DNA Viruses

Viruses can be classified based on proteins encoded within the viral genetic material or genome. Viruses with deoxyribonucleic acid (DNA) genomes are called DNA viruses. Like all viruses, DNA viruses are small when com­pared to the cells they infect and as such are obligate intracellular parasites (parasites that can only replicate within cells). In the appropriate cell, DNA viruses are able to program the cell to replicate the virus using the genes contained within the viral DNA genome.

The extracellular form of a virus is known as a virion. For a DNA virus, the virion is composed of a set of DNA genes protected by a protein- containing coat called a capsid. The coat is often characterized by regular­ity and symmetry in its structure and is capable of binding to and invading cells. In the case of some DNA viruses, the capsid can be surrounded by a membrane that is formed from cellular membranes. On invasion of a sus­ceptible cell the virion is disassembled to release the viral genome into the cell, at which time the genes within the viral DNA are transcribed, pro­ducing viral messenger ribonucleic acid (mRNA).

The viral mRNA is translated into protein. These “early” proteins are responsible for altering normal cellular functions which in some cases allow the infected cell to evade the immune system. These “early” proteins are also important for promoting “late” viral gene synthesis and preparing the cell for the production of progeny virus. Following late gene synthesis, which includes proteins that are important for replicating and encasing the virus, progeny virions are then released by the infected cell to invade other cells so that the process can be repeated.

There are six different DNA virus families that infect and may cause significant disease in humans. These can be further subdivided into those with “small” DNA genomes or “large” DNA genomes. DNA viruses with small DNA genomes have genome sizes of less than 10 kilobasepairs, whereas DNA viruses with large genomes are over 30 kilobasepairs. Small DNA viruses generally have less than ten genes encoded within the viral genome, whereas large DNA viruses can have anywhere from fifty genes to well over one hundred genes. Viruses with small DNA genomes include hu­man papillomavirus (HPV). HPV infects epithelial cells of the skin. It causes common warts on hands and feet and in some cases is important for the de­velopment of cervical cancer in women. Hepatitis B is another small DNA virus that infects the liver, causes hepatitis, and is associated with liver can­cer. Adenovirus, herpesvirus, and poxvirus are all examples of large DNA viruses that infect humans. Adenoviruses, of which there are many types, cause gastroenteritis and respiratory disease in humans.

A scanning electronic micrograph of herpes simplex virus (HSV6).

Herpesviruses are a very diverse family of viruses. There are a total of eight herpesviruses that infect humans and establish latent infection. Her- site, HSV-1 and HSV-2 then establish a latent (“resting”) infection in sensory neurons, which may erupt in times of stress.

Other herpes viruses that infect humans include Epstein-Barr virus, which causes mononucleosis and is important in a variety of human cancers, and varicella-zoster virus, which causes chickenpox in children and shingles in adults. The final large DNA virus that can infect humans is smallpox. Prior to vaccination and eradication of smallpox in 1970s, smallpox caused signifi­cant morbidity in human populations with anywhere from 1 to 25 percent of the cases resulting in death.

References

Flint, S. Jane, et al., eds. Principles of Virology: Molecular Biology, Pathogenesis, and Con­trol. Washington, DC: ASM Press, 2000.

Knipe, David M., and Peter M. Howley, eds. Fields’ Virology, 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2001.