Download Chapter 19 – Viruses

BIOL 1406
HCC-SW/Stafford Campus
J.L. Marshall, Ph.D
Chapter 19 – 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.
A Borrowed Life
Figure 19.1 shows Escherichia coli, a bacterium, being attached by a virus called T4 bacteriophage. This virus
infects the bacterial cell and causes the bacterium to make more viral particles. A virus is an infectious particle
that is composed of at least a nucleic acid center surrounded by a protein coat. Viruses are known to cause
disease in the cells they infect. Yet, viruses do not display the characteristics of life, so they are considered
non-living entities. But, viruses have been used in research to understand basic molecular biology.
Concept 19.1 : A virus consists of a nucleic acid surrounded by a protein coat
Viruses were discovered near the end of the 19th century.
The Discovery of Viruses: Scientific Inquiry
Research was done during the late 1800’s and early 1900’s on the tobacco plant that lead to the discovery of
viruses (Figure 19.2).
Structure of Viruses
Viruses are not cells. They are infectious particles consisting of a nucleic acid core and protein coat; some
viruses have a membranous envelope.
Viral Genomes The genomes of viruses can be dsDNA, ssDNA, dsRNA, and ssRNA. A virus is commonly
referred to as a DNA virus or RNA virus, based on its genome.
Capsids and Envelopes The protein shell surrounding the nucleic acid core is called the capsid. The
capsid gives a virus its overall shape. The protein units that comprise the capsid is called capsomeres
(Figure 19.3a and b). Some viruses contain viral envelopes, which are derived from the host cell
membrane (Figure 19.3c). Some viruses also have glycoprotein spikes. And, some viruses carry
accessory enzymes that aid in their infectious process.
Viruses that infect bacteria are called bacteriophages, or phages (Figure 19.3d).
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BIOL 1406
HCC-SW/Stafford Campus
J.L. Marshall, Ph.D
Concept 19.2 : Viruses replicate only in host cells
Viruses are obligate intracellular parasites as they are completely dependent on infecting a host cell to male
more viral particles. Each virus has a host range, a specific species the virus can infect. Viruses identify their
host by the host cell’s membrane proteins. In multicellular organisms like humans, viruses also identify specific
cells they will infect, like HIV and its specific white blood cell.
General Features of Viral Replicative Cycles
Once viruses enters cells (cross the cell membrane), the virus takes over the cell causing it to re-direct its
cellular efforts to making more viral particles (Figure 19.4). Viruses use the host cell to make the viral
components, then the virus is assembled and is released from the host cell. Often, when the virus exits the
host cell, it causes destruction of the host cell.
Replicative Cycles of Phages
Phages have two alternative replicative cycles: lytic and lysogenic.
The Lytic Cycle The phage cycle that causes death of the host cell is called the lytic cycle. When these
phages exit the cell, they do so by lysing the bacterial cell membrane and cell wall. A phage that only
replicates by a lytic cycle is called a virulent phage (Figure 19.5). Bacteria are not completely
susceptible to phages. Bacteria can mutate such that viruses no longer recognize their receptor
protein. And, bacteria have enzymes called restriction enzymes that will “cut” foreign nucleic acids,
such as viral nucleic acids.
The Lysogenic Cycle The lysogenic cycle of a phage infection is where the phage DNA is incorporated
into the host’s chromosome. This type of phage is called a temperate phage. The most well studied
temperate phage is phage  (lambda) that infects E. coli (Figure 19.6). Lambda phage can go through a
lytic cyle, or incorporate its DNA into E. coli’s DNA, so that now the viral DNA is known as a prophage.
As long as the environment is positive for  it will remain part of the E. coli chromosome.
Replicative Cycles of Animal Viruses
There are a number of different viruses that cause disease in humans. Viruses are classified based on the type
of nucleic acid they have, and whether it is single stranded or double stranded (Table 19.1).
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BIOL 1406
HCC-SW/Stafford Campus
J.L. Marshall, Ph.D
Viral Envelopes Animal viruses with an envelope use it to enter its host’s cell. These viruses use their
glycoproteins to gain entry (Figure 19.7). When some viruses are ready to exit, they do so by a process
similar to exocytosis. RNA viruses make use of the ribosomes in the host cell cytoplasm to begin the
viruses synthesis process. DNA viruses make use of the host cell nucleus to maintain their viral DNA.
RNA as Viral Genetic Material Some RNA viruses are “seen” by the host cell as mRNA that needs to be
translated. Some RNA viruses must make a copy of their RNA before it can being the translation
process. Yet, there are some RNA viruses that go through a DNA intermediate. These viruses are called
retroviruses and carry with them an enzyme that is significant in their infectious process called reverse
transcriptase. Their replication process is RNA --> DNA (Figure 19.8). An example of this type of virus is
the human immunodeficiency virus (HIV) that is known to cause acquired immunodeficiency
syndrome (AIDS).
Concept 19.3 : Viruses, viroids, and prions are formidable pathogens in animals and plants
Complex entities that are smaller than viruses can cause infections, such as viroids and prions, in plants and
animals.
Viral Diseases in Animals
Viruses can cause a wide array of symptoms from the common cold, to polio. Some viral infections are
temporary, and some are permanent.
A vaccine is a harmless variant or a defective pathogen that is used to stimulate the immune system to mount
protection against the more virulent form of the virus. Viral infections can not be cured by antibiotics;
antibiotics are only effective against bacteria. But, there are antiviral medicines that work to inhibit viral
replication steps.
Emerging Viruses
Emerging viruses suddenly become apparent. These are viruses that seem to come from nowhere, like HIV.
Viral diseases that occur in high frequency in a population (higher than normal) cause an epidemic, like the
H1N1 flu virus of 2009 (Figure 19.9a). If the viral disease spreads world wide, then it becomes a pandemic
(Figure 19.9b).
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BIOL 1406
HCC-SW/Stafford Campus
J.L. Marshall, Ph.D
Viral Diseases in Plants
Viral diseases in plants affect agriculture by causing brown spots, stunted growth, and damage to flowers and
roots(page 405). Plants acquire the virus by horizontal transmission, where it acquires the virus from the
environment, usually as a result of tissue destruction of the plant. Plants can also acquire the viral infection by
vertical transmission, where they inherit the viral infection from a parent.
Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA particles that infect plants. Viroids tend to stunt plant growth.
Infectious proteins are called prions, which tend to cause degenerative brain disease in various animals. In
cows its called mad-cow disease. In humans its called Creutzfeldt-Jakob disease. Prions are most likely
transmitted in food. Prions have a slow incubation time, and they are indestructible (Figure 19.10). Today,
there is no known cure.
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