Scientists have long sought to uncover the structure and function of viruses. Viruses are unique -- they have been classified as both living and nonliving at various points in the history of biology. What makes them so interesting? Let's look at the structure of viruses.
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 15 - 25 nanometers in diameter.
Adenovirus - Images courtesy of Linda M. Stannard, University of Cape Town.
Viruses may have double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. In different viruses, which of the four is the "genetic material," depends on the nature and function of the specific virus. 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.
The protein coat that envelopes the genetic material is known as a capsid. It can have several shapes: polyhedral, rod or "complex." The protein subunits of the capsid are called capsomeres.
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.
Influenza virus Image courtesy of Linda M. Stannard, University of Cape Town.
Capsid additions are also found in bacteriophages. For example, bacteriophages can have a protein "tail" attached to the capsid that is used to infect the host bacteria.
Bacteriophage - Image copyright Dennis Kunkel.