Viruses are tiny infectious entities that do not have their own metabolism and can only reproduce within living cells.
What are viruses?
Viruses are infectious particles without their own metabolism that can only reproduce within living host cells.
Discovery and characteristics
Even before their structure was discovered, it was known that there must be another infectious agent besides bacteria and parasites. In 1897, Loeffler and Frosch discovered that this agent could pass through bacteria-proof filters – it is ultrafiltrable and ultravisible. Since it could not be reproduced on culture media, it was initially considered uncultivable.
The term “virus” probably comes from the Sanskrit word for “poison.” Viruses depend on living host cells because they have no metabolism of their own and can only reproduce with the help of their hosts' cellular organelles.
As genetic material, they contain either DNA or RNA, which provide the blueprint for the so-called virion – the infectious virus particle. Viruses are strictly host-specific and can infect plants, animals or even bacteria (phages).
Structure of viruses
Inside a virus, the genetic information is stored in the form of single- or double-stranded DNA or RNA. These nucleic acids are surrounded by a protein shell, the capsid, which consists of subunits (capsomers).
The capsid together with the nucleic acid is called the nucleocapsid. The outer shape of the virus can vary depending on the arrangement of the capsomers: cubic, icosahedral, helical, or filamentous. Complex structures occur, for example, in bacteriophages.
Some viruses also have an envelope consisting of a membrane and spikes (peplomers). These glycoproteins are crucial for host recognition and binding, among other things.
Virus replication in the host cell
To replicate, viruses insert their genetic information into the host cell. The cellular synthesis apparatus is then “reprogrammed” to produce virus components. These assemble into new viruses and are eventually released.
Some viruses carry structure-bound enzymes that are necessary for virus replication, including:
- Neuraminidase
- Reverse transcriptase
- DNA polymerase
Virus families
Like animals and plants, viruses have also been classified. Modern classifications are based on physical and chemical properties, the genome type (DNA or RNA) and the virus structure.
A milestone in virus research was the X-ray structural analysis and the decoding of viral genomes. Today, a basic distinction is made between DNA viruses and RNA viruses.
Viroids
Viroids are infectious RNA molecules with a very low molecular weight (70,000–120,000 daltons). They consist exclusively of ring-shaped RNA and have no envelope or proteins.
Unlike viruses, viroids are completely dependent on cellular mechanisms and are considered “naked” genetic information. So far, they have only caused plant diseases, e.g., the Tomato Spindle Tuber Viroid. No diseases caused by viroids are known to humans to date.
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