When is a life form not a life form? When it's a virus.
Viruses are strange things that straddle the fence between living and non-living. On the one hand, if they're floating around in the air or sitting on a doorknob, they're inert. They're about as alive as a rock. But if they come into contact with a suitable plant, animal or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.
What They Are
A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses have an additional layer around this coat called an envelope. That's basically all there is to viruses.
The polio virus, left, once crippled millions. Courtesy of the MicrobeLibrary.org; © Jean-Yves Sgro, University of Wisconsin. The T4 bacteriophage, middle, is a virus that invades bacterial cells. Courtesy of the MicrobeLibrary.org; © Dennis Kunkel. Gold clusters bound to the knob protein of Adenovirus, right. Courtesy of Brookhaven National Laboratory.
Microbes are single-celled organisms that can perform the basic functions of life — metabolism, reproduction, and adaptation.
Except viruses. Viruses can’t metabolize nutrients, produce and excrete wastes, move around on their own, or even reproduce unless they are inside another organism’s cells.
They aren’t even cells.
Yet viruses have played key roles in shaping the history of life on our planet by shuffling and redistributing genes in and among organisms and by causing diseases in animals and plants. Viruses have been the culprits in many human diseases, including smallpox, flu, AIDS, certain types of cancer, and the ever-present common cold.
When viruses come into contact with host cells, they trigger the cells to engulf them, or fuse themselves to the cell membrane so they can release their DNA into the cell.
Once inside a host cell, viruses take over its machinery to reproduce. Viruses override the host cell’s normal functioning with their own set of instructions that shut down production of host proteins and direct the cell to produce viral proteins to make new virus particles.
Some viruses insert their genetic material into the host cell’s DNA, where they begin directing the copying of their genes or simply lie dormant for years or a lifetime. Either way, the host cell does all the actual work: the viruses simply provide the instructions.
Viruses may be able to infect and reproduce in more than one kind of animal, but the same virus can cause different reactions in different hosts.
For example, flu viruses infect birds, pigs, and humans. While some types of flu viruses don’t harm birds, they can overwhelm and kill humans.
Plant viruses do not infect animals or vice versa. Viruses that infect bacteria do nothing to animal or plant cells.
Viruses can act as miniature couriers. When they infect, they may inadvertently take up a bit of their host’s DNA and have it copied into their progeny. When the offspring viruses move on to infect new cells, they may insert this bit of accidentally pilfered DNA into the new hosts’ genome. This process is called transduction.
This can sometimes create a happy outcome. For example, the soil-dwelling bacterium Bacillus subtilis has viral genes that help protect it from heavy metals and other harmful substances in the soil.
Other times, viruses can wreak havoc when they bring in new genes. For example, Vibrio cholerae, the bacterium that causes cholera, is harmless in itself. The disease-causing toxin that causes illness is actually made by a virus that at some point smuggled itself into its host’s genome.
Viruses can also influence host genes by where they insert themselves into their host’s DNA. Recent decoding of the human genome shows that viral DNA sequences have been reproducing jointly with our genes for ages.
Some of these DNA sequences stay put, but others seem to move about our genome, jumping from place to place on a chromosome or from chromosome to chromosome. These “mobile elements” take up nearly half of the human genome.
Hemophilia and muscular dystrophy are two human diseases that researchers now believe resulted from mobile elements that, while skipping about the genome, ungraciously barged right into the middle of key human genes.
Viruses exist for one purpose only: to reproduce. To do that, they have to take over the reproductive machinery of suitable host cells.
Upon landing on an appropriate host cell, a virus gets its genetic material inside the cell either by tricking the host cell to pull it inside, like it would a nutrient molecule, or by fusing its viral coat with the host cell wall or membrane and releasing its genes inside. Some viruses inject their genes into the host cell, leaving their empty viral coats sitting outside.
If a virus is a DNA virus, its genetic material then inserts itself into the host cell's DNA. If the virus is an RNA virus, it must first turn its RNA into DNA using the host cell's machinery before inserting into the host DNA. The viral genes are then copied many, many times, using the machinery the host cell would normally use to reproduce its own DNA. The virus uses the host cell's enzymes to build new viral capsids and other viral proteins. The new viral genes and proteins then come together and assemble into whole new viruses. The new viruses are either released from the host cell without destroying the cell or eventually build up to a large enough number that they burst the host cell like an overfilled water balloon.
Viruses may be referred to often as the smallest infectious things. But there are some smaller contenders. Some of the agents of plant disease lack even a viral coat and are merely small strings of plain, or "naked," RNA. These particles are called viroids. They are believed to be a more primitive version of ordinary viruses.
Scientists in the 1970s began discovering even simpler and smaller virus-like organisms that can cause disease.
Viroids contain only RNA, but lack an envelope and capsid. Agricultural researchers found they caused problems in potatoes, tomatoes, and some fruit trees, and recently a viroid has been linked to hepatitis D.
But maybe viroids aren't the smallest infectious agents all.
Do you recall hearing about Mad Cow Disease? This is an ailment that affects the animals' brains and is also called bovine spongiform encephalopathy <boh-vine sponge-ee-form en-sef-uh-la-puth-ee> because it makes the brain appear holey, like a sponge. There is a human form of this disease called Creutzfeldt-Jakob <kroits-feld ya-cob> disease. Some scientists now believe these brain illnesses are among a few diseases caused by an infectious agents called prions <pree-ons>. Prions are not even DNA or RNA, but simply proteins. They are thought to be misshapen or abnormal versions of proteins normally found in animals or people. Very little is known about prions. Scientists suggest that they spread when a prion comes into contact with the normal version of the protein and causes the normal protein to change shape and become a prion, too. Visit All About Prions in the News section to learn more details about prions.
Unlike other infectious agents such as bacteria, viruses, and viroids, prions do not have the nucleic acids DNA or RNA. Prions are proteins that have the ability to transmit diseases, a finding that defied scientific expectations. There is still much debate about how they work, but scientists think these rogue proteins direct the host to create abnormal proteins that can cause serious neurological disease in animals and humans.
Prions are blamed for scrapie in sheep, and bovine spongiform encephalopathy (“Mad Cow Disease”) in cattle, and its human variant Creutzfeldt-Jakob disease. Some scientists suspect that prions may be responsible for Alzheimer’s disease.
© Jean Yves-Sgro
Type A flu virus
Many more interesting facts about viruses can be found throughout this website. You can also find a lot more detailed information and several photos of viruses at Hidden Killers: Deadly Viruses. And you'll find a great collection of virus portraits at The Big Picture Book of Viruses.