Download Chapter 17 2015 - Franklin College

Chapter 17-Virus
• What do you know about virus?
Virus characteristics
• Small
• Only view with an electron microscope
LE 18-2
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 µm
Virus characteristics
• Not alive
• Obligate intracellular parasites. Infect
cells-turn into a virus factory.
• Computer virus analogy
Virus Characteristics
• Contain only protein and nucleic acid (either
DNA or RNA)
• Nucleic acid can be SS or DS-makes up virus
core
• Core surrounded by a protein capsid. Shape of
capsid gives the virus its characteristic shape.
Some virus have envelope (animal virus onlyproduced by budding)
• Some virus have enzymes.
LE 18-4a
Capsomere
of capsid
RNA
18  250 mm
20 nm
Tobacco mosaic virus
LE 18-4b
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
50 nm
Adenoviruses
LE 18-4d
Head
Tail
sheath
Tail
fiber
DNA
80  225 nm
50 nm
Bacteriophage T4
Figure 18.02x2 Phages
LE 18-4c
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
50 nm
Influenza viruses
LE 18-9
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA
(two identical
strands)
Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid
Virus characteristics
•
•
•
•
•
Virus are host specific
A. animal virus
B. plant virus
C. bacterial virus (phage)
Sometimes a virus can mutate so it can
infect a new host (avian flu)
Figure 18-01
Virus characteristics
•
•
•
•
•
Virus cause diseases
Rabies
Influenza
Colds
Etc.
Figure 18-03
LE 18-11
Young ballet students in Hong
Kong wear face masks to
protect themselves from the
virus causing SARS.
The SARS-causing agent is a
coronarvirus like this one
(colorized TEM), so named for
the “corona” of glyco-protein
spikes protruding form the
envelope.
Figure 17.8
1 m
(a) 2009 pandemic H1N1
influenza A virus
(b) 2009 pandemic screening
Virus Life cycle
Simplified virus life cycle (lytic). Virulent
virus. What kind of cell can be infected by
a virus (any-plant, animal, bacteria)
Typical cycle-20-30 minutes.
• A. Attachment (specific-received e-mail)
• B. Entry (don’t open that e-mail)
• C. Synthesis
• D. Assembly (spontaneous)
• E. Release
LE 18-5
Entry into cell and
uncoating of DNA
VIRUS
DNA
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new virus particles
and their exit from cell
Enveloped Virus life cycle
LE 18-8
Capsid
Capsid and viral genome
enter cell
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
ER
Glycoproteins
Capsid
proteins
Copy of
genome (RNA)
New virus
LE 18-10
HIV
Membrane of
white blood cell
HOST CELL
Reverse
transcription
Viral RNA
0.25 µm
HIV entering a cell
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New HIV leaving a cell
mRNA
Treatment of viral diseases
Infectious cycle becomes an uncontrolled
chain reaction. How can you deal with it?
A. Antibiotics? No
B. Immune system. Can’t respond quick
enough first time you see virus
(immunization). Colds, flu.
C. Antiviral drugs-AIDS (AZT, protease
inhibitors); Tamiflu (neuramindase inhibitor)
D. Restriction enzymes-if you’re a bacteria.
Why do virus infections cause
disease symptoms?
• Damage to tissue-cell destruction (some cells
regenerate (epithelium of upper respiratory
tract), some can’t (nerves-polio)
• Cause infected cells to produce toxins (infected
cell becomes transformed)
• Viral cancer-oncogenes. Hep B-liver cancer;
Herpes-Burkitt’s lymphoma; papovaviruscervical cancer
• Immune system-inflammation, lymphokines
Emerging virus
• Typically not new virus-Existing virus that:
a. Evolve and infect individuals who are only
resistant to ancestral virus (influenza B and C)
b.Spread from small populations to larger. AIDS
c.Environmental disturbances can increase
emergence. Roads into tropical rainforest, global
warming, airplanes.
d.Spread from species to species. Hanta virusdeer mice to humans. Influenza A- Avian virus.
Porcine virus.
Triple Re-assorted
H1N1Influenza virus
• Three processes contribute to the emergence
of viral diseases
– The mutation of existing viruses, which is
especially high in RNA viruses
– Dissemination of a viral disease from a small,
isolated human population, allowing the disease
to go unnoticed before it begins to spread
– Spread of existing viruses from animal
populations; about three-quarters of new human
diseases originate this way
Lysogenic life cycle (temperate
virus)
Characteristics of the Lysogenic life cycle
• Steps Provirus inserts into infected cell’s
genome. Repressor gene usually
represses viral gene expression.
Induction.
• Differences in lytic and lysogenic life cycle
LE 18-7
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Daughter cell
with prophage
Many cell divisions
produce a large
population of
bacteria infected with
the prophage.
Phage DNA
circularizes
Phage
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle or Lysogenic cycle
is induced
is entered
New phage DNA and proteins are
synthesized and assembled into phages.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Prophage
Phage DNA integrates into the
bacterial chromosomes, becoming a
prophage.
Consequences of Lysogeny
• none-(viral genetic info lies dormant)
• Transformation-makes cell cancerous
(viral gene is an oncogene)
• Induction-Herpes-
Prions
• Infectious proteins
• Proteins are misfolded-cause other
proteins to be misfolded
• Misfolded proteins impair cellular function
• “Mad cow” disease
LE 18-13
Prion
Original
prion
Many prions
Normal
protein
New
prion
Origin of Virus?
There are three main hypotheses regarding the
origins of virus:
a. The progressive, or escape, hypothesis states
that viruses arose from genetic elements that
gained the ability to move between cells;
b.The regressive, or reduction, hypothesis asserts
that viruses are remnants of cellular organisms;
c. The virus-first hypothesis states that viruses
coevolved with their current cellular hosts.
Progressive hypothesis
Genetic material moved from cell to cell via
injured cell surfaces?
Evolution of capsid proteins may have
facilitated infection of undamaged cells.
Candidates for sources of viral genomes?
a. plasmids?
b. Transposons? (Mobile genetic elements)
Mirror image rna sequences
Often associated with rna virus
AAAAAAUUUUUXXXXXAAAAAAUUUUUU
RNA folds back on itself
DS RNA recognized by a system of
enzymes and proteins (the COP)
The COP then destroys an DS m-rna with
these mirror image sequences
RNai
Fig. 18-13
Hairpin
miRNA
Hydrogen
bond
Dicer
miRNA
5 3
(a) Primary miRNA transcript
mRNA degraded
miRNAprotein
complex
Translation blocked
(b) Generation and function of miRNAs
RNAi Video
• http://www.nature.com/nrg/multimedia/rnai
/animation/index.html