Viruses are extremely small and cannot survive on their own, relying on living cells to reproduce. They are not considered true living beings and are often referred to as “organisms on the edge of life.” They are bits of genetic material covered in a protein shell and reproduce by infiltrating cells and using their protein synthesis machinery. They do not evolve in the same way as conventional living things and may originate from mobile genetic elements. Studying their past is difficult and requires an electron microscope.
There are several properties that distinguish viruses from other organisms. They are extremely small, ranging in size from 10 to 300 nanometers. This is about ten times smaller than bacteria. Viruses cannot survive on their own and depend on hijacking the protein synthesis machinery of living cells to reproduce. For this reason, they are sometimes not considered true living beings, but are instead called “organisms on the edge of life.” The domain name “Acytota” (meaning “without cells”) has been attributed to these organisms, but is not widely used. Most scientists don’t consider them alive.
Viruses are bits of genetic material, like an instruction tape, covered in a small protein shell called a capsid. Sometimes, they have very simple “appendages,” like tail filaments or fibers, as in bacteriophages (bacteria-killing viruses), but often they’re just a small package. Their shape can be helical, like a screw; icosahedral, such as a geodesic dome; pleomorphic, like a small sponge; or resemble a bizarre spider robot straight out of science fiction, as in bacteriophages.
Instead of typical organisms, which reproduce through cell division, viruses reproduce at a hyper-exponential rate by infiltrating cells and using their protein synthesis machinery to pump out copies of the virus. In just ten minutes, a virus can take over a cell, copy itself hundreds of times, and kill the cell. Some viruses have a calculated replication time of around 70 seconds. By comparison, the fastest bacterial replicators double their biomass only about every 20 minutes.
Viruses do not evolve or develop in ways similar to conventional living things. They mutate and evolve, but some may originate as rogue mobile genetic elements (transposons) from the genomes of bacteria, plants, or animals. This means they may lack a conventional “family tree” that other organisms have. Because these organisms don’t fossilize well, studying their past can be very difficult. Examining them directly requires an expensive electron microscope.
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