Viruses

For something so small, viruses have a huge impact on the world. They are easily the most abundant organisms on Earth (though the use of the word “organisms” is controversial), found in every environment in vast, mind-numbingly large numbers. They are very ancient, probably played a pivotal role in the origin of life, and continue to be an important driver of evolution.

The vast majority of viruses are little more than strands of genetic material packaged up in a protein container called a capsid. Around a quarter are also surrounding by an envelope, which is usually made of a lipid membrane stolen from its host plus some virus-encoded proteins. A single virus is called a virion; most are around 100 nanometres across. Their sole aim in (quasi-)life is to invade the cells of their host, release their genetic material and hijack the cell’s biochemistry to make more copies of themselves. The new virions then burst out of the cell, killing it, and begin the cycle anew. Viruses are genetic parasites.

They are hugely varied. Some are so simple and formulaic that they can be described using a chemical formula – a polio virion, for example, consists of a short piece of RNA wrapped in a single-protein capsid and has the chemical formula C_332,652 H_492,388 N_98,245 O_131,196 P₇₅₀₁ S₂₃₄₀. Others are as large and complex as a bacterium, with hundreds of genes and a complex capsid and envelope. These giant viruses were discovered quite recently and shook up the world of virology, challenging the idea that viruses are merely non-living bags of chemicals. Some even have pathogenic viruses themselves.

The new coronavirus SARS-CoV-2, aka the Covid-19 virus, is fairly typical. Like about 65 per cent of known viruses its genome is made of RNA (the rest have a DNA genome), which encodes four structural proteins. One of these forms the capsid and the others are components of a double envelope. One of them, the spike protein, is used to break into its host’s cells.

Are viruses alive?

A question that is frequently asked is “are viruses alive?” There is no simple answer. On the one hand they are biological entities that reproduce themselves, which fulfils at least a minimal definition of life. On the other hand they do not have an independent metabolism and are totally unable to reproduce outside of their host. Evolutionary biologists do not consider them to be part of the universal tree of life. It is probably best to think of them as quasi-alive.

The world of viruses – called the virosphere – remains largely unexplored. Only about 9000 viruses have been well characterised; there are probably billions more.

We usually think of viruses as pathogens of both animals and plants, and indeed many of the well-known ones cause diseases, including some of the worst ever: smallpox, polio, influenza, Ebola, rabies, AIDS, Zika and, of course, Covid-19. Measles, chickenpox, herpes and the common cold are also viral diseases. But the vast majority of viruses are probably benign parasites that “live” and reproduce inside of us and other life forms without ever coming to our attention.

The origin of viruses is contentious. One hypothesis is that they predate cells, emerging from the primordial soup as free-living, self-replicating entities in a pre-cellular “virus world”, only later becoming parasitic. Another is that cells came first and that viruses were cellular components that escaped to become autonomous parasites. Both may be true, as viruses do not seem to have a single common ancestor but instead originated several times.

Whatever their origin, viruses have played a central role in evolution. They may have enabled the transition from a primordial RNA world to the modern DNA world. The nucleus of eukaryotic cells may have started life as a parasitic virus. And virus genes that integrated into their hosts’ genomes introduced many evolutionary novelties; for example the ERVW-1 gene that plays a central role in the formation of the mammalian placenta is viral in origin. We may fear and loathe viruses – especially right now – but without them we would not be here.