CHAPTER 24
Viruses
SECTION 1 VOCABULARY PRETEST
• Virus
• Capsid
• Envelope
• Provirus
• Retrovirus
• Reverse transcriptase
• Bacteriophage
A.
B.
C.
D.
E.
Protein coat of a virus
Virus that copies DNA from RNA
Enzyme used by retroviruses
Virus that infects bacteria cells
Nonliving particle that replicates
inside living cells
F. Bilipid membrane surrounding
some viruses
G. Viral DNA inserted into host DNA
• Lytic cycle
H. Active viral cycle
• Virulent
I.
• Lysis
J. Hidden viral cycle
• Lysogenic cycle
K. Rupturing of a cell
• Temperate virus
L. Virus that uses the lytic
cycle
• Prophage
Viral DNA inserted into
bacterial chromosome
M. Virus that uses the
lysogenic cycle
ANSWER KEY
• Virus
E
Lytic Cycle
H
• Capsid
A
Virulent
M
• Envelope
F
Lysis
K
• Provirus
G
Lysogenic Cycle
J
• Retrovirus
B
Temperate virus
L
• Reverse transcriptase
C
Prophage
I
• Bacteriophage
D
DISCOVERY OF VIRUSES
• Virus: Nonliving particle made up of nucleic acid and
a protein coat (or lipid-protein coat)
• Cause disease in living organisms
• Useful tools for genetic research
• Very small (need electron microscope to see)
• Can be crystallized
CHARACTERISTICS OF VIRUSES
• Lack cytoplasm and cellular organelles
• Cannot carry out the cellular functions of metabolism,
homeostasis, respiration or photosynthesis
• Can only reproduce inside a living cell by utilizing the
host cell’s ribosomes, ATP, enzymes and proteins.
SIZE AND STRUCTURE
• Viruses vary in size
and shape
• Most are surrounded
by a protein coat
known as a capsid.
Capsid shapes
include:
Helical
Polyhedral (Icosahedral)
BACTERIOPHAGES
• The smallest viruses
are those that infect
bacteria cells.
• They are called
bacteriophages.
• They have a robot-like
shape.
ENVELOPED VIRUSES
• Some viruses have a
bilipid membrane called
an envelope that
surrounds the capsid.
• It forms from the host cell
membrane as the virus
buds out from the host.
• Ex: chickenpox,
influenza, and HIV
• The proteins on the
envelope help the virus
recognize host cells.
CLASSIFICATION OF VIRUSES
• Most viruses are classified according to the type of
nucleic acid they contain: DNA or RNA
VIRAL REPLICATION
• Viruses are obligate intracellular parasites —they
replicate only by using host cell enzymes and
organelles to make more viruses.
REPLICATION IN DNA VIRUSES
1. Virus enters and is
uncoated, releasing viral
DNA and capsid proteins
Virus
Capsid proteins
2. Host enzymes replicate
the viral genome.
Viral DNA
mRNA
3. Host enzymes transcribe
the viral genome into viral
mRNA, which host
ribosomes use to make
more capsid proteins
4. Viral genomes and capsid
proteins self-assemble
into new viruses which
exit the cell.
REPLICATION IN RNA VIRUSES
• The genome of RNA
viruses serves as a
template for the
synthesis of mRNA
which is then
translated to make
viral proteins and as
a template for
making more copies
of the viral genome.
RETROVIRUSES
• Retroviruses are RNA viruses
that also contain the enzyme
reverse transcriptase
• This enzyme uses RNA as a
template to make DNA.
• The flow of information is
backwards (RNA
DNA):
hence the name “retro” virus.
• HIV is a retrovirus.
• The viral DNA that is copied by reverse transcriptase is
then integrated into the host genome and becomes a
provirus.
• The provirus remains a permanent resident of the
host cell.
• The host’s enzymes transcribe the proviral DNA into
RNA molecules that function both as mRNA to make
viral proteins and as genomes for new viruses.
1. HIV is enveloped and
has an RNA core with
2 molecules of reverse
transcriptase.
Viral RNA
DNA
6. Other RNA strands
become the RNA genome
for the new viruses
7. New viruses assemble
and bud out of the cell
2. Virus fuses w/ cell. Capsid
proteins are removed.
RNA and reverse
transcriptase enzymes are
Reverse transcriptase released.
Provirus
3. Reverse transcriptase
synthesizes a dbl. stranded
DNA molecule that is
incorporated as a provirus
4. Proviral genes are
transcribed into viral RNA
5. Some RNA acts as mRNA
to get the cell to make viral
capsids and other proteins.
VIRAL REPLICATION IN PROKARYOTES
• Viruses that infect
prokaryotes are called
bacteriophages, or phages.
• Robot-shaped
• Replicate using either the
lytic cycle or the lysogenic
cycle.
LYTIC CYCLE
• Viruses that use the lytic cycle are called virulent
viruses.
• Infection results in an immediate replication of up to
200 new phage viruses and the complete destruction
(lysis) of the host cell.
Lytic Cycle of the T4 phage virus.
LYSOGENIC CYCLE
• Viruses that use the lysogenic cycle are called
temperate viruses.
• After infection, these viruses can remain hidden in
their host cell for days, months, or years.
• The viral genome becomes integrated into the
bacteria’s DNA chromosome. It is now called a
prophage.
• Unlike proviruses, which are permanently integrated, a
prophage can exit the host chromosome and then
enter the lytic cycle.
Lysogenic/Lytic Cycle of the phage Lambda virus
VIRUSES: TOOLS FOR BIOTECHNOLOGY
• Phage viruses have become very useful in
recombinant DNA technology.
• Phage DNA is replaced with DNA of interest (ex:
human gene for insulin)
• Phage is allowed to infect bacteria cells.
• Bacteria then produce large amounts of either insulin
or copies of the gene for insulin.
ORIGIN OF VIRUSES
• It is believed that viruses evolved from early cells
(since they cannot reproduce without cells)
• They were probably tiny pieces of naked nucleic acid
that could travel between cells.
• Genes for protein coats evolved on a few mutant
pieces of DNA and ….the first virus evolved.
• Many viruses mutate very quickly (especially RNA
viruses). This makes it difficult for the immune system
to recognize them as well as to develop vaccines to
fight them.
SECTION 2 VOCABULARY PRETEST
• Vector
• Protease inhibitor
• Oncogene
• Proto-oncogene
• Emerging disease
• Inactivated virus
• Attenuated virus
• Viroid
• Prion
A.
B.
C.
D.
E.
Infections protein
Cancer causing gene
Virus that cannot replicate
Infectious piece of RNA
Weakened virus that cannot cause
disease
F. Intermediate viral host that transfers
a virus
G. Drug that blocks the synthesis of new
viral capsids
H. Illnesses caused by new viruses
I. Gene that controls cell replication
ANSWER KEY
• Vector
F
• Protease inhibitor
G
• Oncogene
B
• Proto-oncogene
I
• Emerging disease
H
• Inactivated virus
C
• Attenuated virus
E
• Viroid
D
• Prion
A
VIRAL DISEASES
• Most viruses must
be spread by an
intermediate host
known as a vector.
• Vectors can include:
humans, animals,
mosquitoes, ticks,
and fleas.
HUMAN VIRAL DISEASES
• Chickenpox and Shingles
• Caused by the varicella-zoster
herpesvirus.
• Multiplies in the lungs and
travels to blood vessels in the
skin causing fever and skin
rash
• Spread through direct contact
with the skin rash and through
the air
• If the virus stays as a episome
in nerve cells, it can later
cause shingles
Varicella-zoster virus
Chickenpox
Shingles
• Viral Hepatitis
• Hepatitis: inflammation of the liver;
causing fever, nausea, jaundice and
liver failure
• Five viruses can cause it:
• Hepatitis A and E —spread by
fecally contaminated food and
water
• Hepatitis B, C and D —spread
by sexual contact; and blood to
blood contact (contaminated
needles)
Hepatitis C virus
• Acquired Immune Deficiency Syndrome
(AIDS)
• Caused by HIV virus (human
immunodeficiency virus)
• Destroys immune system
• Spread by sexual contact (contact with
infected body fluids: blood, semen,
vaginal fluid; and from mother to fetus)
• RNA retrovirus that can infect T cells of
the immune system
• It will lyse many of the T cells. Others, it
will infect and stay hidden as a provirus
to avoid detection.
• Efforts to fight AIDS have focused on
drugs that interfere with the
replication cycle of the virus.
• AZT inhibits the ability of reverse
transcriptase to make a DNA
copy of the viral RNA.
• Protease inhibitors block the
synthesis of new viral capsids.
• Most effective treatments involve
a complex combination of
several drugs (AIDS cocktail)
• With treatment, many people
have manage to remain
symptom free for many years.
VIRUSES AND CANCER
• Cancer: results from cells that cannot stop dividing
• Some viruses can cause cancer because they contain
oncogenes: genes that block the normal controls of cell
reproduction.
• Other viruses cause cancer because they insert into a host
cell’s chromosome near a proto-oncogene which normally
controls cell growth. They disrupt the function of this portion of
DNA.
• Cancers caused by viruses include: Cervical cancer (caused
by HPV: human papillomavirus); Liver cancer (caused by
Hepatitis B); Leukemia (caused by HTLV: human Tlymphotophic virus); Burkitt’s lymphoma (caused by EpsteinBarr virus)
EMERGING VIRAL DISEASES
• Emerging Diseases: illnesses
caused by new or reappearing
infectious agents that typically
exist in animal populations—
often in isolated habitats—and
can infect humans who interact
with these animals.
• Hemorrhagic fever (Ebola
virus)
• Fatal pneumonia caused by
Hantavirus
• SARS
Ebola Virus
PREVENTION AND TREATMENT
• Antibiotics DO NOT work on viruses. Your immune
system is your main defense after you have been
infected.
• Vaccinations: a harmless version of the virus is
injected in order to trigger the immune response.
These are used to prevent viral infections…not cure
them. They make use of:
• Inactive viruses: cannot replicate
• Attenuated viruses: weakened form that cannot
cause disease
• Successful vaccines exist for:
Measles, mumps, rubella,
polio, hepatitis A and B, and
chickenpox
• Smallpox has been eradicated
using a worldwide vaccine
campaign carried out by the
World Health Organization.
• Last known case of smallpox
was in 1977 in Somalia
• Declared eradicated in 1980
• Vector Control:
involves controlling the
animal vectors used by
viruses.
• Ex: Mosquito-control
programs help stop the
spread of yellow fever:
Rabies vaccinations
of pets help stop the
spread of rabies.
• Drug Therapy: this approach is limited to drugs that
interfere with viral replication inside cells.
• Acyclovir blocks DNA polymerase of herpes viruses
and chickenpox virus. They do not destroy the
virus.
VIROIDS AND PRIONS
• Viroids: smallest known
particles that can replicate
• Short strand of RNA: no
capsid
• Infect plants: such as
coconuts, potatoes and
oranges
• Prions: infections proteins.
• Cause Mad Cow disease,
Creutzfeldt-Jakob disease
and Kuru