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Viruses: Still on the fence between life and death? Maybe not...
Until now, we all thought…
Virus particles are incredibly simple, consisting of either DNA or RNA to carry genetic information, and a protein coat that protects it. Their replication cycle generally consists of six stages.
First, the virus attaches to the outside of the cell. Next the virus has to penetrate the cell membrane in order to enter the cell. Since some cells have tough cell walls and structure protein in the membrane, many viruses drill a hole and then inject their DNA/RNA instead of entering the cell.
Once inside, the protein coat is removed and the DNA/RNA is allowed to replicate, using the cell’s metabolism. In a sense, viruses hijack the cell’s machinery in order to reproduce.
By definition, a virus is a small infectious agent that can only replicate inside living organisms; a definition obscured by the term “biological entity”.
I’ve been fascinated by viruses for years because they are not classified as “alive”. In describing them to people, I often use the term “living dead”. Other scientists have described them as “organisms on the edge of life”.
They resemble life because they have genes, and can reproduce, although they require a host cell to do so. They do not, however, have a cellular structure, which is often seen as the basic unit of life.
I only gave you one side of the argument, which isn’t quite fair. So here’s the other side: Many scientists hold to the belief that viruses are living creatures, since they have their own complex genome, they replicate, and a population of virions changes over time.
A study published this past month in the journal Nature is the first to show that a virus can have an immune system, which doesn’t prove that viruses are living, but it does strengthen the argument.
The discovery was actually an accident.
The group that published the paper studies a bacteriophage, a virus that is capable of infecting bacteria. Cholera, in this case.
Once Upon a Time
Here's the story: A postdoc in the lab was sifting through DNA sequences isolated from bacteriophages found in cholera patients in Bangladesh.
To her surprise, genes existed in these viruses that are known to comprise a functional immune system. Excited, she tested the ability of the bacteriophage to kill cholera bacteria with and without those immune system genes.
Only the virus with the immune system was able to kill the bacteria!
Okay, but where did this immune system come from? The scientists found that viruses are capable of acquiring the immune system genes from the bacteria that they infect, during the replication phase.
How does it work?
The immune system enables the bacteriophage to disarm the bacterial cell, much like our immune systems disarm viruses. Ironic, no? Looks like the virus is on our side this time!
So where does that put us? On the fence?
Viegas, Jennifer. "Some Viruses, Like Us, Have Immune Systems." Discovery News: Health. N.p., 27 Feb. 2013. Web. 3 Mar. 2013. <http://news.discovery.com/human/health/viruses-with-immune-systems-130227.htm>.