- Discuss the basics of virus structure
Viruses are acellular, meaning they are biological entities that do not have a cellular structure. They therefore lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane. Viruses are sometimes called virions: a virion is a ‘complete’ virus free in the environment (not in a host). A virion consists of at least a nucleic acid core and an outer protein coating or capsid; sometimes a virus will have an outer envelope made of protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins, such as enzymes. The most obvious difference between members of viral families is their morphology, which is quite diverse. An interesting feature of viral complexity is that the complexity of the host does not correlate with the complexity of the virion. Some of the most complex virion structures are observed in bacteriophages, viruses that infect the simplest living organisms, bacteria.
Types of Nucleic Acid
Unlike nearly all living organisms that use DNA as their genetic material, viruses may use either DNA or RNA as theirs. The virus core contains the genome or total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot get from the host cell. This genetic material may be single- or double-stranded. It may also be linear or circular.
DNA viruses cause human diseases, such as chickenpox, hepatitis B, and some venereal diseases, like herpes and genital warts. Human diseases caused by RNA viruses include hepatitis C, measles, and rabies.
Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family. All virions have a nucleic acid genome covered by a protective layer of proteins, called a capsid. The capsid is made up of protein subunits called capsomeres. Some viral capsids are simple polyhedral “spheres,” whereas others are quite complex in structure.
Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors (Figure 1).
Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.
Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification (Figure 2).
Which of the following statements about virus structure is true?
- All viruses are encased in a viral membrane.
- The capsomere is made up of small protein subunits called capsids.
- DNA is the genetic material in all viruses.
- Glycoproteins help the virus attach to the host cell.