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What Are Viruses?

Characteristics of Viruses

Viruses are unique and nonliving infectious particles that invade cells. The viral genome is surrounded and protected by a protein coat called a capsid.
A virus is an infectious agent consisting of a nucleic acid strand within a protein coat. It is a unique and nonliving particle that replicates only in cells of other organisms. Most viruses are smaller than prokaryotic cells, although they range in size and shape. Each virus is essentially a deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) genome surrounded by a protein coat. A genome is the genetic material of an organism. Viruses invade the host cell and use the host cell's system and energy to replicate. Viruses also have an extracellular form, which functions independently of a host and is called the virion or viral particle. The virion moves a virus from one host to another.

Sizes of Viruses

Viruses are small, and viroids, and prions are very small in comparison to cells.
The nucleic acid that makes up the virus genome—DNA or RNA—can be single-stranded or double-stranded, as well as linear or circular. Both of these characteristics affect the viral replication process. A viral capsid is the protein coat surrounding and protecting the viral genome. The viral genome directs the synthesis of capsid proteins and some enzymes necessary for viral penetration in some viruses. The capsid often includes multiple proteins, with molecules arranged around the DNA or RNA into capsomeres. A capsomere is the collection or assembly of protein molecules making up a viral capsid. Many capsomeres can make up a single virion. The entire complex of capsid and nucleic acid is the nucleocapsid. Some viruses are "naked," with no layers around the nucleocapsid, whereas others have a viral envelope. The viral envelope is a structure that consists of lipid-containing layers that surrounds the nucleocapsid of a virus. Viruses with a viral envelope are described as "enveloped." Capsomeres are arranged into two types of symmetry, helical and icosahedral, that determine viral shape. Helical is the type of symmetry of capsomere arrangement associated with spiral-shaped viruses. Icosahedral is the type of symmetry of capsomere arrangement associated with spherical viruses. An icosahedron is a roughly spherical geometric structure with 20 triangular faces and the most efficient arrangement of capsomeres in a viral capsid. Some viruses, including large bacteriophages—viruses that infect bacteria—have a complex structure. Many bacteriophages, for instance, have icosahedral heads and helical tails.

Structure of Viruses

Viruses have a range of shapes and symmetries but share similar components: a capsid composed of capsomeres surrounding nucleic acid.

Classification of Viruses

Because of the multitude of unique properties that different viruses contain, classification of viruses is complex. With no kingdom, viruses are classified according to seven orders and over 100 families.
Because viruses are unique and highly diverse, their classification is complex. Viruses are not grouped into a kingdom but are typically classified into ten orders and 134 families. A species is a unique member of a monophyletic group (descended from the same ancestor) that shares distinctive properties different from other species. Viruses are classified primarily by which nucleic acid they carry and by the nucleic acid strandedness (single or double). Genomes of most double-stranded (ds) RNA viruses are segmented, whereas dsDNA viruses typically consist of one large molecule. Single-stranded (ss) DNA viruses tend to be small. RNA viruses are also subdivided by the different roles RNA plays in transcription: directly as mRNA, as a template for mRNA synthesis, and as a template for DNA synthesis. ssDNA viruses and some ssRNA viruses replicate through an intermediary nucleic acid, which acts as a template for generating new viral genomes. These include a unique group of RNA viruses, called retroviruses, that use reverse transcriptase, an enzyme that uses RNA as a template to make a DNA copy.

Viral Classification by Genetic Material Present

The viral mRNA produced in a host cell depends on the viruses genetic material, particularly whether it is DNA or RNA, single-stranded or double-stranded. Viral groups are classified by these differences.

Classifying Representative Viruses

Group Class/ Family Nucleic Acid Type Nucleic Acid Strandedness Role of RNA
I Herpesviridae DNA Double
Papovaviridae DNA Double
II Geminiviridae DNA Single
Inoviridae DNA Single
III Reoviridae RNA Double
IV Flaviviridae RNA Single mRNA
Picornaviridae RNA Single mRNA
V Filoviridae RNA Single Template, mRNA synthesis
Orthomyxoviridae RNA Single Template, mRNA synthesis
VI Retroviridae RNA-RT (DNA intermediate) Single Template, DNA synthesis
VII Hepadnaviridae DNA-RT (RNA intermediate) Double

Viruses are classified by the type of nucleic acid in their genome, the role of RNA in those with RNA genomes, and the presence or absence of reverse transcriptase.

Viruses can be categorized based on additional properties including symmetry, presence of an envelope, tissue tropism, and host range. A host range is the range of cells—the types of organisms—that a particular virus can infect. Many viruses are bacteriophages, which specifically infect bacteria. Others infect animals, plants, archaea, and other groups of organisms. The tissue tropism of a virus, or other pathogen, describes the cell and tissue types that it typically infects. Some viruses are able to infect many cell types within a multicellular host, while others only infect one cell type. For example, herpes simplex viruses can infect skin, mucosal, and neural cells. In contrast, rhinoviruses infect only the epithelial cells of the respiratory tract.

Characteristics of Viral Groups

Group Class/ Family Example and/or Disease Caused Symmetry Viral Envelope Present? Host Range
I Herpesviridae Herpes simplex; varicella-zoster (chicken pox, shingles) Icosahedral Yes Animals: humans, cows, apes, birds, reptiles
Papovaviridae Papillomavirus (cervical cancer, warts), tumors Icosahedral No Animals, mostly mammals
II Geminiviridae Yellow mosaic, stunting Icosahedral No Plants
Inoviridae M13 Helical No Bacteria
III Reoviridae Rotavirus (diarrhea), rice ragged stunt virus Icosahedral No Animals (humans, fish arthropods, etc.), plants, fungi, protists
IV Flaviviridae Yellow fever, West Nile, hepatitis C Icosahedral, helical Yes Animals: humans, other mammals, birds
Picornaviridae Rhinovirus (common cold), hepatitis A, poliovirus Icosahedral No Animals: humans, most vertebrate classes
V Filoviridae Ebola Helical Yes Animals: humans, apes, other mammals
Orthomyxoviridae Influenza Helical Yes Animals: humans, other mammals
VI Retroviridae HIV/AIDS, leukemia Helical Yes Animals: vertebrates
VII Hepadnaviridae Hepatitis B Icosahedral Yes Animals: humans, birds, apes

Viruses within groups typically share symmetry, envelope characteristics, and host range.