Eukaryotes and Viruses
Chapters 12 and 13
Viral Characteristics and
Structure
Why Viruses aren’t Alive
General Characteristics of Viruses
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Name derives from the Latin for “poison”
Obligatory intracellular parasites
Referred to as filterable
Contain a single type of nucleic material
The nucleic material is covered in a
protein coat.
• Use the synthesis machinery of the host to
multiply.
Why are they not Alive?
• No independent metabolism or
reproduction
• No single phylogenetic origin
• No cellular structure
• No ribosomes
• Though they DO evolve and reproduce.
Host Range
• Viruses have a specific subset of cell types
they will infect, referred to as Host Range.
• Most viruses can only infect a single species
• Some viruses can cross species barriers
• Numerous factors influence host range
• Viruses that infect bacteria are referred to as
bacteriophage or simply phage.
Viral Particle Size
Poxviridae
Lentiviruses
Staphylococcus Bacteria
1 μm in diameter
Picornaviridae
Bacteriophage
Viral Structure
• A Virion is a complete, infectious viral
particle and is composed of…
– Nucleic Acid
– Capsid and Envelope
Nucleic Acid
• Only a single type of nucleic acid (RNA or
DNA) is present in any species of virus.
• Unlike cellular life, viral nucleic acid can be
either single or double-stranded (again
only a single type per species)
• Size of the genetic structure can range
from a few thousand base pairs to a
quarter of a million
Capsid and Envelope
• Capsids are regular repeating protein
structures composed of capsomeres.
• Some viral species also have a hostderived envelope surrounding the capsid
• Some viral species have protein/
carbohydrate “spikes” rising from the
surface that can be used for identification
Viral Morphology
Viral Taxonomy
How do you do a taxonomy of something that isn’t alive?
Viral Taxonomy
• Without a shared phylogeny there is no use
for the higher taxons (Domain, Kingdom,
Phylum, and Class)
• Typically viral species are referred to by
Order, Family, Genus and a descriptive
common name (in place of a species epithet)
• Based on
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Nucleic Acid type
Strategy of replication
Morphology
Host range
Viral Replication
No, not 1 becomes 2, more like 1 becomes 1000.
Growing Bacteriophage
Growing Animal Viruses
Viral Identification
• Polyphasic Identification
– Morphology
– Detection of Antibodies
– Western Blotting of known viral proteins
– Nucleic Methodologies
• PCR
• RFLP
• RNA PCR
Lytic Bacteriophage Cycle
Attachment
Release
Penetration
Biosynthesis
Maturation
Lysogenic Bacteriophage Cycle
Lytic Cycle
Animal Unenveloped DNA Viral
Replication
DNA vs. RNA Viral Replication
• There are various types of RNA viruses.
• Replication of the Genetic Material can be
simple or a multistep process.
– +RNA, direct translation and replication by viral
protein
– -RNA, indirect translation and replication by viral
protein
– dsRNA, direct translation and relication by viral
protein
– Retroviruses, conversion of RNA to DNA,
integration and then production by host.
Comparison
Bacteriophage
• Attachment to Cell Wall
proteins.
• Viral DNA is injected into
cell
• No removal of capsid
required
• Biosynthesis in cytoplasm
• Lysogeny
• Host cell lysed for release
Animal
• Attachment to plasma
membrane proteins and
glycoproteins.
• Capsid enters cells
• Capsid removed by
enzymes
• Biosynthesis in nucleus or
cytoplasm
• Latency
• Enveloped viruses bud and
nonenveloped rupture.
Viruses and Cancer
One of many factors
Oncogenic Viruses
• Some viruses and known to help trigger
cancers, called oncogenic viruses.
• These viruses affect oncogenes, natural
parts of our genetic structure that can
cause cancer.
• The process of becoming cancerous is
termed transformation.
• Oncogenic Viruses integrate into the host
genetic material.
Known Oncogenic Viruses
Type of Virus
Viral Species
Associated Cancer
DNA
Human Papillomavirus
Cervical Cancer
Epstein-Barr Virus
Burkitt’s Lymphoma
Hepatitis B Virus
Liver Cancer
Kaposi Sarcoma-Associated
Hepervirus
Kaposi Sarcoma
Merkel Cell Polyomavirus
Merkel Cell Carcinoma
Human T-Lymphotropic Virus 1
Leukemia
Hepatitis C Virus
Liver Cancer
RNA
Latency, Persistence and the
Prions
Acute, Latent, and Persistent Viral
Infections
• Acute Infections are those that cause
immediate proliferation.
• Latent infections can occur by itself or
after an acute infection, where the viral
load remains undetected for a long period
of time before reemerging quickly.
• Persistent Infections are ones where the
viral load build over a long period of time.
Graph of Acute, Latent and
Persistent Viral Infections
Prions
• Prions are infectious protein particles
• Prions are altered forms of a normal protein
in the host that can catalyze the alteration of
other “normal” protein particles to the “prion”
state
• They cause neurological degradation and
death with no known treatment.
• Since each prion protein is infectious, they
are extremely resistant to control measures.
Prion Reaction
C
PrP
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Sc
PrP
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Sc
PrP