Download Chapter 13 – Viruses

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Chapter 6 – Viruses
Characteristics of Viruses
 Infectious Particles -- Not considered to be living organisms (outside of living host cells, they are inert)
 Obligatory intracellular parasites - multiply inside living cells – use the machinery of the cell
 Lack many enzymes (e.g. for protein synthesis or ATP synthesis)
 Most drugs that interfere with viral multiplication also interfere with functioning of the host cell
Host Range (the range of host cells the virus can infect) – viruses are usually specific for one host species
1. bacteriophages (phages) – infect specific bacteria
2. host cell must have receptor sites on the cell membrane or cell wall to which virus can attach.
3. host cell must have right cellular factors for viral multiplication
Viral Size - varies (20 – 14,000 nm) – need electron microscope to see
Viral Structure
Virion = a complete viral particle
Nucleic Acid
1. single kind – either DNA or RNA – never both
2. either kind can be single or double-stranded  ssDNA, dsDNA, ssRNA, dsRNA
Capsid – the protein coat
1. surrounds the nucleic acid and protects it from the host cell’s enzymes
2. Assists in binding to and penetrating the host cell
3. makes up most of the mass of a virus
4. composed of protein subunits called capsomeres
5. These proteins can stimulate the host’s immune system (act as antigens); immune response might be
production of antibodies or activated immune cells.
Envelope
1. not present in all viruses
2. combination of lipids, proteins, and carbohydrates
3. usually derived from the host cell membrane, with some viral proteins added.
4. The host cell membrane coats the viral particle as it buds from the host cell (becomes the envelope)
5. spikes (viral carbohydrate & protein on the membrane); used to attach virus to host cell
6. Some spikes are associated with ability of virus to clump red blood cells
7. Spikes are surface proteins that can act as antigens – trigger immune system
8. Mutations in the genes that code for the spikes alter surface protein – previous antibodies won’t bind to
mutated spikes – virus evades immune system ( frequent in influenza virus)
 Antigenic drift – minor changes in surface antigens
 Antigenic shift – major changes in surface antigens
General Morphology – based on structure of capsid
1. Helical Viruses
a. long rods – hollow, cylindrical capsid shaped like a helix
b. e.g., rabies virus, Ebola virus
2. Icosahedral Viruses
a. capsid with 20 triangluar faces & 12 corners
b. e.g. adenovirus & poliovirus
3. Enveloped viruses
a. roughly spherical, but the underlying capsid still has shape of a helix or polyhedron
b. e.g., influenza virus, herpes simplex virus
4. Complex viruses
a. complicated structures – e.g., bacteriophage, poxviruses
b. additional structures are attached to the capsids ( tail sheath, tail fibers, baseplate, pins, etc)
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Taxonomy of Viruses - Grouped into Families based on nucleic acid type ; strategy for replication ;
morphology
viral species - group of viruses sharing the same genetic information and ecological niche. - usually
designated by a descriptive common name ( e.g. human immunodeficiency virus )
Some Important Human Virus families are:
1. Parvoviridae
8. Calciviridae
15. Deltaviridae
2. Adenoviridae
9. Togaviridae
16. Orthomyxoviridae
3. Papovaviridae
10. Flaviviridae
17. Bunyaviridae
4. Poxviridae
11. Coronaviridae
18. Arenaviridae
5. Herpesviridae
12. Rhabdoviridae
19. Retroviridae
6. Hepadnaviridae
13. Filoviridae
20. Reoviridae
7. Picornaviridae
14. Paramyxoviridae
Viral Multiplication
Virus Nucleic Acid – has very few genes –only codes for a few structural components & few enzymes
Most enzymes needed for protein synthesis, ribosomes, tRNA, energy (ATP), etc. – must be supplied by the
host cell.
Multiplication of Animal Viruses – DNA Viruses
General Phases
1. Attachment (Adsorption)
a. Normal proteins on the animal cell membrane can act as specific receptors for the virus
b. Attachment proteins ( e.g. spikes) on the surface of the virus attach to the receptor proteins.
c. They must be specific for each other – have complementary shapes
 Viruses tend to be host-specific and species-specific
d. Receptor sites vary from person to person - ? determine your susceptibility to virus?
e. Antiviral drugs – can block host’s receptor sites or block virus’ attachment site
2. Penetration
a. Penetration is by endocytosis – plasma membrane folds inward – forms vesicle around virus
particle & brings it inside the cell.
b. Fusion – an alternative method for enveloped viruses. Envelope fuses with plasma membrane &
releases capsid into cell cytoplasm ( HIV uses this)
3. Uncoating
a. Uncoating – the separation of viral nucleic acid from its protein coat.
b. Inside the cell – the viral envelope is destroyed; capsid is digested by host cell; nucleic acid is
released
4. Synthesis
a. After uncoating, viral DNA is released into the nucleus of the host cell
b. Some genes are transcribed & translated. – These are mostly viral enzymes needed for the
multiplication of viral DNA. Most of the enzymes used by the virus are the host cell enzymes
c. The viral DNA is replicated.
d. Additional genes are transcribed & translated. These are mostly for the capsid proteins which are
synthesized in the cytoplasm.
5. Assembly
a. The viral DNA & capsid proteins assemble to form complete viruses.
6. Release
a. Viruses are released from the cell ( by budding or by lysis, a rupture of membrane)
Some DNA viruses:
 Adenoviruses –- causes acute respiratory disease ( common colds)
 Poxviruses – e,g, smallpox, cowpox
 Herpesviruses –
 Herpes simplex I – fever blisters, cold sores
 Herpes simplex 2 – genital ulcers
 Varicella virus - chickenpox and shingles
 Epstein-Barr Virus – (EBV) infectious mononucleosis
 Papovaviruses – Papillomavirus (gemital warts; assoc. w/ cancer)
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RNA viruses – Replication Cycle is similar to the DNA viruses, but varies; Not required to know all of the
modes of replication of RNA viruses for the exam
Retroviruses -- RNA viruses
 Retroviruses infect vertebrates, including humans
 Include the Lentivirus genus – which includes HIV-1 & HIV-2
 Carry their own enzyme – reverse transcriptase ( copies RNA  DNA)
 The single strand of DNA, is then replicated to form dsDNA.
 The viral DNA enters the nucleus & gets integrated into the host cell DNA – this stage is called a provirus.
 Latent state – the provirus is inactive, but gets replicated whenever host cell DNA is replicated. OR
 Provirus DNA gets transcribed into mRNA – produces new viruses OR
 Provirus can convert host cell into a tumor cell
Cytopathic Effects (CPE)
Cytopathic Effects are the damage to the host cell due to a virus infection
1. Inclusion bodies – compacted masses of viruses or damaged cell organelles found in virally infected
cells
2. Syncytia – result of the ability of a virus to fuse cell membranes ; infected cells mass together and fuse
into a multinucleate giant cell called a syncytia; RSV – Respiratory Syncytia Virus causes acute
respiratory disease, esp. in children in institutionalized settings.
3. Persistent infections (slow virus infections)
 develops gradually over long period of time.
 Can be caused by some common viruses ( measles -- years later  encephalitis)
 Number of infectious viral particles builds slowly ( In latent infection, a high number appears
suddenly)
4. Chronic Latent Infections
 Virus remains in the host for long period ( can be years) – doesn’t produce disease.
 Some latent viruses can exist in a lysogenic state ( incorporated into the host DNA) in the host
cell.
 Chronic Epstein-Barr Infection (EBV)
 Human herpesviruses – herpes simplex virus – fever blisters, cold sores – remains latent in
host nerve cells.
 Chickenpox virus (Varicellavirus) – latent in nerve cells – reactivation shingles
o Reactivation can be triggered by immune suppression, or a stimulus like fever,
sunburn, etc.
5. Transformation – Viruses and Cancer
 Viruses and Cancer – Approx. 10% of cancers are known to be virally induced
 Oncovirus or oncogenic virus – a virus capable of inducing tumors in animals
 Oncogenes – a normal “housekeeping” gene that can be activated to abnormal functioning,
and can induce tumor formation. ( activators include – chemical mutagens, radiation & viruses)
 Both DNA and RNA viruses are capable of inducing tumors in animals.
 Transformation – the process in which cells acquire properties that are distinct from those
found in normal cells ( They become transformed).
DNA Oncogenic Viruses
1. Epstein-Barr virus (EBV) -- infectious mononucleosis, Burkitt’s Lymphoma, nasopharyngeal
cancer, possibly Hodgkin’s Disease ( about 90% of U.S. population carry this virus in latent
form, but show no disease)
2. Papillomavirus (HPV) –assoc w/ development of uterine /cervical cancer
3. Hepatitis-B virus – liver cancer
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RNA Oncogenic Viruses
1. Only the retroviruses . – Provirus DNA gets incorporated into host cell genome -- (HTLV-1 &
HTLV-2 – T-cell leukemia), Sarcoma viruses of cats, chickens, Feline leukemia virus.
Multiplication of Bacteriophages –
The Lytic Cycle ( ends with lysis & death of host cell)a. Attachment – phage attaches to host cell
 Attachment site on virus – tail fibers matches complementary receptor site on
bacterial cell wall
b. Penetration
 Tail releases lysozyme – breaks down bacterial cell wall.
 Tail sheath contracts, tail core punctures cell wall & contacts cell membrane
 DNA passes through tail core – is injected into bacterial cell
c. Synthesis
 Host cell nucleotides & enzymes are used to make many copies of phage DNA.
 Phage DNA is transcribed into mRNA – codes for phage enzymes & capsid
 Host cell ribosomes, enzymes & amino acids used for translation.
d. Assembly
 Bacteriophage DNA & capsids spontaneously assemble into complete virions
 Head is filled with phage DNA and attached to the newly-assembled tail.
e. Release – the final stage
 Lysis – plasma membrane breaks open.
 Phage produced lysozyme breaks open the bacterial cell wall
 Bacteriophages are released from the host cell
 Host cell is dead.
1. The Lysogenic Cycle
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lysogeny – a state in which phage DNA is incorporated into the host cell’s chromosome; The e
phage remains latent or inactive – host bacterial cell survives, but harbors the phage.
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Temperate phage ( lysogenic phage)– can multiply by either the lytic or lysogenic method
Phage attaches to host cell & injects DNA
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If it follows the Lytic Pathway - new phage DNA & proteins are synthesized & assembled into
virions – Cell lyses, releasing virions
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Lysogenic Cycle – phage DNA gets incorporated into the bacterial chromosome; the inserted
phage DNA is now called a prophage.
Every time the host cell replicates, it also replicates the prophage DNA, so all the descendants
get a copy of the viral DNA. ( The prophage remains latent in these cells).
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Some events can trigger excision – the phage DNA pops out of the host chromosome, and the
lytic cycle is initiated. ( events – UV or chemical exposure, spontaneous event)
Importance
 The host cell may exhibit new properties (e.g. can produce a toxin that is coded for
by phage DNA – diphtheria toxin, scarlet fever toxin, botulism toxin, cholera toxin
are all examples of toxins produced by bacteria that are infected with phages in a
lysogenic state.
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Isolation, cultivation and Identification of Viruses
Viruses cannot multiply outside a living host cell – you must provide living cells in order to grow viruses in a lab.
They cannot be grown on artificial media. Even media like blood agar (cells are dead)
Growing Bacteriophages in the Lab
1. in suspensions of bacteria or in bacterial cultures on solid media
2. plaque assay – used to enumerate viral particles
3. Grow bacteriophages on a lawn of bacteria – Count the number of clearings (plaques) on the agar.
4. Theoretically, each plaque corresponds to a single virus in the initial suspension
Growing Animal Viruses in the Lab
1. In living animals ( some will only grow in living animals)
 mice, rabbits, guinea pigs, primates
 used in research, immune system studies, diagnostics
 Some human viruses won’t grow in any other animal – or if they do, do not cause the disease –
problem with HIV research
2. In (fertilized) embryonated eggs
 Virus is injected into fertilized egg – grows in the embryo or one of the membranes surrounding
the embryo.
 Often used to grow viruses for vaccines – Some egg proteins may be present in these
preparations. People who are allergic to eggs may have allergic reaction to the vaccine.
3. In cell cultures
 cells can be grown in an artificial culture media in Petri dishes or flasks.
 Cells will spread evenly to form a single layer (monolayer).
 Viruses cause the cells to round up & pull away, causing tears or “holes” called plaques in the
monolayer – this cytopathic effect (CPE) – can be counted to detect virus.
Viral Identification
1. Serological methods – e.g., Western Blot – are most common – If virus reacts with specific
antibodies against the virus, you can identify it.
2. Molecular methods – PCR to amplify nucleic acid; fingerprinting
Other Non-Cellular Infectious Agents
I.
Prions
 prion – proteinaceous infectious particle (major portion of molecule is a protein – no DNA or RNA)
 The PrP gene codes for the normal protein PrP. An abnormal form of the protein is found in animals
with scrapie.
 When the abnormal form contacts a normal PrP protein, it causes it to fold in an abnormal manner.
 The new abnormal protein attacks more normal PrP and converts it to abnormal form.
 The abnormal PrP accumulates in the brain, forming plaques, but the plaques do not appear to be
the cause of cell damage.
 Associated with neurological diseases of animals:
1. Humans: kuru, Creutzfeldt-Jakob disease (CJD)
2. Sheep: scrapie
3. Cows – bovine spongiform encephalopathy (BSE) – “mad cow disease”
II.
Plant Viruses and Viroids
 morphologically similar to animal viruses
 cause diseases of many economically important crops
 viroids – short pieces of naked RNA ( no protein coat) – doesn’t code for any protein
 viroids are pathogens only of plants – annual crop damage = millions of dollars in losses.