Laffe virus

This is not an argument I am comfortable with. If a virus is alive, should we not also consider a DNA molecule to be alive? Plasmids can transfer as conjugative molecules, or be passively transferred, between cells, and they may carry genes obtained from the host.

What about prions? The argument reductio ad absurdum is that any biologically produced mineral that can act as a crystallisation seed for further mineralisation hence meeting the criterion of reproducibility might also be classified as living! This questioner currently considers viruses to be non-living. Where we draw the line between chemistry and life can seem a philosophical, or even theological argument.

Are viruses able to claim a similar ancestry? The contention that viruses have no place in the tree of life is often supported by the assertion that viruses do not have a comparable history — viruses are polyphyletic.

Viruses are at a terrible disadvantage in this comparison, however. We are aware of only a tiny fraction of the total genetic diversity of viruses. Moreover, their genomes evolve far more rapidly than cellular organisms. So, from the small islands of sequence data we have, it is hard to argue that a coherent phylogeny does or does not exist.

Interestingly, conservation of folds in viral proteins has begun to highlight possible common ancestries that could never be inferred from genome sequence data. A striking example is domain duplication of the beta jelly roll motif which gives rise to the pseudo-sixfold symmetry of trimeric hexon capsomeres in adenovirus. This is also found in viruses that infect insects, Gram-positive and Gram-negative bacteria and extremophile archaea.

Viruses assemble their capsids from surprisingly few distinct protein folds, such that convergent evolution seems highly implausible. A recent study has investigated viral origins by analysis of the evolution and conservation of protein folds in the structural classification of proteins SCOP database.

This work identified a subset of proteins that are unique to viruses. The authors conclude that viruses most likely originated from early RNA-containing cells. If viruses made an evolutionary leap away from the cellular form, casting off its weighty metabolic shackles to opt for a more streamlined existence, did they cease to be life? Have they reverted to mere chemistry? They all have surprisingly complex replication life cycles, however; they are exquisitely adapted to deliver their genomes to the site of replication and have precisely regulated cascades of gene expression.

Viruses also engineer their environment, constructing organelles within which they may safely replicate, a feature they share with other intracellular parasites.

Fundamental to the argument that viruses are not alive is the suggestion that metabolism and self-sustaining replication are key definitions of life. Viruses are not able to replicate without the metabolic machinery of the cell.

No organism is entirely self-supporting, however — life is absolutely interdependent. There are many examples of obligate intracellular organisms, prokaryote and eukaryote that are critically dependent on the metabolic activities of their host cells. Humans likewise depend on the metabolic activity of nitrogen-fixing bacteria and photosynthetic plants along with that of our microbiota.

There are very few if any forms of life on Earth that could survive in a world in which all chemical requirements were present but no other life. So, what does define life?

Some have argued that the possession of ribosomes is a key ingredient. This draws a neat distinction between viruses and obligate intracellular parasites such as Chlamydia and Rickettsia. This definition also confers the status of life on mitochondria and plastids, however. The endosymbiosis that led to mitochondria is thought to have given rise to eukaryotic life. Mitochondria have metabolic activity on which we depend, they have machinery to manufacture proteins and they have genomes.

Most would accept that mitochondria are part of a life form, but they are not independent life. I would argue that the only satisfactory definition of life therefore lies in the most critical property of genetic heredity: independent evolution.

Life is the manifestation of a coherent collection of genes that are competent to replicate within the niche in which they evolve d. Viruses fulfil this definition. It is estimated that there are 10 31 virus particles in the oceans — they vastly outnumber all other organisms on the planet.

Alive or not, viruses are doing rather well! Paid software is more likely to have anti-phishing filters for email clients, and anti-malware filters for Web browsers, both important features now that most malware tends to come through such software. What you don't want to do is have two anti-virus programs running at once.

They'll try to scan new files at the same time, and God forbid they try to do overlapping full-system scans. That will indeed render your computer completely useless.

Many users install or activate Microsoft's free anti-virus products Microsoft Security Essentials in Windows XP through 7; Windows Defender in Windows 8 and then later install a third-party anti-virus product without deactivating the earlier option.

That said, a free anti-virus product is always better than nothing. Combine that with a couple of other essential security tips, such as activating the built-in firewall and operating as a limited user except when installing software, and you'll head off 95 percent of threats to your machine. Follow Paul Wagenseil.

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