Download Chapter 6- An Introduction to Viruses

BIOL 2320
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J.L. Marshall, Ph.D.
Chapter 6- An Introduction to Viruses*
*Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for
the exams.
6.1 Overview of Viruses
Early Searches for the Tiniest Microbes
Viruses are smaller than the cells they infect. The existence of viruses was postulated by Louis Pasteur. In the
late 1890’s Ivanoski and Beijerinck showed the existence of the tobacco mosaic virus, along with Loeffler and
Frosch’s experiment with the virus that causes foot-and-mouth disease in cattle.
Years of experiments after these initial discoveries showed that viruses have unique sizes, shapes, and
chemical structures. Virology is now a sub-discipline of Microbiology. See 6.1 Making Connections
The Position of Viruses in the Biological Spectrum
Viruses can infect all types of cells in each Domain. The early history of viruses is unknown, however, it is
widely accepted that viruses have been in existence for billions of years. It is estimated that there are more
viruses on Earth than even bacterial cells. Viruses have had an important impact on the evolution of the cells
in the three (3) Domains.
Viruses are described as obligate intracellular parasites that use a specific host cell to replicate. As a result,
viruses cause serious damage and disease to the cells they infect. See Table 6.1. There are conflicting theories
on what viruses are and are not, but suffice it to say, viruses can cause disease, so they must be dealt with in
terms of human health.
6.2 The General Structure of Viruses
Size Range
Viruses are ultramicroscopic , you need an electron microscope to see them. Negative staining techniques are
used to visualize their external structure and positive staining techniques can visualize proteins and nucleic
acids. See Monster Viruses.
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Viral Components: Capsids, Nucleic Acids, and Envelopes
Viruses lack many of the components seen in cells, even the simplest cell. Viruses contain a nucleic acid core,
either DNA or RNA, surrounded by a protein coat. Some viruses have an additional outer layer called an
envelope. Also, some viruses carry enzymes in their internal structure that assists them during their infectious
process. A nucleocapsid virus is composed of a nucleic acid core and a surrounding protein capsid. These
viruses are also called naked viruses (figure 6.4a). Many animal viruses have an envelope, and are called
enveloped viruses (figure 6.4b).
The Viral Capsid: The Protective Outer Shell
The capsid is a prominent geometric feature. The capsid is comprised of smaller units called capsomers. The
capsomers can form the shapes helical (rod shaped, figure 6.5 and 6.6), and icosahedral (figure 6.7).
The Viral Envelope
Animal viruses are mostly enveloped viruses, and when they release, they take some of the host cell
membrane with them, with the addition of viral membrane proteins. Spikes are extensions from the surface of
the virus envelope, and are used to attach to the host cell.
Functions of the Viral Capsid/Envelope
-
Protect the nucleic acid core.
Assist penetration of the host cells
Stimulate an immune response
Complex Viruses: Atypical Viruses
The poxviruses are covered by lipoproteins and coarse fibrils and the bacteriophages have polyhedral head
and tail fibers (figure 6.9).
Nucleic Acids: At the Core of a Virus
The genome of organisms is expressed through nucleic acids, DNA, RNA. Viruses contain either DNA or RNA,
but not both in the viral particle. The structure of the nucleic acid in a virus can vary, ds, ss, segmented,
circular. RNA viruses that are immediately translated upon infection of the host cell is called a positive sense
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RNA virus. An RNA virus that must convert its RNA to a translatable form is called a negative sense RNA virus.
Viruses carry just the amount of nucleic acid they need to be infectious in a host cell.
Other Substances in the Viral Particle
Viruses can carry with them specific enzymes they need to be infectious in a host cell. Enzymes such as
polymerases, replicases, and reverse transcriptase in HIV.
6.3 How Viruses Are Classified and Named
Viruses are not classified in a Kingdom and/or Domain systems. Viruses are grouped into orders, families, and
genera. The common English name is widely used to designate a virus. See Table 6.2.
6.4 Modes of Viral Multiplication
Viruses will completely take-over a host cell once it enters. The virus will use the machinery of the host cell to
make more viral particles.
Multiplication Cycles in animal Viruses
Viruses are intracellular obligate parasites unable to replicate outside of a host cell. Fully formed viruses
capable of infecting new host cells are referred to as virions. Once the virion nucleic acid has penetrated the
host cell, the replication cycle can proceed via two pathways: the lytic cycle or the lysogenic cycle.
Figure 6.11,illustrates the one thing fundamental for all viral pathogens: They must enter a living cell before
they can multiply. Viruses have different methods for replicating (once inside a host cell) depending on what
type of nucleic acid they carry and whether or not they will be enveloped.
1. Adsorption and Host Range- Viruses adhere to host cell membranes via recognition between spikes and
receptors of the host cell (fig. 6.12). The range of host cells that viruses can infect is called host range. This
limits the cells viruses can infect. Viruses can also have tropisms, a preference for a cell type.
2. Penetration/Uncoating of Animal Viruses - The entire virus is taken into the animal cell in most cases. With
enveloped viruses, the envelope fuses with the cell membrane and the genome and capsid enter the cell. For
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naked viruses, the virion is endocytosed, then within vesicles (or in the cytoplasm) the capsid breaks down and
the nucleic acid is released (fig. 6.13).
3. Synthesis: Replication and Protein Production - RNA synthesis, protein synthesis, and genome replication
occur. Because viral genomes are ssDNA, dsDNA, ssRNA, or dsRNA, there is a wide variety of methods for
transcription, translation and replication.
4. Assembly of Animal Viruses: Host Cell as Factory - During this phase, newly synthesized viral capsids are
package with the viral DNA, and virion assembly is completed.
5. Release of Mature Viruses - Many animal viruses become enveloped as they escape from the cell (fig.
6.14). Nonenveloped and complex viruses exit the cell through cell lysis. Enveloped viruses can exit by
budding or exocytosis. A virion is a fully formed virus that is ready to infect a host cell.
Visible Damage to the Host Cell
The cytopathic effects (CPE) are defined as viral induced damages to the host cell. Damages such as: inclusion
bodies and cancer. See CLINICAL CONNECTIONS: PERSISTENT VIRAL INFECTIONS and Table 6.3.
6.5 The Multiplication Cycle in Bacteriophages
Viruses that infect bacteria are called bacteriophages(figure 6.17). The most widely studied bacteriophages
infect E. coli (figure 6.18). Bacteriophages go through similar steps of host cell infection like animal viruses. See
Table 6.4.
Lysogeny: The Silent Virus Infection
When a DNA virus is inside the host cell, it can incorporate its DNA into the host chromosome, and now the
virus is called a prophage. The condition in which the bacterial chromosome carries the phage DNA is called
lysogeny. The virus can remove itself from the host chromosome in a process called induction. Some phages
are responsible for bacteria being pathogenic. This is called lysogenic conversion, such as Vibrio cholera.
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Once inside the cell, the viral nucleic can take one of two paths:
(i) Lytic Phase (fig. 6.16)
In the lytic phase, viral DNA is replicated by using host DNA polymerases and other enzymes. As viral DNA
copies are being made, those DNA molecules are also being transcribed and translated (once again by using
the host enzymes).
At this point, the lytic stage proceeds directly to step 4: Assembly & Maturation
Unless . . . a lysogenic state is induced
(ii) Lysogenic State (fig. 6.16)
In this scenario, the viral DNA does not get replicated and transcribed immediately upon viral penetration of
the host cell. Rather, the viral DNA integrates into the host DNA and becomes “silent.” At this point the virus is
called a provirus. Every time the host cell divides, it copies its DNA, and so the viral DNA is copied as well, and
passed to each daughter cell. This can continue for many hundreds of generations. This is especially true of
DNA viruses and retroviruses (like HIV). Note that this incorporation of viral DNA into host cell DNA can have
severe ramifications including cancer in animals.
When the time is right (usually cellular stress or a change in environmental conditions), the provirus will
become active, the genes will be transcribed and translated and a lytic phase will begin.
6.6 Techniques in Cultivating and Identifying Animal Viruses
Methods for culturing viruses are in vitro methods, such as in tissue culture, or in vivo methods, such as
inoculating lab animals. The need for culturing viruses are: 1) isolate clinical samples, 2) prepare vaccines, and
3) research.
Using Cell (Tissue) Culture Techniques
Using isolated animal cells that are grown in sterile chambers called tissue culture flasks. Viruses can affect the
cells in the tissue culture by lysing them and/or forming plaques (figure 6.19).
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Using Bird Embryos
Bird embryos are suitable for culturing viruses because they are an already enclosed, sterile system. Bird
embryos are often used to make vaccines.
Using Live Animal Inoculation
Animals such as mice, rats, rabbits, and non-human primates can be used.
6.7 Viral Infection, Detection, and Treatment
Viruses are a common cause of acute infections. Not all viral infections result in death, but some do have a
high mortality rate such as HIV, Ebola, and rabies. It is essential to have the correct diagnosis for viral
infections. Rapid tests, immunofluorescence, PCR, and cell culture techniques can be used to detect viruses in
a patient sample.
6.8 Prions and Other Nonviral Infectious Particles
Prions are abnormal naked proteins and cause a disease called transmissible spongiform encephalopathies
(TSEs). Prions occur in several mammals, such as sheep and goats, cows and humans.
Satellite viruses are dependent on other viruses for their replication, and can increase the severity of the
primary viral infection.
Plants are susceptible to an altered RNA particle called viroids.
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