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A virus is an infectious particle that displays characteristics of life and non-life. Viruses are different from plants, animals, and bacteria in their structure and function. They are not cells and can't replicate on their own. Viruses must rely on a host for energy production, reproduction, and survival. Although typically only 20-400 nanometers in diameter, viruses are the cause of many human diseases including influenza, chickenpox, and the common cold.01of 07
Some Viruses Cause Cancer.KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images
Certain types of cancers have been linked to cancer viruses. Burkitt's lymphoma, cervical cancer, liver cancer, T-cell leukemia, and Kaposi sarcoma are examples of cancers that have been associated with different types of viral infections. The majority of viral infections, however, do not cause cancer.02of 07
Some Viruses Are Naked
All viruses have a protein coating or capsid, but some viruses, such as the flu virus, have an additional membrane called an envelope. Viruses without this extra membrane are called naked viruses. The presence or absence of an envelope is an important determining factor in how a virus interacts with the host's membrane, how it enters a host, and how it exits the host after maturation. Enveloped viruses can enter the host by fusion with the host membrane to release their genetic material into the cytoplasm, while naked viruses must enter a cell through endocytosis by the host cell. Enveloped viruses exit by budding or exocytosis by the host, but naked viruses must lyse (break open) the host cell to escape.
There Are 2 Classes of Viruses
Viruses can contain single-stranded or double-stranded DNA as the basis for their genetic material, and some even contain single-stranded or double-stranded RNA. Furthermore, some viruses have their genetic information organized as straight strands, while others have circular molecules. The type of genetic material contained in a virus not only determines which types of cells are viable hosts but also how the virus is replicated.
A Virus Can Remain Dormant in a Host for Years
Viruses undergo a life cycle with several phases. The virus first attaches to the host via specific proteins on the cell surface. These proteins are generally receptors that differ depending on the type of virus targeting the cell. Once attached, the virus then enters the cell by endocytosis or fusion. The host's mechanisms are used to replicate the DNA or RNA of the virus as well as essential proteins. After these new viruses mature, the host is lysed to allow the new viruses to repeat the cycle.
An additional phase before replication, known as the lysogenic or dormant phase, occurs in only a select number of viruses. During this phase, the virus can remain inside the host for extended periods of time without causing any apparent changes in the host cell. Once activated, however, these viruses can immediately enter the lytic phase in which replication, maturation, and release can occur. HIV, for example, can remain dormant for 10 years.
Viruses Infect Plant, Animal, and Bacterial cells
Viruses can infect bacterial and eukaryotic cells. The most commonly known eukaryotic viruses are animal viruses, but viruses can infect plants as well. These plant viruses usually need the assistance of insects or bacteria to penetrate a plant's cell wall. Once the plant is infected, the virus can cause several diseases which usually do not kill the plant but cause deformation in the plant's growth and development.
A virus that infects bacteria is known as a bacteriophages or phage. Bacteriophages follow the same life cycle as eukaryotic viruses and can cause diseases in bacteria as well as destroy them through lysis. In fact, these viruses replicate so efficiently that entire colonies of bacteria can be destroyed quickly. Bacteriophages have been used in diagnosis and treatments of infections from bacteria such as E. coli and Salmonella.06of 07
Some Viruses Use Human Proteins to Infect Cells
HIV and Ebola are examples of viruses that use human proteins to infect cells. The viral capsid contains both viral proteins and proteins from the cell membranes of human cells. The human proteins help to 'disguise' the virus from the immune system.07of 07
Retroviruses Are Used in Cloning and Gene Therapy
A retrovirus is a type of virus that contains RNA and that replicates its genome using an enzyme known as reverse transcriptase. This enzyme converts the viral RNA to DNA that can be integrated into the host DNA. The host then uses its own enzymes to translate the viral DNA into viral RNA used for viral replication. Retroviruses have the unique ability to insert genes into human chromosomes. These special viruses have been used as important tools in scientific discovery. Scientists have patterned many techniques after retroviruses including cloning, sequencing, and some gene therapy approaches.
- Coffin JM, Hughes SH, Varmus HE, editors. Retroviruses. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997. The Place of Retroviruses in Biology. Available from: //www.ncbi.nlm.nih.gov/books/NBK19382/
- Liao JB. Viruses and Human Cancer. The Yale Journal of Biology and Medicine. 2006;79(3-4):115-122.