Viruses

Viruses

Viruses are infectious agents that have no organelles or reproductive machinery of their own. Viruses cannot duplicate their DNA or RNA, nor can they translate their genetic information into protein. Essentially, they are small bags of genes that typically encode a comparatively small number of proteins. For example, the human immunodeficiency virus (HIV) is composed of only nine genes, yet with these simple nine bits of protein it can wreak havoc on the human immune system. Others, such as herpes simplex or adenovirus, can have large genomes with dozens of genes. Simple or complex, though, all viruses have the same function. As they cannot make protein or reproduce on their own, viruses must force bacteria or animal cells to do their work for them. A virus is, simply put, a genetic parasite.

As it does not have to sustain other energetically expensive cellular processes, a virus has a very simple structure. It is usually composed of nucleic acids (DNA or RNA) and a protein coat. This coat may be a very primitive covering (called a capsid) or it may be a complex structure derived from a host's membrane (called an envelope). Viral envelopes may possess a variety of receptors and decoys designed to fool the host's immune system. Avoiding detection is one of a virus's main tasks, and viruses may rely on dummy surface molecules or manipulating the immune system to do so. For example, HIV convinces infected cells to stop producing molecular flags that indicate infection to the rest of the body. In order to be successful, a virus must defeat the immune system and reproduce itself efficiently.

A viral life cycle generally has five distinct phases:

1. Attachment: The virus must connect itself to the target cell. Often, this is accomplished by molecules on the virus that mimic cell surface receptors required to interact with other cells. Rhinoviruses, some of which cause the common cold, bind to intercellular adhesion molecules (ICAMs) on the respiratory tract. ICAMs are meant to help white blood cells find their targets, but rhinoviruses have evolved to use them to get into cells. HIV binds to CD4 and CCR5, two surface markers involved in T-cell trafficking. Such binding may restrict the virus to infecting certain species or certain types of cells within a species.
2. Penetration: Once docked, the virus must get its nucleic material inside the cell. Some viruses are taken entirely into the cell. Others inject their DNA or RNA through the cell membrane. Still others fuse their membrane with their hosts' and dump the nuclear contents into the cytosol. In order for the virus to use the host's reproduction capabilities, it needs to be inside the cell.
3. Replication: Once the nucleic acids are inside, the virus will use the host's replication machinery to make more copies of itself. Some viruses, such as HIV, are made of RNA instead of DNA. For this reason, these viruses must first transcribe their genomes into DNA in order for cells to copy them. Whether made of DNA or RNA, though, a virus will also force the host cell to make its protein coat and any other proteins necessary to put the virus together. Rather than expending its precious energy on maintenance, the host cell instead is forced to use its energy to make viral parts.
4. Assembly: The various parts of the virus are put together with its freshly copied DNA. Soon the host cell is full of these virions, or individual viral particles, and the virions are ready to infect another cell.
5. Lysis: The virus ruptures the cell and disperses its viral progeny, all off to infect new host cells and begin the cycle again.

Not all viruses lyse (rupture) their host cells. Some may bud off the host cell. Others may become latent and rest in the host's cytosol or even the host's own chromosomes for a long time without causing damage. For example, herpes simplex 1 infects individuals and causes cold sores, but does so only intermittently. Cytomegalovirus (CMV) and Epstein-Barr virus (EBV), also known as mononucleosis, both infect the human body and remain latent for life. These viruses are held in check by the immune system and cause no harm, but they never go away completely. Persons with acquired immune deficiency syndrome (AIDS) lose immune function, and CMV and EBV have been known to resurface in persons with AIDS.

Because some viruses insert themselves into the host's genome, there is a possibility that they might affect normal gene regulation in the host itself. Viruses can be responsible for certain cellular problems that involve gene regulation. For example, some viruses are thought to be the cause of certain types of cancer. Human papilloma virus, for example, has been associated with cervical cancer. Hepatitis B and hepatitis C cause a majority of the world's cases of liver cancer.

While viruses can be specific for a particular species, cross-species infection happens frequently and sometimes with disastrous results. For example, all fifteen known strains of influenza A virus reside in aquatic birds, preferring the intestinal tracts of ducks in particular. As such, fowl fecal matter, as well as seals, whales, pigs, horses, and chickens, have been implicated in a number of human influenza outbreaks.

As a parasite, the best evolutionary strategy for a virus is for it not to harm the host. It is thought that HIV-1 and HIV-2 were introduced to humanity through the ingestion of uncooked monkeys (a chimpanzee and a sooty mangabey, respectively) sometime in the early twentieth century. These monkeys had been infected with simian immunodeficiency virus (SIV), which is fairly benign to its host. In species such as the mangabeys, African green monkeys, and pigtailed macaques, SIV causes no detectable problems and infection is widespread (it is estimated that some 80 percent of captive sooty mangabeys carry the disease). Infected rhesus monkeys, however, lose their ability to fight disease and waste away. Likewise, when the virus mutated to HIV in humans, it infected human populations and continues to cause widespread sickness and death. If HIV is to be with humanity for a long time, then it must become a little less virulent, lest it kill off all of its hosts.

see also Animal Testing.

Ian Quigley

Bibliography

Cann, Alan J., ed. DNA Virus Replication. New York: Oxford University Press, 2000.

Dalgleish, Angus, and Robin Weiss. HIV and the New Virus. San Diego: Academic, 1999.

Johnson, George B. Biology: Visualizing Life. New York: Holt, Rinehart and Winston, Inc., 1998.