In any case, viruses are small enough, significantly smaller than eukaryotic cells, bacteria and archaea. And this is precisely their "problem".
All viruses contain genetic material, either a small genome (usually for less than ten proteins) consisting of single- or double-stranded DNA or RNA, which is enveloped by proteins (capsid). Some viruses also have an outer lipid membrane. However, all viruses lack the genetic information as well as cellular structures, enzymes and biomolecules for their own metabolism, energy-supplying reactions and independent reproduction. They are therefore dependent on the equipment of host cells.
Many viruses recognise their host cells through specific binding proteins with which they dock to certain structures on the
cell surface. Viruses of bacteria (
bacteriophages) and archaea (archaeviruses) inject their genome through the cell envelope into the microbe's interior. Viruses of plants and animals usually enter the cytoplasm through active uptake (phagocytosis) or fusion of their lipid envelope with the cell membrane.
The first gene products are responsible for reading the viral genome and controlling the cell's synthesis metabolism. In this way, proteins and genetic material of the viruses are formed and defence reactions of the host cell are suppressed. The finished viruses leave the cell, whereby microbes usually perish. Sometimes the viral genome inserts itself into that of the host cell, survives there and is passed on during cell division. This state is called latency or, in the case of bacteria, temperate phage. If the viral genes become virulent again, they sometimes take parts of the genetic information of neighbouring host genes with them and transfer them to other cells upon infection. This natural form of gene transfer has an effect on the evolution of viruses and organisms.
The intact viruses (virions) are not active outside their host cells and are therefore also regarded as an infectious factor, but not as a separate organism and therefore not as microbes. Alternatively, viruses are regarded as simple, obligate cell parasites.
Whether viruses are categorised as living organisms depends on the - unresolved - definition of a living organism. In any case, the evolutionary origin of viruses is linked to living organisms, as their biomolecules, genetic information and the reactions of the encoded enzymes are compatible with those of living organisms on our planet. Whether they evolved before the first biological cells or are descendants of independent genes of organisms is the subject of scientific debate. However, it is becoming clear that viruses are not drastically reduced forms of previously parasitic cells. Viruses do not represent a separate domain of life like bacteria, archaea and eukaryotes. The genetic material of viruses is distributed across all branches of the organism family tree.
Incidentally, not all viruses are small (20 to 300 nm). Giant viruses discovered a few years ago can grow to over 1 µm in size, can themselves be attacked by viruses (virophages) and contain an exceptionally large genome with 300 to over 1000 genes. This corresponds to the number in some bacteria! These include genes from other viruses and from host cells, but the majority are genes with as yet unknown functions that bear no resemblance to genes from microbes or other organisms. Most giant viruses with very large genomes (>500 genes) infect eukaryotes such as protozoa, algae and plants.
Read more:
N. Brandes, M. Linial (2019) Giant viruses – big surprises. Viruses 11, doi:10.3390/v11050404
M.J. Roossinck (2018) Viren! Springer-Verlag GmbH, Deutschland
S. Suerbaum, G.-D. Burchard, S.H.E. Kaufmann, T.F. Schulz (2016) Medizinische Mikrobiologie und Infektiologie. Springer-Verlag, Berlin.
© Text Harald Engelhardt / VAAM, engelhar[at]biochem.mpg.de, Use according to CC 4.0
Figure: schematic 3D representation of an adenovirus, T. Splettstoesser, https://commons.wikimedia.org/wiki/User:Splette
