Viruses – Extreme parasites
Surrounded by predigested food, absorbing nutrients though its “skin,” an intestinal tapeworm lives quite well without a digestive system – or even a mouth. Energy and resources the worm would have spent building mouth and intestines, finding and digesting food, goes instead into reproduction; the tapeworm is one long egg-making machine.
Even more stripped-down than tapeworms are viruses – little more than a dozen or so genes, an instruction set that hijacks a host cell and converts it into a virus-making factory.
Not that the parasitic lifestyle is completely carefree. The tapeworm cannot afford the luxury of a soft skin but must construct a tough, indigestible cuticle. Viruses, too, must counter their host’s defenses.
The mammalian immune system patrols the body to protect us from microbial attack. White blood cells (WBCs) search out and destroy any bacteria they find munching on our cells.
Viruses are endoparasites – they reside inside cells, where WBCs can’t go. The immune system, fortunately, has evolved defenses against such attack.
Seeing a virus within itself, the cell mounts a chemical attack: It deploys molecules that hack the virus to pieces. These molecules can’t do much to stop the infection – they’re hopelessly outnumbered. But that’s not their strategy. Rather, they take the viral fragments, attach them to “flagpole” molecules, and push them to the surface of the cell, where the pieces wave like flags for the WBCs to see. WBCs hone in and destroy all the cells they see waving such flags, destroying the virus in the process.
HIV – human immunodeficiency virus – subverts the immune system by hijacking, and then destroying, the very immune cells that should protect us, leading to acquired immunodeficiency syndrome, or AIDS. Other viruses have evolved more subtle means of subverting the immune system.
To make skin and hair and muscle and enzymes, we use proteins. Nearly every cell in our body is a protein factory.
To do their jobs, proteins must be exactly the right size and shape. As in any factory, though, mistakes are made, resulting in defective proteins. Defective proteins are recognized, shuttled off the assembly line, and tagged for destruction. Roving molecular “machines” within the cell find these tagged proteins and hack them apart.
Evolution has allowed certain viruses to pervert this system. Once inside the cell, they head for the factories where proteins are made. Monitoring the factories’ production, they allow most proteins to complete their journey down the assembly line undisturbed. But when a defensive flag-waving protein is constructed, viral molecules redirect it – to the place where misshapen molecules are tagged for destruction.
The would-be flag-wavers, tagged for destruction, are hacked to pieces. The alarm never sounds.
Viruses are both intimately tied into the way our bodies work and utterly dependent on us for their reproduction. How did they evolve?
They could be (extremely) stripped-down bacteria.
They could have evolved side by side, over billions of years, with their hosts.
Or perhaps viruses are rogue descendants of our own genes – genes that have somehow escaped from our chromosomes, genes that come “home” every now and then to make mischief.
In 1999, West Nile virus – a bird virus that can also infect other animals (including humans) – appeared in New York. The strain, according to Roy Campbell of the Centers for Disease Control, is common in Israel and neighboring areas. Of the several ways the virus could have entered the new world, Campbell believes it most likely hitchhiked, riding the blood of a mosquito or a passenger aboard a flight from the Middle East.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science prgrams can be heard at noon on alternate Tuesday’s on KVMR-FM (89.5).
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