Under the Microscope / Prof William Reville: Viruses are highly interesting little biological organisms that are neither clearly alive nor dead. Under some circumstances the virus behaves as an inert chemical and under other circumstances it can actively replicate itself.
In order to reproduce, which means making copies of themselves, viruses must invade a living cell and take control of its metabolic machinery which it then employs to copy itself. Each type of virus is parasitically dependent on a particular type of living cell (its host) for its replication. Some viruses cause human or animal or plant diseases and this is how viruses were originally discovered.
Viruses are very small, and only one or two types are big enough to be seen in the ordinary light microscope that has a resolution of 0.25 micrometres (millionths of a metre). All other viruses can be seen only in the electron microscope whose resolution exceeds that of the light microscope by 3,000 times. The polio virus is spherical in shape and has a diameter of 27 nanometres (thousand millionths of a metre). The HIV virus that causes AIDS is also spherical and has a diameter of 50 nanometres. To put these figures in perspective, a bacterium will have a diameter of two micrometres and an animal cell a diameter of 20-50 micrometres.
Fundamentally there are two parts in a virus - an outer protective protein coat that encloses an inner core containing nucleic acid. Some viruses have their nucleic acid in the form of DNA, others in the form of RNA. In either case, the nucleic acid comprises the genetic material and contains all the information, coded in genes, to allow the virus to replicate itself.
Because viruses are so small, they can only contain a small amount of nucleic acid and consequently only a small number of genes. The genes contain the information necessary to make the proteins in the virus coat, but because the virus contains few genes, the coat contains only a few types of protein. The proteins, once made, self-assemble to form the protein coat and because there are only a few types of protein, they self-assemble into a regular (para-crystalline) array, and so, viruses are always regular structures with a high degree of symmetry. To illustrate this concept, imagine depositing a collection of marbles onto an up-turned biscuit tin lid and tilting the lid so that all the marbles congregate on one side. The marbles will spontaneously assemble in a regular crystalline array.
Because viruses are regular structures they are classified on the basis of symmetry into three classes - spherical viruses, helical viruses and complex viruses. The protein coat of a spherical virus encloses a hollow sphere. The protein molecules in a helical virus are arranged in a helical fashion in the protein coat that forms the wall of a cylinder. Complex viruses are often a mixture of helical and spherical symmetry.
Examples of spherical viruses include the polio virus and the AIDS virus. Examples of helical viruses include tobacco mosaic virus and para-influenza virus. Many viruses (bacteriophages) that use bacteria as hosts have complex symmetry. The space within the hollow protein coat is called the core and houses the DNA or RNA.
When a virus makes contact with a host cell, its DNA, or RNA, effects entry into that cell and takes over metabolic control of the cell. The information in the invading viral genes dictates the synthesis of new viral coat proteins and the genes also replicate themselves many times over. Many new viruses spontaneously assemble and leave the host cell. For every virus that invades a host, many hundreds of new viruses will be formed. In some cases the new viruses burst the host cell open as they leave, killing it. In others, the new viruses are shed without killing the host cell.
Because viruses are regular structures, they will form crystals in a test-tube if you have a sufficient concentration of them. You can collect these crystals and store them in a jar on the laboratory shelf where the crystals will remain unchanged indefinitely, just like a jar of table sugar. However, if you introduce these crystals to a wet sample of host cells, the viruses spring to "life", invade the host cells and replicate themselves vigorously.
Is the virus living or dead? For something to qualify as living it must exist between two boundaries - birth and death. It must also have its own metabolism and the ability to replicate itself under its own genetic instructions. The virus, out of contact with its host, has no independent metabolism of its own. It can replicate itself under its own genetic instruction, but only with the help of its host's metabolism.
The word virus comes from the Latin meaning "poison". Many viruses cause a wide variety of diseases in humans, animals and plants. The influenza pandemic of 1918 killed 20 to 40 million people worldwide. Included among human virus diseases is the common cold, but others are frequently fatal, including rabies, encephalitic diseases, AIDS, polio and yellow fever. Most viruses however cause diseases that create only acute discomfort unless serious complications arise - these include influenza, measles, mumps, chicken pox, shingles, acute diarrhoea, warts and hepatitis.
The only effective way to prevent virus disease is vaccination. Smallpox was eradicated worldwide in the 1970s through widespread vaccination. Vaccines for rubella, measles, polio and influenza have also been developed. The vaccine stimulates the immune system to produce antibodies that protect the body against infection with the virus.
William Reville is associate professor of biochemistry and director of microscopy at UCC