Download Chapter 10 Viruses

Chapter 10 Viruses
• Viruses contain DNA or RNA
• And a protein coat called a
• The capsid is composed of individual
• Some are enclosed by an envelope
• May have spikes protruding from envelope
• Most viruses infect only specific types of cells in one
host
• Host range is determined by specific host attachment
sites and cellular factors
Viruses
Polyhedral Viruses
Helical Viruses
Causes systemic infection often
with hemorrhagic fever
Complex Viruses
Viral Taxonomy
• Family names end in -viridae
• Genus names end in -virus
•
A group of viruses sharing the
same genes and host. Common names are used for
species
• Subspecies are designated by a number
Viral Taxonomy
• Family : Herpesviridae
• Genus: Simplexvirus
• Species/Subspecies: Human herpes virus 1,
HHV 2
• Family: Retroviridae
• Genus: Lentivirus
• Species/Subspecies: Human
Immunodeficiency Virus 1, HIV 2
Growing Animal Viruses
• Animal viruses may
be grown
• in living animals
• In embryonated
eggs
•
• or in cell culture.
Growing Viruses
• Animal (and plant) viruses may be grown in cell
culture.
• For research purposes
• For clinical identification of virus type
• By observing
Virus Identification
• Some cytopathic effects (not comfirmatory)
• Inclusion bodies - rabies virus
• Syncytia formation – measles, common cold
• Transformation – human papillomavirus
Virus Identification
• Serological tests
• Detect antibodies against viruses in a patient
• Use antibodies to identify viruses
• Western blot
• Viral nucleic acids: RFLPs (i.e. DNA Fingerprinting),
PCR tests
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Virus Identification – a cytopathic effect
Viral Multiplication
• Viruses require host cell “machinery” for replication
•
• Some viruses have some of their own enzymes: mainly
for viral nucleic acid replication
• Most, if not all, enzymes come from host cell
• These enzymes are involved in building new virions
Multiplication of Bacteriophages (Lytic Cycle)
•
Phage causes lysis and death of
host cell
• Attachment
Phage attaches by tail fibers to
host cell
• Penetration
Phage lysozyme opens cell wall;
DNA is then “injected” into cell
• Biosynthesis Production of phage DNA
and proteins
• Maturation
Assembly of phage particles
• Release
Phage lysozyme breaks cell wall
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
1 Attachment:
Phage attaches
to host cell.
Base plate
Pin
Cell wall
Tail
Plasma membrane
2 Penetration:
Phage penetrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3
Biosynthesis:
phage DNA directs
synthesis of viral
components by
host cell.
Tail
DNA
4 Maturation:
Viral components
are assembled into
virions.
Capsid
5 Release:
Host cell lyses and
new virions are
released.
Tail fibers
The Lysogenic Cycle: Phage DNA incorporated into host DNA
Specialized Transduction
Prophage
gal gene
Bacterial DNA
1 Prophage exists in galactose-using host
(containing the gal gene).
Galactose-positive
donor cell
gal gene
2 Phage genome excises, carrying
with it the adjacent gal gene from
the host.
gal gene
3 Phage matures and cell lyses, releasing
phage carrying gal gene.
4 Phage infects a cell that cannot utilize
galactose (lacking gal gene).
Galactose-negative
recipient cell
5 Along with the prophage, the bacterial gal
gene becomes integrated into the new
host’s DNA.
6 Lysogenic cell can now metabolize
galactose.
Galactose-positive recombinant cell
Multiplication of Animal viruses
• Attachment
Viruses attach to cell membrane
• Penetration
By endocytosis or fusion
• Uncoating
By viral or host enzymes
• Biosynthesis
Production of nucleic acid and proteins
• Maturation
Nucleic acid and capsid proteins
assemble
• Release
By budding (enveloped viruses) or
rupture (non-enveloped viruses)
Attachment, Penetration, and Uncoating
Release of an enveloped virus by budding
Nonenveloped DNA virus
•
• Can cause cancer (cervical cancer)
• Hand warts
• Genital warts
• Commonly sexually transmitted
• Prevention: vaccine for HPV
•
• Respiratory infections in humans
• Usually sudden onset and short duration
• Tumors in animals
Enveloped DNA virus
•
• Can cause chronic liver disease
• Can lead to liver cancer
• Body fluid transmisson
• Prevention: vaccine
• Human Herpes Virus 1 and HHV 2 – “cold sores” and
genital herpes
• Sexually transmitted
• Other direct contact transmission
• Prevention: condoms (not 100%), abstinence
Enveloped DNA virus
• Varicella-Zoster virus (HHV 3) –
• Chicken pox is initial infection
• Shingles may occur later
• Contracted by inhaling virus
• Prevention: vaccine
• Epstein-Barr virus (HHV 4) – Infectious Mononucleosis
• Saliva transmission
• Burkitt’s lymphoma in African children
• Prevention: avoid infected body fluids (mainly saliva)
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Multiplication of a Retrovirus
Capsid
Reverse
transcriptase
Envelope
Virus
Two identical strands of RNA
1 Retrovirus penetrates
host cell.
Host
cell
DNA of one of the host
cell’s chromosomes
5 Mature
retrovirus
leaves host
cell, acquiring
an envelope as
it buds out.
Viral RNA
Identical
strands of
RNA
4 Transcription of the
Viral proteins
RNA
Reverse
transcriptase
provirus may also occur,
producing RNA for new
retrovirus genomes and
RNA that codes for the
retrovirus capsid and
envelope proteins.
Provirus
2 Its RNA is
uncoated; reverse
transcription takes
place.
3 The new viral DNA is
tranported into the host cell’s
nucleus and integrated as a
provirus. The provirus may
divide indefinitely with the
host cell DNA.
Retroviruses – enveloped RNA viruses
• Use reverse transcriptase to produce DNA from RNA
viral genome
• HIV - AIDS
• Oncogenic viruses
• Some retroviruses can cause cancer
Cancer
• The genetic material of oncogenic viruses becomes
integrated into the host cell's DNA.
“transform” normal cells into
•
cancerous cells.
• An oncovirus can promote oncogene expression
•
may contain oncogenes
Viral Infections
•
• Virus remains in host cell for long periods,
asymptomatically
• Subsequent activation may lead to: i.e. cold
sores, shingles
• Persistent Viral Infections
• Disease progresses over a long period, generally
fatal
• Subacute sclerosing panencephalitis, SSPE
(measles virus)
• AIDS dementia complex (HIV)
Prions
• Infectious proteins
• Inherited, and transmissible by ingestion, transplant, &
surgical instruments
• Spongiform encephalopathies (fatal): Sheep Scrapie,
Creutzfeldt-Jakob disease, mad cow disease
• Cause:
•
normal cellular prion protein on cell
surface,
• converts to
•
scrapie protein, accumulates in
brain cells forming plaques
Prions
PrPSc
PrPc
1
2
3
4
Lysosome
Endosome
5
6
7
8
Plant viruses and viroids
• Plant Viruses
• enter through
wounds or via
insects
•
are
infectious RNA
• One causes
potato spindle
tuber disease
Potato Spindle Tuber Viroids
RNA virus, nonenveloped
• Poliovirus
• May be paralytic in ~1% of cases
• Transmission:
• Prevention: vaccine
• Human
• Number one cause of the common cold
•
• Attacks liver, kidneys, spleen
• Transmission: fecal-oral route
• Usually not fatal
• Prevention: vaccine (long-term), immune globulin
(short-term)
RNA virus, enveloped
• Influenza viruses A, B, and C
• Type A have caused
• Both type A and B cause seasonal flu
• Subtypes differ based on H and N spike variation
• Influenza viruses continued:
• Mutation of H and N spike can lead to epidemics or
pandemics
• Contracted by inhaling virus
• Prevention: vaccine may prevent
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
RNA virus, enveloped
•
• Infects birds and humans
• Transmitted by mosquitos
• First appeared in U.S. in 1999 in NYC area; now
coast to coast
• Symptoms: Usually “flu-like;” less than 1% get West
Nile Encephalitis
• Prevention: avoid mosquito bites
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings