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Letters

TWiV 59

Nick writes:

I’m a bit behind in my Twiv listening, but I’d like to comment on something you said in Twiv 47. You have reminded us several times in the past that “viruses are not alive” and you embellished on this during Twiv 47 by saying that if they are not alive, they can’t be killed. Actually, viruses can be killed, by treatment with formaldehyde or chlorox, boiling, or irradiation with x-rays or UV light. These are commonly-applied treatments that “kill” viruses. Were they alive before having been killed? Perhaps that is just a semantic question, but it seems to me that you cannot kill something that is not alive!

I think there may be some confusion in our minds about what a virus is. We most often think of viruses as being the virus particle, or virion, which carries the viral genome, wrapped in a protein and sometimes a lipid coat, from one cell to the next. This looks inert and not living. But this is a bit like saying that an oak tree is an acorn, or a sunflower is a sunflower seed. Viruses actually exist not only in the form of virus particles, but also in their intracellular forms, during which they carry out the many intricate and complex activities that lead to their reproduction. Sure enough, they need lots of things that cells provide, including energy, the machinery that makes proteins, and basic building blocks such as amino acids and nucleotides. But if you mean that “being alive” implies coding for those proteins and RNAs needed to synthesize proteins, produce energy, and make amino acids and nucleotides, this becomes a bit of a techical argument.

What about other “obligate intracellular parasites” such as chlyamydia and rickettsia? They can only reproduce inside cells, in part because they cannot synthesize certain amino acids and nucleotides. These organisms do make their own ribosomes and protein-synthesizing machinery, and reproduce by cell growth and division once inside an appropriate host cell. But are chlamydia elementary bodies (the infectious form that transports the chlymadia genome from cell to cell) alive? Are acorns and grains of wheat alive? The answer is yes, because they can reproduce under the right conditions; within certain cells, for chlamydia, and under certain conditions of humidity and temperature, for acorns and grains of wheat.

I argue that viruses are just as alive, simply that they have less of the equipment of life and depend more strongly on the host cell to provide this equipment. The recent sequencing of the genome of a monster virus, mimivirus, as well as the genomes of a number of other big viruses, brings viruses even closer to cellular life. Some of these viruses have more DNA and more genes than the simplest forms of cellular life. Mimivirus has genes for amino acid and nucleotide metabolism and for some parts of the protein-synthesizing machinery (but not ribosomes or energy production).

At any rate, I’m not comfortable with the notion that viruses are not alive. They are certainly part of the “biosphere” of living things. They share with other organisms the basic mechanisms of replication and expression of their genomes. They can reproduce themselves (with a little help from host cells), control many cellular processes, and undergo mutation and Darwinian evolution. Viruses have sex: they can recombine or reassort their genes. They can even have sex with cells, in both directions: viruses can insert their genomes into the cell’s genome, and they can pick up genes from host cells and integrate those genes into the viral genome.

Viruses may have originated from cells, or alternatively could have played a role in the origin of life, before cells as we know them existed. RNA viruses and viroids may even be evolutionary relics of the RNA world, which is thought to have preceeded the present world in which all known cellular organisms have DNA genomes. DNA viruses could have played a role in the transformation of cells from RNA to DNA genomes, by having introduced DNA genomes into RNA-based cells. For a fascinating argument in favor of this theory, see “Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: A hypothesis for the origin of cellular domains” by Patrick Forterre [PNAS March 7, 2006 vol. 103 no. 10, 3669-3674].

If you can kill it, it must have been alive!

I just listened to Twiv 48: your further discussion of this topic was interesting but sort of stalled. I got the impression that Rich agrees with me!

 

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