Viruses are everywhere -- and abundant. Viral infections can pose a mild risk to our health, like the common cold, or a threat to our lives, like an HIV infection. Viruses can be grouped according to their genetic material: DNA or RNA. Both types can infect host organisms and cause disease. However, the ways that DNA and RNA viruses infect host cells and take over the cell’s biochemical machinery are different.
Viruses are small, nonliving parasites, which cannot replicate outside of a host cell. A virus consists of genetic information -- either DNA or RNA -- coated by a protein. A virus injects its genetic information into a host cell and then takes control of the cell’s machinery. This process enables the virus to make copies of its DNA or RNA and make the viral proteins inside the host cell. A virus can quickly make multiple copies of itself in one cell, release these copies to infect new host cells and make even more copies. In this way, a virus can replicate very quickly inside a host.
As their name implies, DNA viruses use DNA as their genetic material. Some common examples of DNA viruses are parvovirus, papillomavirus, and herpesvirus. DNA viruses can affect both humans and animals and can range from causing benign symptoms to posing a very serious health risk.
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DNA viruses enter a host cell,usually when the membrane of the virus fuses with the cell’s membrane. The contents of the virus enter the cell, travel to the nucleus and take over the cell’s biochemical machinery for DNA replication and transcription into RNA. The RNA controls the formation of proteins needed by the virus to coat the viral DNA. This coating of viral DNA is known as a capsid. The capsids accumulate inside the cell until the cell reaches capacity and bursts open, releasing the newlyformed viruses to infect new host cells.
RNA viruses, also known as retroviruses, have RNA as their genetic material. Some examples of retroviruses are hepatitis viruses and HIV. When these viruses enter a host cell, they must first convert their RNA into DNA. This process, called reverse transcription, enables the virus to inject its genetic material into the host cell and use the host's biochemical machinery, similar to a DNA virus.
Often, retroviruses use an enzyme, called integrase, to insert the retroviral DNA into the genome of the host cell. The ability of retroviruses to integrate this DNA into the host cell’s DNA increases the chances of causing cancer or other diseases. For example, if the retroviral DNA is inserted into the middle of one of the host cell’s genes, that gene may no longer be functional, leading to disease.
Vaccines are available for many of the more common DNA viruses. These vaccines work by injecting the patient with an inactive form of the virus, usually the protein coat without the DNA. In the absence of the DNA, there is not any genetic material to copy, and the virus cannot replicate. However, exposing patients to viral proteins makes it more likely that their immune systems will recognize the virus as foreign and destroy it before it has a chance to infect host cells.
Retroviruses, which use the host's biochemical system to reproduce, are more difficult to treat. Treatment for these viruses typically involves treatment with a drug that inhibits the activity of reverse transcriptase, the enzyme that converts retroviral RNA into DNA. Often, patients with retroviral infections such as HIV take a cocktail of many different types of drugs, each of which targets a different step in the viral life cycle.