7.2: Viral Structures

Learning Objectives

• Describe the structural components of viruses and explain how they differ from prokaryotic and eukaryotic cells
• Compare and contrast DNA and RNA viruses in terms of replication strategies
• Describe how viruses are classified

Viral Structures

In general, virions (viral particles) are small and cannot be observed using a regular light microscope. They are much smaller than prokaryotic and eukaryotic cells; this is an adaptation allowing viruses to infect these larger cells (see Figure \(\PageIndex{2}\)). The size of a virion can range from 20 nm for small viruses up to 900 nm for typical, large viruses (see Figure \(\PageIndex{3}\)). Recent discoveries, however, have identified new giant viral species, such as Pandoravirus salinus and Pithovirus sibericum, with sizes approaching that of a bacterial cell.⁶

Figure \(\PageIndex{2}\): (a) In this transmission electron micrograph, a bacteriophage (a virus that infects bacteria) is dwarfed by the bacterial cell it infects. (b) An illustration of the bacteriophage in the micrograph. (credit a: modification of work by U.S. Department of Energy, Office of Science, LBL, PBD)

In 1935, after the development of the electron microscope, Wendell Stanley was the first scientist to crystallize the structure of the tobacco mosaic virus and discovered that it is composed of RNA and protein. In 1943, he isolated Influenza B virus, which contributed to the development of an influenza (flu) vaccine. Stanley’s discoveries unlocked the mystery of the nature of viruses that had been puzzling scientists for over 40 years and his contributions to the field of virology led to him being awarded the Nobel Prize in 1946.

Figure \(\PageIndex{3}\):The size of a virus is small relative to the size of most bacterial and eukaryotic cells and their organelles.

Query \(\PageIndex{1}\)

 

As a result of continuing research into the nature of viruses, we now know they consist of a nucleic acid (either RNA or DNA, but never both) surrounded by a protein coat called a capsid (see Figure \(\PageIndex{4}\)). The interior of the capsid is not filled with cytosol, as in a cell, but instead it contains the bare necessities in terms of genome and enzymes needed to direct the synthesis of new virions. Each capsid is composed of protein subunits called capsomeres made of one or more different types of capsomere proteins that interlock to form the closely packed capsid.

There are two categories of viruses based on general composition. Viruses formed from only a nucleic acid and capsid are called naked viruses or nonenveloped viruses. Viruses formed with a nucleic-acid packed capsid surrounded by a lipid layer are called enveloped viruses (see Figure \(\PageIndex{4}\)). The viral envelope is a small portion of phospholipid membrane obtained as the virion buds from a host cell. The viral envelope may either be intracellular or cytoplasmic in origin.

Extending outward and away from the capsid on some naked viruses and enveloped viruses are protein structures called spikes. At the tips of these spikes are structures that allow the virus to attach and enter a cell, like the influenza virus hemagglutinin spikes (H) or enzymes like the neuraminidase (N) influenza virus spikes that allow the virus to detach from the cell surface during release of new virions. Influenza viruses are often identified by their H and N spikes. For example, H1N1 influenza viruses were responsible for the pandemics in 1918 and 2009,⁷ H2N2 for the pandemic in 1957, and H3N2 for the pandemic in 1968.

Figure \(\PageIndex{4}\): (a) The naked atadenovirus uses spikes made of glycoproteins from its capsid to bind to host cells. (b) The enveloped human immunodeficiency virus uses spikes made of glycoproteins embedded in its envelope to bind to host cells (credit a “micrograph”: modification of work by NIAID; credit b “micrograph”: modification of work by Centers for Disease Control and Prevention)

Viruses vary in the shape of their capsids, which can be either helical, polyhedral, or complex. A helical capsid forms the shape of tobacco mosaic virus (TMV), a naked helical virus, and Ebola virus, an enveloped helical virus. The capsid is cylindrical or rod shaped, with the genome fitting just inside the length of the capsid. Polyhedral capsids form the shapes of poliovirus and rhinovirus, and consist of a nucleic acid surrounded by a polyhedral (many-sided) capsid in the form of an icosahedron. An icosahedral capsid is a three-dimensional, 20-sided structure with 12 vertices. These capsids somewhat resemble a soccer ball. Both helical and polyhedral viruses can have envelopes. Viral shapes seen in certain types of bacteriophages, such as T4 phage, and poxviruses, like vaccinia virus, may have features of both polyhedral and helical viruses so they are described as a complex viral shape (see Figure \(\PageIndex{5}\)). In the bacteriophage complex form, the genome is located within the polyhedral head and the sheath connects the head to the tail fibers and tail pins that help the virus attach to receptors on the host cell’s surface. Poxviruses that have complex shapes are often brick shaped, with intricate surface characteristics not seen in the other categories of capsid.

Figure \(\PageIndex{5}\): Viral capsids can be (a) helical, (b) polyhedral, or (c) have a complex shape. (credit a “micrograph”: modification of work by USDA ARS; credit b “micrograph”: modification of work by U.S. Department of Energy)

Query \(\PageIndex{1}\)

Query \(\PageIndex{1}\)

Classification and Taxonomy of Viruses

Although viruses are not classified in the three domains of life, their numbers are great enough to require classification. Since 1971, the International Union of Microbiological Societies Virology Division has given the task of developing, refining, and maintaining a universal virus taxonomy to the International Committee on Taxonomy of Viruses (ICTV). Since viruses can mutate so quickly, it can be difficult to classify them into a genus and a species epithet using the binomial nomenclature system. Thus, the ICTV’s viral nomenclature system classifies viruses into families and genera based on viral genetics, chemistry, morphology, and mechanism of multiplication. To date, the ICTV has classified known viruses in seven orders, 96 families, and 350 genera. Viral family names end in -viridae (e.g, Parvoviridae) and genus names end in −virus (e.g., Parvovirus). The names of viral orders, families, and genera are all italicized. When referring to a viral species, we often use a genus and species epithet such as Pandoravirus dulcis or Pandoravirus salinus.

The Baltimore classification system is an alternative to ICTV nomenclature. The Baltimore system classifies viruses according to their genomes (DNA or RNA, single versus double stranded, and mode of replication). This system thus creates seven groups of viruses that have common genetics and biology.

Link to Learning

Explore the latest virus taxonomy at the ICTV website.

Aside from formal systems of nomenclature, viruses are often informally grouped into categories based on chemistry, morphology, or other characteristics they share in common. Categories may include naked or enveloped structure, single-stranded (ss) or double-stranded (ds) DNA or ss or ds RNA genomes, segmented or nonsegmented genomes, and positive-strand (+) or negative-strand (−) RNA. For example, herpes viruses can be classified as a dsDNA enveloped virus; human immunodeficiency virus (HIV) is a +ssRNA enveloped virus, and tobacco mosaic virus is a +ssRNA virus. Other characteristics such as host specificity, tissue specificity, capsid shape, and special genes or enzymes may also be used to describe groups of similar viruses. Table \(\PageIndex{2}\) lists some of the most common viruses that are human pathogens by genome type.

Table \(\PageIndex{2}\): Common Pathogenic Viruses

Genome	Family	Example Virus	Clinical Features
dsDNA, enveloped	Poxviridae	Orthopoxvirus	Skin papules, pustules, lesions
	Poxviridae	Parapoxvirus	Skin lesions
	Herpesviridae	Simplexvirus	Cold sores, genital herpes, sexually transmitted disease
dsDNA, naked	Adenoviridae	Atadenovirus	Respiratory infection (common cold)
	Papillomaviridae	Papillomavirus	Genital warts, cervical, vulvar, or vaginal cancer
	Reoviridae	Reovirus	Gastroenteritis severe diarrhea (stomach flu)
ssDNA, naked	Parvoviridae	Adeno-associated dependoparvovirus A	Respiratory tract infection
	Parvoviridae	Adeno-associated dependoparvovirus B	Respiratory tract infection
dsRNA, naked	Reoviridae	Rotavirus	Gastroenteritis
+ssRNA, naked	Picornaviridae	Enterovirus C	Poliomyelitis
	Picornaviridae	Rhinovirus	Upper respiratory tract infection (common cold)
	Picornaviridae	Hepatovirus	Hepatitis
+ssRNA, enveloped	Togaviridae	Alphavirus	Encephalitis, hemorrhagic fever
	Togaviridae	Rubivirus	Rubella
	Retroviridae	Lentivirus	Acquired immune deficiency syndrome (AIDS)
−ssRNA, enveloped	Filoviridae	Zaire Ebolavirus	Hemorrhagic fever
	Orthomyxoviridae	Influenzavirus A, B, C	Flu
	Rhabdoviridae	Lyssavirus	Rabies

Classification of Viral Diseases

While the ICTV has been tasked with the biological classification of viruses, it has also played an important role in the classification of diseases caused by viruses. To facilitate the tracking of virus-related human diseases, the ICTV has created classifications that link to the International Classification of Diseases (ICD), the standard taxonomy of disease that is maintained and updated by the World Health Organization (WHO). The ICD assigns an alphanumeric code of up to six characters to every type of viral infection, as well as all other types of diseases, medical conditions, and causes of death. This ICD code is used in conjunction with two other coding systems (the Current Procedural Terminology, and the Healthcare Common Procedure Coding System) to categorize patient conditions for treatment and insurance reimbursement.

For example, when a patient seeks treatment for a viral infection, ICD codes are routinely used by clinicians to order laboratory tests and prescribe treatments specific to the virus suspected of causing the illness. This ICD code is then used by medical laboratories to identify tests that must be performed to confirm the diagnosis. The ICD code is used by the health-care management system to verify that all treatments and laboratory work performed are appropriate for the given virus. Medical coders use ICD codes to assign the proper code for procedures performed, and medical billers, in turn, use this information to process claims for reimbursement by insurance companies. Vital-records keepers use ICD codes to record cause of death on death certificates, and epidemiologists used ICD codes to calculate morbidity and mortality statistics.

Key Concepts and Summary

• Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid.
• Viruses are obligate intracellular parasites.
• Viruses are known to infect various types of cells found in plants, animals, fungi, protists, bacteria, and archaea. Viruses typically have limited host ranges and infect specific cell types.
• Viruses may have helical, polyhedral, or complex shapes.
• Classification of viruses is based on morphology, type of nucleic acid, host range, cell specificity, and enzymes carried within the virion.
• Like other diseases, viral diseases are classified using ICD codes.

Footnotes

1. 1 H. Lecoq. “[Discovery of the First Virus, the Tobacco Mosaic Virus: 1892 or 1898?].” Comptes Rendus de l’Academie des Sciences – Serie III – Sciences de la Vie 324, no. 10 (2001): 929–933.
2. 2 US Department of Health and Human Services, Centers for Disease Control and Prevention. “Antibiotic Resistance Threats in the United States, 2013.” www.cdc.gov/drugresistance/pd...s-2013-508.pdf (accessed September 22, 2015).
3. 3 M. Clokie et al. “Phages in Nature.” Bacteriophage 1, no. 1 (2011): 31–45.
4. 4 A. Sulakvelidze et al. “Bacteriophage Therapy.” Antimicrobial Agents and Chemotherapy 45, no. 3 (2001): 649–659.
5. 5 US Food and Drug Administration. “FDA Approval of Listeria-specific Bacteriophage Preparation on Ready-to-Eat (RTE) Meat and Poultry Products.” www.fda.gov/food/ingredientsp.../ucm083572.htm (accessed September 22, 2015).
6. 6 N. Philippe et al. “Pandoraviruses: Amoeba Viruses with Genomes up to 2.5 Mb Reaching that of Parasitic Eukaryotes.” Science 341, no. 6143 (2013): 281–286.
7. 7 J. Cohen. “What’s Old Is New: 1918 Virus Matches 2009 H1N1 Strain. Science 327, no. 5973 (2010): 1563–1564.