Viral Diseases

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. 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. DNA Disrupter 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.