Chapter 19
Viruses
In the 1800s a parallel between viruses and bacteria was apparent and considered them
the simplest of life forms. However, viruses cannot reproduce or carry out metabolic
activities outside of a host cell, so most biologist agrees that they are not alive. A current
phrase about viruses is that they lead “a kind of borrowed life”. Very useful for
Molecular Biology.
I. A virus consists of a nucleic acid surrounded by a protein coat.
A. Discovery of Viruses:
1. 1883 Adolf Mayer, a German scientist was seeking the cause of tobacco
mosaic disease. Because he could not find the microbe in the infectious sap,
he concluded that the “bacteria” was unusually small. Fig. 19.2
2. 1935 an American scientist Wendell Stanley crystallized the infectious
particle now known as tobacco mosaic virus (TMV). This and others were
later seen with the help of the electron microscope.
B. Structure of Viruses
Viruses are infectious particles consisting of nucleic acid enclosed in a
protein coat and in some case a membranous envelope. Very small in size,
only measured in nanometers.
C. Viral genomes
1. May be single or double stranded DNA or RNA
2. The genome is usually organized as a single linear or circular molecule
of nucleic acid.
3. As small as only 4 genes up to several hundred, maybe a thousand.
4. Basics of the genome (DNA/RNA) is used in classification of viruses.
D. Capsids and Envelopes
1. Capsid: protein shell that encloses the viral genome/ Built from a large
number of protein subunits called capsomeres.
2. Shapes: helical, polyhedral, or more complex Fig. 19.3
3. Viral envelopes: accessory structures that are membranes cloaking
their capsids. Derived from the membrane of the host cell but also
including viral proteins and glycoproteins.
4. The most complex capsids are found among viruses that infect
bacteria, called bacteriophages, or simply phages. First phages studied
included seven that infect E. coli.
II. Viruses reproduce only in host cells.
A. General Feature of Viral Reproductive cycles
1. Obligate intracellular parasites
2. Have limited host range. Identify their host by a “lock-and-key” fit.
Using proteins on the outside of the virus and specific receptor molecules
on the surface of the cell, it makes its way into the host.
Fig. 19.4 page 384: A simplified viral reproductive cycle.
1. After entering the cell, the viral DNA uses host nucleotides and enzymes
to replicate itself.
2. It uses other host materials and machinery to produce its capsid proteins.
3. Viral DNA and capsid proteins then assemble into new virus particles,
which leave the cell.
B. Reproductive Cycles of Phages (Bacteriophage)
*The Lytic Cycle Fig. 19.5 page 396 (culminates in death of the host cell)
1. Virulent bacteriophages (phages) reproduce by a lytic cycle
2. Lytic cycle - viral replication cycle that results in the death or lysis of
the host cell.
Steps:
 Phage attaches to cell surface.
 Phage contracts sheath and injects DNA
 The empty capsid of the phage is left as a “ghost”
outside the cell.
 Hydrolytic enzymes destroy host cell's DNA
 Phage genome directs the host cell to produce phage
components: DNA (made using cell’s degraded DNA)
and Capsid proteins.
 Cell lyses as a result of an enzyme produced by the
virus and releases phage particles.
Time - approximately 20 - 30 minutes at 37 C.
Defenses used by bacteria
 Can change receptor sites used by phages for
recognition
 Restriction enzymes can recognize and cut up foreign
DNA
* The Lysogenic Cycle Fig. 19.6 page 397 (study this diagram)
1. Temperate virus - A virus that can reproduce without killing the host.
2. Lysogenic cycle – A type of phage replication cycle in which the viral
genome becomes incorporated into the bacterial host chromosome as a
prophage.
3. Prophage - A phage genome that has been inserted into a specific site
on the bacterial chromosome.
4. Lysogenic cell - host cell carrying a prophage in its chromosome.
C. Reproductive Cycles of Animal Viruses
1. Viruses with envelops Fig. 19.7
a. Outside the capsid is an envelope
b. Consist of host cell's plasma membrane containing certain virus
specified proteins.
c. They bud off from the cell's surface.
d. Other viruses can become a provirus coexisting in the cell's
nucleus and have envelops of the nuclear membrane.
2. RNA as Viral Genetic Material
All possible type of RNA genomes are represented among animal
viruses.
The retrovirus belongs to a more complicate reproductive cycle.
They use reverse transcriptase to transcribe DNA from the viral
RNA genome. This unusual phenomenon gave rise to the name
retroviruses meaning backwards. HIV belongs to this group.
Fig.19.8 shows the structure of HIV and its reproductive cycle.
Provirus – integrated viral DNA which never leaves the Host’s
genome.
4. Evolution of Viruses
Because the genome of viruses follows the same rules of other living
organism, how could they have originated?
Plasmids: are small, circular DNA molecules usually found in
bacteria that can replicate independently of the genome of the cell.
Transposons: are DNA segments that can move from one portion
to another in a cell’s genome.
III. Viruses, viroids, and prions are formidable pathogens in animals and plants.
A. Viral Diseases in Animals
1. Our immune system is our major defense against viral infections.
2. Vaccines – harmless variant or derivative of a pathogen that stimulates
a host’s immune system to mount defenses against the pathogen. The
term vaccine is derived from vacca, the Latin word for cow.
3. Edward Jenner developed the first vaccine against smallpox in 1796.
4. Antibiotics kill bacteria by inhibiting enzymes or processes specific to
the pathogens. Viruses have few or no enzyme of their own.
5. AZT is a drug, which inhibits HIV reproduction by interfering with the
action of reverse transcriptase.
B. Emerging Viruses - make a sudden appearance, but in reality are usually not
new viruses. (Skimming the topic)
Three processes contribute to the emergence of viral diseases:
1. The mutation of existing viruses (Flu viruses)
2. The dissemination of a viral disease from a small, isolated
population can lead to widespread epidemics. Ebola, AIDS
3. The spread of existing viruses from one host species to another.
SARS virus is likely to have been in a particular species of bats.
C. Viral Diseases in Plants
They stunt plant growth and diminish crop yields.
Most plant viruses contain the RNA genome.
D. Viroids and Prions: The Simplest Infectious Agents
Viroids and Prions: The simplest infectious units.
Viroids - small circular RNA molecules that infect plants.
Prions are pathogens that are proteins. They are infectious proteins.
So far they seem to cause degenerative brain diseases (the mad
cow disease) in various animal species. Primarily they are
transferred through food.
Two characteristics of prions have been noted. One, they
have a long incubation period (at least ten years before symptoms
develop). This prevents sources of infection from being identified
until long after the first case appears. Second, prions are virtually
indestructible. Normal cooking does not affect them. Propagation
is shown in Fig. 19.10