CHAPTER
13
Characterizing and Classifying
Viruses, Viroids, and Prions
Chapter Outline
Characteristics of Viruses (pp. 382–386)
Genetic Material of Viruses
Hosts of Viruses
Sizes of Viruses
Classification of Viruses (pp. 387–388)
Viral Replication (pp. 388–397)
Lytic Replication of Bacteriophages
Lysogenic Replication of Bacteriophages
Replication of Animal Viruses
The Role of Viruses in Cancer (pp. 397–399)
Culturing Viruses in the Laboratory (pp. 399–401)
Culturing Viruses in Mature Organisms
Culturing Viruses in Embryonated Chicken Eggs
Culturing Viruses in Cell (Tissue) Culture
Are Viruses Alive? (p. 3401)
Other Parasitic Particles: Viroids and Prions (pp. 401–404)
Chapter Summary
Viruses, viroids, and prions are acellular (noncellular) disease-causing agents that lack cell
structure and cannot metabolize, grow, reproduce, or respond to their environment. To increase
their numbers, they must utilize the cell’s metabolic chemicals and ribosomes.
Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions
Characteristics of Viruses (pp. 382–387)
A virus is a minuscule, acellular, infectious agent having one to several pieces of nucleic acid—
either DNA or RNA. Viruses have no cytoplasmic membrane and lack organelles and cytosol.
Genetic Material of Viruses
The genomes of viruses are either DNA or RNA, in a variety of forms. Viruses are classified on
the basis of the genetic material of their genome. They may be double-stranded (ds) or single-
Hosts of Viruses
Most viruses of multicellular organisms infect only particular kinds of cells in a host. This speci-
ficity is due to the affinity of viral surface proteins or glycoproteins for complementary proteins
Sizes of Viruses
Viruses are so small that most cannot be seen by light microscopy. The smallest have a diameter
of 17 nm, whereas the largest are approximately 500 nm, about the size of the smallest bacterial
cell. The first virus isolated was the plant pathogen Tobacco Mosaic Virus (TMV). The inven-
tion of electron microscopy allowed scientists to visualize viruses.
Capsid Morphology
The capsid of a virus is composed of proteinaceous subunits called capsomeres (capsomers).
Viral Shapes
Virion shapes are also used in classification. There are three basic types of viral shapes. Helical
viruses have capsomeres that encircle the nucleic acid, forming a tube-like structure. Polyhedral
viruses are roughly spherical, with a shape similar to a geodesic dome; the most common type is
the icosahedron (20 faces). Complex viruses have capsids of many different shapes.
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
The Viral Envelope
Some viruses have a membrane similar in composition to a cell membrane surrounding their
capsids. This membrane is called an envelope, and thus a virus with a membrane is called an en-
Classification of Viruses (pp. 387–388)
Virologists classify viruses by their type of nucleic acid, presence of an envelope, shape, and
size. They have recognized viral family and genus names. Family names describe characteristics
or are based on the name of important members. At this time, specific epithets for viruses are
their common English designations written in italics, such as rabies virus.
Viral Replication (pp. 388–397)
Viruses cannot reproduce themselves because they have neither the genes for all enzymes nec-
essary for replication nor functional ribosomes for protein synthesis. Instead, they depend on a
specific host cell type for the organelles and enzymes to produce new virions.
Viral replication that results in lysis of the cell near the end of the cycle is termed lytic repli-
cation. The cycle consists of five stages:
2. During entry, the virion or its genome enters the host cell. In bacteriophages, only the nucleic
acid enters the cell.
4. During assembly, new virions are spontaneously assembled in the host cell, typically as cap-
someres surround replicated or transcribed nucleic acids to form new virions.
5. During release, new virions are released from the host cell which lyses.
Lytic Replication of Bacteriophages
Bacteriophages are important tools in the study of viruses. Bacteriophage T4, which infects E.
coli, is one of the most extensively studied.
2. Entry. Lysozyme is released from the capsid to weaken the cell wall, and contraction of the
3. Synthesis. Viral enzymes degrade the host DNA, whereupon the cell synthesizes viral pro-
teins specified by the viral genome. The viral genome is replicated.
Lysogenic Replication of Bacteriophages
Not all viruses follow the lytic pattern. Some bacteriophages have a modified replication cycle
in which infected host cells grow and reproduce normally for many generations before they lyse.
Such a replication cycle is called a lysogenic replication cycle or lysogeny, and the phages in-
volved are called temperate phages or lysogenic phages. Another E. coli phage, lambda phage,
is a well-studied temperate phage. After entry into the host cell, the viral genome does not im-
Replication of Animal Viruses
Animal viruses have the same five basic replication pathways as bacteriophages, but some dif-
Attachment of Animal Viruses
Animal viruses lack both tails and tail fibers, and they typically attach via glycoprotein spikes or
other molecules on their capsids or envelopes.
Entry and Uncoating of Animal Viruses
There are three basic methods by which animal viruses enter cells. Some naked viruses enter
their host’s cells by direct penetration—a process in which a viral capsid attaches and sinks into
the cytoplasmic membrane. This creates a pore through which the viral genome alone enters the
Synthesis of DNA Viruses of Animals
Each type of animal virus requires a different strategy for synthesis that depends on the kind of
nucleic acid involved: DNA or RNA, and ds versus ss.
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
dsDNA. Synthesis of new dsDNA virions is similar to the normal replication of cellular DNA
and translation of proteins. Each strand of viral DNA is used as a template for its comple-
Synthesis of RNA Viruses of Animals
Positive sense ssRNA. Some ssRNA viruses have positive strand RNA (+ssRNA), which
can be directly translated by ribosomes to synthesize protein. From the +RNA, complemen-
tary negative strand RNA (–ssRNA) is also transcribed to serve as a template for more
+ssRNA. Poliovirus is a +ssRNA.
Assembly and Release of Animal Viruses
Though there are several notable exceptions, most DNA viruses assemble in the nucleus while
most RNA viruses replicate and assemble in the cytoplasm. Enveloped animal viruses are often
Latency of Animal Viruses
Some animal viruses, including HIV, chickenpox and herpesviruses, may remain dormant in
cells in a process known as latency; the viruses involved are called latent viruses or provirus-
es. In the case of HIV, reverse transcriptase uses the RNA genome as a template to generate
Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions
The Role of Viruses in Cancer (pp. 397–399)
Neoplasia is uncontrolled cellular reproduction in a multicellular animal. A mass of neoplastic
cells, called a tumor, may be relatively harmless (benign) or invasive (malignant). Malignant
tumors are also called cancer. The term metastasis describes the spreading of malignant cells,
Culturing Viruses in the Laboratory (pp. 399–401)
Because viruses cannot metabolize or replicate by themselves, they cannot be grown in standard
broths or on agar plates. Instead, they must be cultured inside suitable host cells.
Culturing Viruses in Mature Organisms
Most of our knowledge of viral replication has been derived from research on bacteriophages,
which are relatively easy to culture because bacteria are easily grown and maintained. Phages
Culturing Viruses in Embryonated Chicken Eggs
Chicken eggs are a useful culture medium for viruses because they are inexpensive, are among
Culturing Viruses in Cell (Tissue) Culture
Viruses can also be grown in cell culture (sometimes called tissue culture), which consists of
cells isolated from an organism and grown on the surface of a medium or in broth. They are of
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
Are Viruses Alive? (p. 397)
The characteristics of life are growth, self-reproduction, responsiveness, and the ability to me-
tabolize. According to these criteria, viruses seem to lack the qualities of living things. For some
Other Parasitic Particles: Viroids and Prions (pp. 397–400)
Two molecular particles also infect cells: viroids and prions.
Characteristics of Viroids
Viroids are very small, circular pieces of RNA with no capsid that infect and cause disease in
plants. Their pathology appears to involve binding to complementary mRNA in the cell, causing
Characteristics of Prions
Prions are infectious protein particles that lack nucleic acids and replicate by converting similar
normal proteins into new prions. Mammalian brain cells produce a protein called PrP. When
folded into proper shape, cellular PrP (c-PrP) has a role in normal brain function. PrP can also