What Are Viruses?

Are Viruses Living or Nonliving?

Scientists have long sought to uncover the structure and function of viruses. Viruses are unique in that they have been classified as both living and nonliving at various points in the history of biology. Viruses are particles that are capable of causing a number of diseases including cancer. They not only infect humans and animals, but also plants, bacteria, and archaeans. What makes viruses so interesting? They are about 1,000 times smaller than bacteria and can be found in almost any environment. Viruses can not exist independently of other organisms as they must take over a living cell in order to reproduce.

Viruses: Structure

A virus particle, also known as a virion, is essentially a nucleic acid (DNA or RNA) enclosed in a protein shell or coat. Viruses are extremely small, approximately 20 - 400 nanometers in diameter. The largest virus, known as the Mimivirus, can measure up to 500 nanometers in diameter. By comparison, a human red blood cell is around 6,000 to 8,000 nanometers in diameter. In addition to varying sizes, viruses also have a variety of shapes. Similar to bacteria, some viruses have spherical or rod shapes. Other viruses are icosahedral (polyhedron with 20 faces) or helical shaped.

Viruses: Genetic Material

Viruses may have double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. The type of genetic material found in a particular virus depends on the nature and function of the specific virus. The genetic material is not typically exposed but covered by a protein coat known as a capsid. The viral genome can consist of a very small number of genes or up to hundreds of genes depending on the type of virus. Note that the genome is typically organized as a long molecule that is usually straight or circular.

Viruses: Replication

Viruses are not capable of replicating their genes by themselves. They must rely on a host cell for reproduction. In order for viral replication to occur, the virus must first infect a host cell. The virus injects its genetic material into the cell and uses the cell's organelles to replicate. Once a sufficient number of viruses have been replicated, the newly formed viruses lyse or break open the host cell and move on to infect other cells.

Next > Viral Capsids and Disease

Viral Capsids

The protein coat that envelopes viral genetic material is known as a capsid. A capsid is composed of protein subunits called capsomeres. Capsids can have several shapes: polyhedral, rod or complex. Capsids function to protect the viral genetic material from damage. In addition to the protein coat, some viruses have specialized structures. For example, the flu virus has a membrane-like envelope around its capsid. The envelope has both host cell and viral components and assists the virus in infecting its host. Capsid additions are also found in bacteriophages. For example, bacteriophages can have a protein "tail" attached to the capsid that is used to infect host bacteria.

Viral Diseases

Viruses cause a number of diseases in the organisms they infect. Human infections and diseases caused by viruses include Ebola fever, chicken pox, measles, influenza, HIV and herpes. Vaccines have been effective at preventing some types of viral infections, such as small pox, in humans. They work by helping the body to build an immune system response against specific viruses. Viral diseases that impact animals include rabies, foot-and-mouth disease, bird flu, and swine flu. Plant diseases include mosaic disease, ring spot, leaf curl, and leaf roll diseases. Viruses known as bacteriophages cause disease in bacteria and archaeans.