Viruses

The first virus to be discovered was the tobacco mosaic virus (TMV). In the 1880s, researchers figured out that tobacco plants could “catch” what appeared to be a contagious “germ” from other, infected tobacco plants. Subsequent researchers knew enough about bacteria to know how to search for a bacterial pathogen by filtering extracts through special filter paper, etc., but none of those methods worked to find the cause. Someone figured out that it wasn’t just toxins produced by an infected plant, but that there was some agent that was reproducing, and that generation after generation, could still infect tobacco plants. Attempts to grow the pathogen on petri dishes were unsuccessful. Researchers were also puzzled by the fact that, unlike any bacteria (living cells) that were known, this pathogen could not be killed by alcohol, thus they were beginning to suspect some kind of chemical that could only reproduce when inside an appropriate host. Finally in the 1930s, TMV was found by crystalizing it! Indeed, it was a “chemical” that could reproduce like it was alive, yet needed the cells of a host organism to do so, and could be crystalized, but remained “viable” and infectious even then.

Virion The smallest viruses are smaller than ribosomes in cells, and the largest are so big that they’re just barely visible under the highest power of magnification possible with a regular light microscope. A single virus particle is called a virion, and is made of nucleic acid (either single or double-stranded RNA or DNA depending on what kind of virus it is) in a protein shell. The viral nucleic acid (RNA or DNA) is one molecule (one “chromosome”), consisting of from four to several thousand genes in length. Much current DNA technology research is aimed at inserting “good” genes into otherwise harmless viruses, then letting these infect animals/humans as a way of inserting the needed gene into the host’s cells. The surrounding protein coat that encloses the viral nucleic acid is called a capsid, and its shape, its protein structure, is specific to each kind (“species”?) of virus. An isolated virion is inert: it has no metabolic equipment, thus cannot do any chemical reactions on its own. Virions can be separated into separate nucleic acid and protein components, each separately crystallized and stored, and yet if mixed back together, can reassemble and be just as “viable”, as infective, as before.

Viruses are obligate intracellular parasites, that is, they can express their genes, do chemical synthesis of more viral nucleic acid and protein, and replicate only within a living cell of the correct host species. When a virus enters a host cell, it “takes over” the host cell’s metabolic machinery and chemical pathways and uses these to make more virus protein and nucleic acid, which then spontaneously come together to form not two, but MANY new virions. Viruses are capable of replicating many copies of themselves, which then go infect other cells. However, each type of virus has a limited range of host cells. For example, TMV won’t infect humans, but because humans are closely related to gorillas, many human viruses can be transferred from humans to zoo gorillas and make them very sick (hence one of the reasons why people aren’t allowed to feed zoo animals). Viruses use a sort of chemical “lock and key” mechanism to join to receptor sites on the surface of their host cell, thus the host may be only one or or several closely-related species. Some viruses can insert themselves (their DNA) into their host’s genetic material, stay there, and replicate along with host DNA when the host cells do mitosis. Herpes viruses are especially known for this, and it is thought that some forms of cancer may be caused in this way.

Cloaking Device Bacterial viruses (viruses that infect bacteria — yes, such things really do exist!) are known as bacteriophages. When these replicate, they then burst out of the host bacterium cell, killing it as many viruses are released. Many plant viruses are passed in the usual way by contamination from another infected plant (don’t work in the garden when the leaves are wet), but some others are passed to the next generation in the seed. Many animal viruses have a slightly different way of entering or leaving their host cells: many have an external membrane “cloaking device” which is derived from the cell membranes of their host cells. This bit of borrowed cell membrane helps the virus to enter/leave a host cell unnoticed, and without “exploding” the host cell. To infect a new cell, the membrane surrounding the virus joins with the cell membrane of a host cell, and the capsid and nucleic acid sneak inside. When the new virions leave, each wraps a bit of cell membrane around itself on the way out. Viruses like Rubella, Rabies, and HIV can also cross the placenta of an infected female mammal and infect her growing baby. If the baby lives, it may be deformed (Rubella) or be born with a viral infection (HIV and Rabies). Interestingly, because Rabies can be transferred transplacentally, many zoos will no longer accept donations of “pet” skunks because there is a question as to how many generations of captive-rearing with no signs of Rabies are necessary to insure that an animal is, indeed, free of Rabies.

One of the biggest questions about viruses is, “Are they alive?” Consider that viruses