Viruses
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Explaim txhy viru*es are rc*t *lasslfled into amy klragd*rn,
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Describa *hanacteristics cf a representative virus.
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Explain tlre life cycle of a reBresentative virus.
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diseases that plague members from each
kingdom are caused by bacteria or viruses. You might
think, because of this, that bacteria and viruses are
roughly similar kinds of micro-organisms. Yet
bacteria are classified as living organisms, while
viruses are not.
What are viruses? You know that scientists
consider cells to be the basic units of life. Viruses
have no cellular structure. By this definition,
therefore, viruses are not organisms and they are
not classified in any kingdom of living things.
Viruses have no cytoplasm, organelles, or cell
membranes. They do not carry out respiration or
many other common life processes. Viruses consist of
Iittle more than strands of DNA or RNA surrounded
by a protective protein coat called a capsid shown
in Figure 4.19. In effect, viruses are mobile genes
that parasitize cells. The capsid protects the virus
from attack by the host cell enzymes, and it helps
the virus attach itself to specific receptors on the
host cell.
Classitying Viruses
Since viruses were first identified in 1935,
scientists have described more than 160 maior
groups. Members of different groups differ in their
size and shape as shown in Figure 4,20. The shape
is determined by the type and arrangement of
proteins in the capsid, Polyhedral viruses such as
the polio virus resemble small crystals and may
have as many as 20 sides. The HIV virus that
causes AIDS, has a spherical shape. The tobacco
mosaic virus has a cylindrical shape. The T4 virus
infects bacteria. It has a polyhedral head attached
to a protein tail and several tail fibres, Some groups
of viruses are able to replicate only in a particular
species, while others may be found, for example,
in both animals and plants, or in both plants and
fungi. Viruses are also grouped by the types of
diseases they cause, Viruses that infect humans are
currently classified into 21 groups. These groups
differ in their genomes (set of genes) and their
method of replication,
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Figure 4.19 HIV viruses budding from the surface of a host
T-lymphocyte white blood cell. The viruses are acquiring
their protein coat from the host cell's membrane.
122 r
MHR. Unit 2 Biodiversity
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Figure 4.2o Virus particles have a variety of shapes. The
viruses shown here, include (A) the polio virus, (B) the HIV
virus, (C) the tobacco mosaic virus, and (D) the T4 virus.
Viral Reproduction
One characteristic viruses do share with living
things is the ability to multiply. However, a virus
cannot do this on its own. It depends entirely on
the metabolism of a eukaryotic or prokaryotic cell
to replicate its DNA or RNA and to make protein
coats for each newly formed virus particle.
Before a virus can enter any cell, it must attach
to a specific receptor site on the cell membrane of
the host cell, The proteins on the surface of the
virus act as keys that fit exactly into a matching
shape on the host cell membrane. For example, a
protein in the tail fibres of the T4 virus shown in
Figure 4.20 recognizes and attaches the T4 to the
specific host ceII. In other viruses, the attachment
protein is in the capsid or in the envelope. This
attachment sequence, where the virus recognizes
and attaches to the host cell, is like two jigsaw
pieces fitting together. Because each virus has a
specifically shaped attachment protein, the virus
can only attach to a few specific types of cells. The
T4 virus mentioned above, for example, can only
VITUS
infect certain bacterial cells. It cannot attach
to a plant or animal cell. Some viruses are even
specialized to the type of cells within an organism.
The polio virus, for example, infects human nerve
and intestinal cells. This specificity is very
important for controlling the spread of viral
diseases. Therefore, each virus can only enter
particular cells with specific receptor sites. Outside
of their host ceII, viruses are completely inert.
Viruses can enter cells in two different ways. In
the first, once attached to the host celi, the virus
can inject its nucleic acid into the ceII. Figure 4.21
shows the steps to this cycle of viral replication,
called the lytic cycle. The lytic cycle is typical of
bacteriophages
- viruses that attack bacterial cells.
A typical lytic cycle takes about 30 min, and
may produce up to 200 new viruses. If a virus is
contained in an envelope, it may enter in a second
way. After the virus attaches, the membrane of the
host cell surrounds the virus. This creates a vacuole
inside in the host cell's cytoplasm that contains the
virus. When the virus breaks out of the vacuole, it
releases its nucleic acid into the cell.
bacterial DNA
nucleic
acid
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Attachment
bacterial
host cell
Figure 4.21 The lytic cycle occurs when a virus inserts its nucleic acid into a cell,
such as bacterium, and then uses the cell's metabolism to replicate its DNA or RNA
and make new viruses.
Chapter 4 Patterns of Life
. MHR * 123
ln the 1970s, the World Health 0rganization off icially
declared that it had rid the world of the smallpox virus. The
disease produced by this virus had been a major cause of
human deaths throughout history. The extermination of the
virus was possible because the virus rnfected only humans
and no other species.
Viruses and Disease
In the lytic cycle of a virus, newly formed viruses
burst from the host cell, usually killing it. In
multicellular hosts, these new viruses then infect
neighbouring cells, thereby causing damage to their
host. The amount of damage and its effects on the
host vary,
Human immunodeficiency virus (HIV) is an
example of a type of RNA virus called a retrovirus.
Retroviruses contain an enzyme called reverse
transcriptase. As you can see in Figure 4.22, this
enzyme causes the host cell to copy the viral RNA
into DNA. In this form, the viral genome can enter
the chromosomes of the host cell and be copied
when the cell divides. You wiII learn more about
RNA viruses at the end of this section.
In other cases, viruses can invade a cell but not
kill it. These viruses undergo a different type of
replication cycle in which their DNA becomes
integrated with the host cell chromosomes. Once
inserted into the host chromosomes, the viral DNA
is called a provirus. When the host cell divides
through the process of mitosis, it replicates the
retrovirus
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provirus along with its own DNA. Every descendant
of the host cell will carry a copy of the provirus in
its chromosomes. This process can continue for
years, with no harm to the host. As part of the host
chromosomes, the virus cannot be easily detected
by medicai tests. At any time, however, the provirus
can separate from the host chromosomes and
complete the more damaging lytic cycle shown
in Figure 4.21.
The replication strategies of viruses help explain
certain patterns of disease. For example, the herpes
simplex virus causes cold sores in people. These
sores may appear and disappear on the skin of an
infected person throughout her or his lifetime. The
sores appear when the viral cycle destroys cells, and
they disappear when the virus is in its provirus
stage. The exact trigger that causes the switch from
one phase to another is not known.
Other viruses follow variations of the replication
strategies already described. For example, HIV
forms a provirus in the host cell chromosomes, but
it also produces small numbers of new viruses
while the cell continues to function normally. This
explains why people may test positive for HIV but
remain healthy for many years. Only when the
infection spreads to more and more cells do the
symptoms of AIDS (Acquired Immune Deficiency
Syndrome) eventually appear. The symptoms result
from infections by other micro-organisms because
the HIV virus has destroyed the body's
T-lymphocytes, which help the immune
system fight off other diseases.
DNA is made from
the viral RNA
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Retrovirus cycle
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new virus
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124 r
MHR. Unit 2 Biodiversity
Figure 4.22 Reproductive
cycle of a retrovirus.
Retroviruses contain an
enzyme that causes the
host cell to copy viral
RNA into DNA. This DNA
becomes a provirus that
continues to produce new
viruses without destroying
the cell.
www. mcgrawhill.callinks/atlbiology
To view animations
that demonstrate the lytic cycle and the
retrovirus reproductive cycle, go to the web site above and
click on Electronic Learning Partner.
Viruses and Biotechnology
Because viruses enter host cells and direct the
activity of the host cell's DNA, they can be useful
tools for genetic engineers. For example, if
researchers want to clone a gene, they first splice
the gene into the genome of a virus. The virus then
enters a host cell and directs the cell to make
multiple copies of the virus. Each new virus in
each new cell contains the added gene that the
researchers wanted copied. This process is
illustrated in Figure 4.23.
ln 1997, a Nobel Prize was awarded to American researcher
Stanley Prusiner for his discovery of an entirely new type
of disease-causing agent called prions. The discovery was
remarkable for two reasons. First, prions are proteins that
are found normally in the body. Second, they are the only
disease-causing agents known not to have RNA or DNA.
Diseases result when prions convert from their normal
form into harmful particles that have the same chemical
composition but a different molecular shape. Prions cause
several deadly brai n diseases, i ncl udi n g Creutzfeldt-Jakob
disease (CJD) in humans, scrapie in sheep, and Bovine
Spongiform Encephalopathy (BSE) or "mad cow disease"
in cows.
recombinant DNA (DNA that contains
genes from more than one source)
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viral DNA
recombinant DNA
host cell
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foreign gene
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Figure 4.23 Genetic engineers use viruses to introduce new genes into a cell and
to clone copies of genes.
Viral Replication
Viruses are very successful at invading the cells of
organisms because they can only reproduce using the
metabolism of a host cell. ln this lab you will diagram the
steps involved in viral replication in cells. Your teacher will
give you 5 photocopies of a drawing showing a bacterial
cell. Put one of the following labels on each sheet:
Attachment; Penetration; Biosynthesis; Maturation; Release.
Using these headings as a guide, on each figure draw the
different stages in the process of viral replication, At the
bottom of each diagram, summarize each step in words.
Analyze
How do replication and protein synthesis occur in a cell? ln
what way is viral replication different from cell reproduction?
Examine your completed diagrams of viral replication. What
two processes are directed by viral genes that are activated
inside the host cell? Describe the stage that occurs before
viruses are released from the cell. To summarize and
enhance what you have learned in this lab, write an essay
that explains the different ways viruses invade host cells
and replicate. Use your library or the lnternet to gather
your information.
Chapter 4 Patterns of Life
. MHR r 125
-
The Viral Genome
Origin of Viruses
The genome of a virus consists of either DNA or
RNA. The entire genome may occur as a single
nucleic acid molecule or several nucleic acid
segments. The DNA or RNA may be single-stranded
or double-stranded, and either linear or circular.
Because viruses are so small, the size of the
genome is limited. For example, the genome
includes coded instructions for making only a
few different proteins that are needed to make the
capsid. In contrast, the human genome codes for
over 30 000 different proteins. Most DNA viruses
have their genome on a single, linear, doublestranded DNA molecule. Some $oups of these
viruses require the presence of helper viruses to
reproduce themselves. They are said to be
replication defective.
RNA viruses comprise Tooh of all viruses. The
process of RNA replication frequently involves
emors, and as a result these viruses usually have
much higher mutation rates than do DNA viruses.
Mutations provide new variants of the virus, some
of which may be better adapted to invade new hosts.
Where do viruses fit into the history of life?
Because they cannot replicate without host cells,
they must have evolved after the first cells came
into existence. They probably originated as
fragments of nucleic acid that escaped from their
original cell. They survived by becoming parasites
of the same or similar types of ceII. Viruses and
their hosts evolved together, and each type of virus
is probably more closely related to its host cells
than to other viruses in different groups.
Viruses are much smaller than prokaryotic cells and cannot
be seen with a light microscope. They vary in size from
about one half to one one-hundredth the size of the smallest
bacterium. While bacteria were first observed in the 1670s,
viruses were not identified until 1935, after electron
microscopes had been invented.
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1. Describe the structure of a virus. Why are viruses
considered to be non-living?
Measles
9-1 1 days
Shingles
years
Warts
months
Cold
2-4 days
HIV
2-5 years
2. Why must a virus enter a host cell in order to
reproduce itself?
3. What are the different ways a virus can reproduce
using a host cell?
4. Sketch the lytic and retrovirus reproductive cycles to
show the similarities and differences between these
two processes.
5. A doctor tells a patient that an antibiotic will not help
cure a cold sore. Explain the doctor's reasoning.
6. How is viral replication similar to the making of a
product in a factory? How does it differ from the
making of a product in a factory?
7. Explain how viruses might be used to copy the gene
for producing human insulin.
8. Consider the data table above right, on incubation
time of different viral diseases. Use the data to
predict which diseases are caused by viruses that
undergo the lytic cycle, versus diseases that include
a provirus stage. What is a possible public health
consequence of the incubation time for diseases
caused by proviruses?
126 r
MHR.Unit2Biodiversity
9. The following table
records the estimated
numbers of viruses
found in samples taken
from a bacterial culture
at hourly intervals.
Plot the data on a graph
and Interpret them to
explain what was
happening in the
bacterial culture.
1
15
2
17
J
49
4
128
5
385
6
386
7
386
a
387
10. lf you were a scientist developing a drug that would
block viral replication, which steps would you choose
to block? Explain your answer in detail.