Some viruses literally "inject" their genetic material into the host cell. The viral genome then reprograms the host cell in such a way that it produces the viral genomes and capsid proteins. When the host cell has produced the viral nucleic acid molecules and capsid proteins, they assemble into hundreds and thousands of new virus particles - the virus has thus greatly multiplied. Usually, the cell is then destroyed and the released viruses can infect new cells. This is how a viral infection spreads.

But what exactly are RNA viruses?

As already mentioned in the first section of this article, different types of viruses also use different types of nucleic acid to store their genetic material. This property is used to classify viruses: for example, one speaks of DNA viruses, RNA viruses, double-stranded and single-stranded, plus and minus. The genes of RNA viruses consist of RNA and not - like our human cells, for example - of DNA. RNA can be double-stranded (similar to the DNA double helix of our chromosomes) or single-stranded. Finally, in the case of single-stranded RNA viruses, a distinction is also made as to which of the two strands (compared to a double strand) is present. This is referred to as the polarity of the RNA strand, which is indicated as "plus" or "minus". Different virus types also differ in their reproduction cycle within a cell.

"Emerging" viruses:

Viruses frequently re-emerge and trigger epidemics. HIV, for example, spread in the early 1980s, Ebola fever broke out in West Africa between 2014 and 2016, the Spanish flu was triggered by an influenza virus in 1918-1920, and, very recently, the coronavirus Covid-19 led to a global pandemic.

Where do such "newly emerging" viruses come from?

There are various possibilities of origins of "new" viruses. Often, an already existing virus changes (mutates), which can be the source of a new viral spread and disease. RNA viruses have the property to change their gene sequences very frequently. The reason for this is that no "proofreading" takes place during the replication of the viral genome (as it happens, for example, during the replication of human DNA). Incidentally, the high mutation rates are one of the reasons why it is so difficult to produce suitable vaccines that protect against viral diseases. For this reason, it happens frequently that during the time when vaccines are developed, tested and produced, the virus has already slightly changed. Then the newly developed vaccine can already be less effective, possibly even ineffective against the new viral variant! A well-known example of an RNA virus that mutates very frequently is the flu virus (influenza). Another origin of novel viruses is the spread of viruses from one host organism to another. For example, viruses can be transmitted from animals to humans - especially if the viruses slightly mutate or if humans have contact with unusual wild animals. An example of this is the H1N1 swine flu virus, which originally came from pigs but can also be transmitted from human to human. Or the new coronavirus, which was probably transmitted from a wild animal to humans.

"Newly emerging" viruses are therefore not really "new", but they already existed before and could now suddenly spread extensively - mutations, evolution, and adaptation to a new host species play an important role in this.

For the development of drugs and vaccines against viral diseases, research into viruses and their molecular mechanisms is of fundamental importance.