Download Viruses

S E C T I O N
11.2
Viruses
E X P E C TAT I O N S
Describe characteristics of a
representative virus.
Learn the life cycles of
representative viruses.
Explain the relevance of
current studies of viruses to
the field of biotechnology.
Figure 11.6 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 plasma membrane.
Many diseases of plants and animals (including
humans) are caused by bacteria or viruses that
invade the body. 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 have probably learned
that scientists consider cells to be the basic units of
life. Viruses have no cellular structure, so by this
definition, 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 little more than strands of DNA or RNA
surrounded by a protective protein coat called a
capsid (Figure 11.6). 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.
Classifying Viruses
Since viruses were first identified in 1935,
scientists have described more than 160 major
groups. Members of different groups differ in their
size and shape as shown in Figure 11.7. 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.
A
C
B
D
Figure 11.7 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.
Patterns of Life • MHR
385
Viral Reproduction
can only 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 cell, viruses are completely inert.
Viruses can enter cells in two different ways. In
the first, once attached to the host cell, the virus
can inject its nucleic acid into the cell. Figure 11.8
shows the steps to this cycle of viral replication,
called the lytic cycle. 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.
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 plasma 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 11.7 on the previous page, recognizes and
attaches the T4 to the specific host cell. 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,
virus
bacterial DNA
nucleic
acid
B Entry
The virus injects its
nucleic acid into the
bacterial cell.
bacterial
host cell
A Attachment
D Assembly
New virus particles
are assembled.
E Lysis and Release
The host cell breaks
open and releases
new virus particles.
Figure 11.8 The lytic cycle occurs when a virus inserts its nucleic acid into a cell, and
then uses the cell’s metabolism to replicate its DNA or RNA and make new viruses.
386
MHR • Diversity of Living Things
C Replication
The host’s metabolism
replicates the viral
DNA or RNA.
BIO
FACT
In the 1970s, the World Health Organization officially
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 infected only humans
and no other species.
Viruses and Disease
In the lytic cycle of a virus, shown in Figure 11.8,
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. This enzyme causes the host cell to
copy the viral RNA into DNA (Figure 11.9). In this
form, the viral genome can enter the chromosomes
of the host cell and be copied when the cell
divides. You will learn more about RNA viruses in
the viral genome section on page 389.
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
RNA
retrovirus
RNA
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 medical tests. At any time,
however, the provirus can separate from the host
chromosomes and complete the more damaging
lytic cycle shown in Figure 11.8.
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
DNA
reverse
transcriptase
entering
cell
provirus in
host chromosome
mRNA
Retrovirus cycle
new virus parts
exiting
cell
new virus
forming
Figure 11.9 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.
Patterns of Life • MHR
387
BIO
PLAY
Your Electronic Learning Partner has animations to
demonstrate the lytic cycle and the retrovirus
reproductive cycle.
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 11.10.
FACT
In 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 brain diseases, including Creutzfeldt-Jakob
disease (CJD) in humans, scrapie in sheep, and Bovine
Spongiform Encephalopathy (BSE) or “mad cow disease”
in cows.
virus
recombinant DNA
recombinant DNA
host cell
viral DNA
foreign gene
Figure 11.10 Genetic engineers use viruses to introduce new genes
into a cell and to clone copies of genes.
MINI
LAB
Viral Replication
Analyze
Viruses are very successful at invading the cells of
organisms because they can only reproduce using the
metabolism of a host cell. In 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.
How do replication and protein synthesis occur in a cell? In
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 Internet to gather
your information.
388
MHR • Diversity of Living Things
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 groups of these
viruses require the presence of helper viruses to
reproduce themselves. They are said to be
replication defective.
RNA viruses comprise 70% of all viruses. The
process of RNA replication frequently involves
errors, 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 cell. 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.
SECTION
FACT
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.
REVIEW
1.
Describe the structure of a virus. Why are
viruses considered to be non-living?
2.
Why must a virus enter a host cell in order to
reproduce itself?
K/U
Disease
Incubation time
Measles
9-11 days
Shingles
years
Warts
months
Cold
2-4 days
HIV
2-5 years
K/U
3.
What are the different ways a virus can
reproduce using a host cell?
4.
Draw 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.
BIO
K/U
C
9.
K/U
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?
K/U
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?
The following table
records the estimated
numbers of viruses
found in samples taken
from a bacterial culture
at hourly intervals.
I
Plot the data on a graph
and interpret them to
explain what was
happening in the
bacterial culture.
MC
K/U
10.
Hour
Number of viruses
1
15
2
17
3
49
4
128
5
385
6
386
7
386
8
387
MC If you were a scientist developing a drug that
would block viral replication, which steps would you
choose to block? Explain your answer in detail.
Patterns of Life • MHR
389