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Chapter 13
Viruses, Viroids, and Prions
• Viruses
– an ultramicroscopic - 20 to 300 nm in diameter
– replicates only within the cells of living hosts (bacteria, plants, and
animals)
– composed of an RNA or DNA core, a protein coat, and, in more
complex types, a surrounding envelope.
• Viroids
– consists solely of a strand of RNA
– smaller than a virus
– an infectious particle - capable of causing disease
• Prions
– an infectious protein particle similar to a virus but lacking nucleic
acid;
Viruses
• Acellular
• Viral size is ascertained by electron
microscopy
• Consist of DNA or RNA core
• Core is surrounded by a protein
coat
• Coat may be enclosed in a lipid
envelope
• Reproduction – use host resources
– Viruses are replicated only
when they are in a living host
cell
– Obligate cellular parasite
Figure 1.1e
Virus structure
• Virion - a fully assembled infectious virus
1. Core- Nucleic acid – viral genome
• Single- or double-stranded RNA or DNA
• Encodes for only a few structural proteins (Capsid proteins )
• Non-structural regulatory proteins involved in virus replication
2. Protein coat - capsid
• Single or double protein shells
• Consist of only one or a few structural protein species
– Capsomer - The single protein unit
– Multiple protein copies must self assemble to form the continuous
three-dimensional capsid structure.
• Shell Function
– Protect the viral genome from nucleases.
– Attaches the virion to specific receptors exposed on the prospective
host cell during infection.
Virus structure
3. Envelope
• Some virus families have an additional lipid bilayer cover closely surrounding
the shell of the virus
• It is usually derived in part from modified host cell membranes.
• Virus-encoded membrane-associated proteins
– The exterior of the bilayer is studded with virus-coded glycosylated transmembrane proteins (glycoproteins)
• Spikes or knobs, also called peplomers.
Virus structure
•
Helical viruses
– The protein subunits and the nucleic
acid are arranged in a helix.
• Polyhedral viruses
– The protein subunits assemble into a
symmetric shell that covers the nucleic
acid-containing core.
• Complex Viruses
– Often have architecture consisting of
both helical and polyhedral parts
confined to different structural
components
Viruses reproduction
• The main function of the virion is to deliver its DNA or RNA
genome into the host cell so that the genome can be expressed
(transcribed and translated) by the host cell.
• For propagation viruses depend on specialized host cells supplying
the complex metabolic and biosynthetic machinery of eukaryotic or
prokaryotic cells.
• Host range is determined by specific host attachment sites and
cellular factors
• Most viruses infect only one host
• Viruses may be viewed as mobile genetic elements, most probably of
cellular origin and characterized by a long co-evolution of virus and
host.
Viral Taxonomy
• Family names end in -viridae
• Genus names end in -virus
• Viral species: A group of viruses sharing the same genetic
information (DNA or RNA) and ecological niche (host).
– Common names are used for species
• Subspecies are designated by a number
• Example:
– Herpesviridae
• Herpesvirus
– Human herpes virus HHV-1, HHV-2, HHV-3
– Retroviridae
• Lentivirus
– Human immunodeficiency virus HIV-1, HIV-2
The most important taxonomic criteria
• Genome Type
• Particle Morphology – symmetry and viral core
structure, envelope.
• Host organism(s) - Hosts of viruses include all
classes of cellular organisms described to date.
Genomic diversity among viruses
• Nucleic acid
– DNA
– RNA (RNA viruses, comprising 70% of all viruses, vary remarkably in genome structure.)
• Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much
higher mutation rates than do the DNA viruses.
• Mutation rates of 10-4 lead to the continuous generation of virus variants which show great adaptability
to new hosts
– Both DNA and RNA (at different stages in the life cycle)
• Shape
– Linear (adenoviruses )
– Circular (polyomaviruses )
– Segmented (some RNA viruses )
• Strandedness
– Single-stranded
– Double-stranded
– Double-stranded with regions of single-strandedness (Hepadnaviridae)
• Sense
– Positive sense (+)
– Negative sense (antisense)
– Ambisense (+/−)
Schemes of 21 virus families infecting humans showing a number of distinctive criteria:
an envelope or (double-) capsid
nucleic acid genome:
+, Sense strand;
-, antisense strand;
±, dsRNA or DNA;
0, circular DNA;
C, number of capsomers, where known;
nm, dimensions of capsid, or envelope when present;
Multiplication of Bacteriophages
• Bacteriophage – bacterial viruses – complex viruses
• Lytic cycle
Phage causes lysis and death of host cell
• Lysogenic cycle Prophage DNA incorporated in host DNA
Multiplication of Bacteriophages (Lytic cycle)
1. Attachment
2. Penetration
3. Biosynthesis
4. Maturation
5. Release
Multiplication of Bacteriophages (Lysogenic Cycle)
Figure 13.12
Generalized Transduction
Specialized Transduction
2
3
4
5
6
Figure 8.28
Multiplication of Animal viruses
1. Attachment
2. Penetration
# by endocytosis
# by fusion
3. Uncoating
by viral or host enzymes
Figure 13.14a
Multiplication of Animal viruses
4. Biosynthesis - Production of nucleic acid and proteins (In
nucleus (DNA viruses) or cytoplasm (RNA viruses)
5. Maturation - Nucleic acid and capsid proteins assemble
6. Release
- By rupture
- By budding (enveloped viruses)
Multiplication of DNA-Containing Virus
1.Viral DNA is replicated in the
host nucleus
2.Viral proteins are made in the cytoplasm
3. Viral assemble into virions in the nucleus
4. The virions are then transported
through the host ER for release
*Viral DNA is usually, but not always, integrated into the cell
chromosomes at random sites. Only part of the viral genome is expressed
*The family of Papovaviridae is not longer used in recent taxonomy, but is split into the
Papillomaviridae and the Polyomaviridae.
Multiplication of RNA-Containing Viruses
-Both the viral RNA and proteins are synthesized in the cytoplasm
-RNA dependent RNA polymerase
-Virions assemble in the cytoplasm
Figure 13.17
Multiplication of Retroviruses
• RNA +strand
• Reverse transcription
– Synthesis of dsDNA
– Transport to the nuclei
– Integrates in the host
genome –Provirus DNA
– The provirus can then remain latent
• Transcription activation
– Genome, or the genes can be expressed to create virions
– Assembly
– Release
Figure 13.19 - Overview
Virus Families That Affect Humans
• Single-stranded DNA, nonenveloped viruses
– Parvoviridae - Anemia in immunocompromised patients
• Double-stranded DNA, nonenveloped viruses
– Adenoviridae - Respiratory infections in humans, tumors in
animals
– Papovaviridae
• Papillomavirus - Human wart virus
• Polyomavirus - Cause tumors; some cause cancer
• - Double-stranded DNA, enveloped viruses
– Poxviridae
• Orthopoxvirus (vaccinia and smallpox viruses)
– Herpesviridae
• Simplexvirus (HHV-1 and HHV-2)
• Varicellovirus (HHV-3)
• Lymphocryptovirus (HHV-4)
• Cytomegalovirus (HHV-5)
• Roseolovirus (HHV-6)
• HHV-7
• Kaposi's sarcoma (HHV-8)
• Some herpesviruses can remain latent in host cells
– Hepadnaviridae Hepatitis B virus
• DNA single strand
• Blood borne pathogen
Virus Families That Affect Humans
• Single-stranded RNA, + strand, nonenveloped
– Picornaviridae - enterovirus
• Hepatitis A virus
• Single-stranded RNA, + strand, enveloped
– Togaviridae
• Rubivirus (rubella virus)
– Flaviviridae
• West Nile viruses,
• Hepatitis C virus
• Single-stranded RNA, – strand, one RNA strand
– Rhabdoviridae
• Lyssavirus (rabies virus)
– Filoviridae
• Ebola and Marburg viruses
– Paramyxoviridae
• Morbilli virus, Parainfluenza,Mumps
• Single-stranded RNA, – strand, multiple RNA strands
– Orthomyxoviridae - Envelope spikes can agglutinate RBCs
• Influenzavirus (influenza viruses A and B)
• Influenza C virus
Avian Influenza
(Lectin)
Clinical Focus, p. 371
Virus Families That Affect Humans
• Single stranded RNA, 2 RNA strands, produce DNA
– Retroviridae
• Use reverse transcriptase to produce DNA from viral genome
• Lentivirus (HIV)
• Oncogenic viruses
– Includes all RNA tumor viruses
Cancer
• Tumor is a general term used to describe the uncontrollable
growth of cells.
– Tumors can be Benign
– Malignant (cancerous)
• An oncogene is a gene that, when mutated or expressed at high levels,
helps turn a normal cell into a cancer cell
• Activated oncogenes transform normal cells into cancerous cells:
– increased growth
– loss of contact inhibition
– tumor specific transplant and T antigens.
• A proto-oncogene is a normal gene that can become an oncogene
– Proto-oncogenes code for proteins that help to regulate cell growth and
differentiation.
• The region of the viral genome that can cause a tumor (DNA in DNA
tumor-viruses or RNA in RNA-tumor viruses).
– This foreign gene can be carried into a cell by the virus and cause the host cell to
take on new properties.
Oncogenic Viruses
• Oncogenic DNA Viruses
– Adenoviridae
– Herpesviridae
– Poxviridae
– Papovaviridae
– Hepadnaviridae
• Oncogenic RNA viruses
– Retroviridae
• Viral RNA is transcribed to DNA which can integrate into
host DNA
• Examples:
1.
2.
3.
4.
5.
EBV ------> Nasopharyngeal cancer
RNA virus -----> breast cancer
Genital Herpes ------> cervical cancer
Genital warts -----> cervical cancer (specifically HPV-16)
unknown virus -----> cervical cancer carried in sperm
Viral infection
• Acute Infection
– During an acute infection, the virus essentially hijacks the cellular
machinery
• Influenza virus infection.
• Latent Viral Infections
– After infection the viral genome is replicated in conjunction with host
DNA;
• expression of viral genes is absent or inefficient; the viral genome remains
intact
• an acute infection may occur at a later time.
• Herpes virus - Cold sores, shingles
• Persistent Viral Infections
– Involve stages of both silent and productive
infection without rapidly killing or even
producing excessive damage of the host cells.
• Disease processes occurs over
a long period, generally fatalSubacute sclerosing panencephalitis
(measles virus)
Growing Viruses - Viruses must be grown in living cells.
• Bacteriophages form plaques on
a lawn of bacteria.
• Animal viruses may be grown in
living animals or in embryonated eggs.
• Animal and plants viruses may be grown in cell culture.
– Continuous cell lines may be maintained indefinitely.
Figure 13.6
Virus Identification
• Cytopathic effects
• Serological tests
– Detect antibodies against viruses in a patient
– Use antibodies to identify viruses in neutralization tests, viral
hemagglutination, and Western blot
• Nucleic acids
– RFLPs - A Restriction Fragment Length Polymorphism (or RFLP,
often pronounced as "rif-lip") is a variation in the DNA sequence of a
genome which can be detected by a laboratory technique.
– PCR
Plant Viruses
Plant viruses enter through wounds or via insects
Viroids
Viroids are infectious peaces of RNA; potato spindle tuber disease
Figure 13.22
Prions
• Prion (protein-infection) is an infectious agent composed only
of protein in a misfolded form.
• Inherited and transmissible by ingestion, transplant, & surgical
instruments
• Spongiform encephalopathies
–
–
–
–
–
– because of the post mortem appearance of the brain with large
vacuoles in the cortex and cerebellum
Sheep scrapie
Creutzfeldt-Jakob disease
Gerstmann-Sträussler-Scheinker syndrome
Fatal familial insomnia
Mad cow disease
Prions
• Prions propagate by transmitting a misfolded protein state. When a prion
enters a healthy organism, it induces existing, properly folded proteins to
convert into the disease-associated, prion form; the prion acts as a template to
guide the misfolding of more protein into prion form.
Learning objectives
• Differentiate a virus from a bacterium.
• Describe the chemical composition and physical structure of an enveloped and a
nonenveloped virus.
• Define viral species.
• Give an example of a family, genus, and common name for a virus.
• Describe the lytic cycle of bacteriophages.
• Describe the lysogenic cycle of bacteriophage lambda.
• Compare and contrast the multiplication cycle of DNA- and RNA-containing animal
viruses.
• Define oncogene and transformed cell.
• Discuss the relationship between DNA- and RNA-containing viruses and cancer.
• Differentiate between persistent viral infections and latent viral infections.
• Describe how bacteriophages are cultured.
• Describe how animal viruses are cultured.
• List three techniques that are used to identify viruses.
• Discuss how a protein can be infectious.
• Differentiate virus, viroid, and prion.