What exactly are these microscopic hijackers?
Observing thin slices of a Martian meteorite under the microscope, scientists were intrigued to find clusters of tiny bubbles. The simplest interpretation was that they were bubbles of gas trapped in the rock as it solidified. Some researchers, however, interpreted the bubbles as fossils of bacteria that had once lived on Mars … fossil Martians.
A major objection to this hypothesis was that the bubbles were smaller than any bacteria found on Earth, smaller than any bacteria that could exist on Earth. Bacteria, like people, carry their bodies’ blueprints in molecules of DNA and, small as it is, DNA takes up space.
Assuming Martian microbes also encoded their genes in some sort of chemical, there would not have been enough room within the bubbles to pack all the genes necessary to keep a bacterium alive, growing, reproducing.
From a Greek word for “small,” we refer to bacteria as microorganisms; the putative Martian bacteria, even smaller than any Earthly bacteria, have been denoted “nanobacteria,” from the Greek word for “dwarf.” It will take more study, and especially more samples, to decide whether these “bubbles” were ever alive.
The need to carry its blueprints puts a limit on how small an earthly organism can be. But there exist on Earth other entities – I hesitate to call them life forms – that have found ways to evade this limit.
Large enough to pack their genetic material, bacteria are also large enough to be removed from water by filtration (a fact exploited by backpackers). Late in the 19th century, however, scientists found diseases whose causative agents could slip through their finest filters.
Many disease organisms at that time were referred to by a word derived from the Latin for “poison.” These especially tiny ones were special – they were filterable “viruses.”
Like a traveler who carries only his wallet and toothbrush, knowing he can get whatever else he needs upon arrival, viruses travel light.
Biologists lump parasites into two broad categories. Those that are “free-living” (e.g., mosquitoes) must find their host, grab a meal, digest what they’ve eaten, and do all the other tasks necessary to keep themselves alive and allow themselves to reproduce. The free-living lifestyle is, ironically, “expensive” in terms of energy and resources the parasite must use to survive and reproduce.
Endoparasites, on the other hand, take up permanent residence within their host. Such a lifestyle frees up a lot of resources. A tapeworm, once it’s found its way into a host’s gut, latches on for life. With no need to ever look for food, it can dispense with eyes, ears and nose. Furthermore, its food is practically predigested. All the resources it would have put into eyes, ears, nose and digestive system can be invested in reproduction. The tapeworm has become one giant egg-making machine.
Viruses have gone tapeworms, and other endoparasites, one better. The typical virus is not much more than a small bit of nucleic acid, coding for a dozen or so genes, surrounded by a protective protein coat. Making its way into a host cell, the virus hijacks the machinery inside, commanding the cell to forget most of its duties to its owner’s body and concentrate on one simple task – following the blueprints in the dozen or so viral genes, blueprints that describe how to make more viruses.
Viruses are packages of rogue genes, exquisitely tuned to hijack our bodies. Where have they come from?
(To be continued.)
Trained as a biologist, Alan Stahler is also an amateur astronomer. He teaches biology and geology at Bitney Springs Charter High School. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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