Viruses (singular: virus) are the smallest, infectious particles and also parasites, i.e. living organisms that cannot reproduce independently without a host organism. On average, a virus particle is between 20 and 400 nm in size, many times smaller than human cells or bacteria or fungi.
Structure of viruses
The structure of viruses is not particularly complex. The most important component of viruses is their genetic material. This can be present in viruses either in the form of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid).
This characteristic also makes it possible to distinguish DNA viruses from RNA viruses (there are also the so-called retroviruses, which are a subgroup of the RNA viruses). The genetic material can be either ring-shaped or thread-shaped inside viruses. If the virus has not yet implanted itself into a cell, it is called a virion.
In almost all cases, the genetic material is surrounded by a capsid, which serves to protect the genetic material. This capsid is a structure of many identical subunits (capsomers) consisting of proteins. Consequently, the capsid is often referred to as a protein shell, together with DNA or RNA it is called a nucleocapsid.
In addition, some viruses are surrounded by a further envelope, the virus envelope, which is made up of a lipid bilayer in which proteins and glycoproteins are partially embedded. The glycoproteins protrude from the envelope in a spiky shape, which is why they are also called “spikes”, such viruses are called enveloped. If the virus envelope is missing, they are called unenveloped viruses.
In addition, some viruses have other components, but never a cytoplasm with cell organelles as in human, animal or plant cells, which would enable them to have their own metabolism. Since both mitochondria and ribosomes are missing, viruses are not capable of protein biosynthesis on their own and cannot produce their own energy. It has to nestle in a so-called host cell, i.e. a cell of a human being, for example, which has the necessary material at its disposal. There the virus is then able to manipulate the cell metabolism in such a way that it adapts to the needs of the virus and, instead of producing its own proteins, produces those proteins that the viruses need to survive.