Download VIRUSES Chapter 19 What is a virus?

VIRUSES
Chapter 19
What is a virus?
• A virus is a submicroscopic infectious
particle composed of a protein coat
(capsid) and a nucleic acid core (either
DNA or RNA).
• Viruses are similar in size to a large
protein macromolecule, generally
smaller than 200 nm in diameter.
Discovery of Viruses
• Search for cause of tobacco
mosaic disease led to viruses
• Beijerinck proved that the
disease was caused by a virus.
• The elusive virus was
crystallized in 1935 by Wendell
Stanley.
Fig. 19-2
RESULTS
Beijerinck’s experiment
1 Extracted sap 2 Passed sap
from tobacco
plant with
tobacco
mosaic
disease
3 Rubbed filtered
through a
porcelain
filter known
to trap
bacteria
4 Healthy plants
became infected
sap on healthy
tobacco plants
Viral Capsids
• Capsids are built from protein
subunits called capsomeres
• May be rod-shaped (helical viruses),
polyhedral (icosahedral viruses) or
more complex
• Some viruses have membranous
envelopes that help them infect
hosts (flu virus)
• Bacteriophages, also called phages,
infect bacteria
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Glycoproteins
Glycoprotein
18  250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
virus
50 nm
Tail
fiber
80  225 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
• Viruses are obligate intracellular parasites,
which means they can reproduce only
within a host cell
• Each virus has a host range, a limited
number of host cells that it can infect
Viral Reproduction
• Once a viral genome has entered a
cell, the cell begins to manufacture
viral proteins using the host cell’s
materials (enzymes, ribosomes,
tRNAs, amino acids, ATP, etc.)
• ** RNA viruses may have codes for
their own enzymes however.
Fig. 19-4
VIRUS
1 Entry and
DNA
uncoating
Capsid
3 Transcription
and manufacture
of capsid proteins
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
• Phages are the best understood of all
viruses
• Phages have two reproductive
mechanisms: the lytic cycle and the
lysogenic cycle
The Lytic Cycle
• The lytic cycle culminates in the death of
the host cell by producing new phages
and digests the host’s cell wall,
releasing the progeny viruses
• A phage that reproduces only by the
lytic cycle is called a virulent phage
• Bacteria have defenses against phages,
including restriction enzymes that
recognize and cut up certain phage DNA
Fig. 19-5-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
The Lysogenic Cycle
• The lysogenic cycle replicates the phage
genome without destroying the host
• The viral DNA molecule is incorporated
into the host cell’s chromosome and is
called a prophage.
• Every time the host divides, it copies the
phage DNA and passes the copies to
daughter cells
• Viruses that can be lysogenic or lytic are
called temperate phages.
Fig. 19-6
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and proteins
are synthesized and
assembled into phages.
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Animal Viruses
• Classified as DNA or RNA viruses, single
or double-stranded
• Many have envelopes with glycoproteins
that are specific for receptors.
• The glycoproteins are made by the ER
and added to the host cell’s membrane
which envelopes the emerging viruses.
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Notice the viral
ER
mRNA codes for
Glycoproteins that
are added to
the cell membrane.
RNA viruses often have
Codes for their own
enzymes unlike DNA
Viruses.
Capsid
proteins
Glycoproteins
Copy of
genome (RNA)
New virus
Table 19-1a
Table 19-1b
RNA Viruses
• The broadest variety of RNA genomes is
found in viruses that infect animals
• Retroviruses use reverse transcriptase to
copy their RNA genome into DNA (HIV is
ex.)
• The viral DNA that is integrated into the
host genome is called a provirus
• Unlike a prophage, a provirus remains a
permanent resident of the host cell
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120088/micro4
1.swf::HIV Replication
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HOST CELL
HIV
RNA
Reverse
transcriptase
Viral RNA
DNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
http://highered.mcgrawhill.com/olcweb/cgi/pluginpo
p.cgi?it=swf::535::535::/sites/
dl/free/0072437316/120088/
micro41.swf::HIV Replication
New virus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
Fig. 19-8b
HIV
Membrane of
white blood cell
0.25 µm
HIV entering a cell
New HIV leaving a cell
Evolution of Viruses
• Since viruses can reproduce only within
cells, they probably evolved as bits of
cellular nucleic acid
• Candidates for the source of viral
genomes are plasmids and transposons
(small mobile DNA segments)
• Mimivirus, a double-stranded DNA virus,
is the largest virus yet discovered…. not
any more…. Mega Virus
Mimivirus and megavirus
Mimivirus was first
isolated in 1992 from
amoeba growing in a
water tower.
Megavirus was
isolated from infecting
amoeba with
mimiviruses.
Which came first,
the cell or the mimivirus?
How fast can viruses evolve?
• When viruses face an obstacle to
infecting the cells they normally infect,
how long does it take for them to
evolve to successfully invade them
again? A new study has a frightening
answer: just a little more than two
weeks.
• how fast viruses evolve – lambda virus
Viral diseases in animals
• Symptoms caused by
- toxins
- body’s defense mechanisms
• Vaccines – weakened or derivatives of
viral particles capable of causing an
immune response
• Antibiotics not effective
• Some antiviral medications interfere
with viral nucleic acid synthesis
Why are antibiotics
ineffective against viruses?
• They target 70s ribosomes, cell walls, or
bacteria-specific enzymes
• High rates of mutation in viral protein
coats and enzymes make it difficult to
develop vaccines and drugs against
viruses
Where do new viruses come
from?
• Mutations of existing viruses
• The dissemination of an existing
virus to a more widespread
population
• Or spread between species
• Epidemic – general outbreak of a
disease
• Pandemic – global epidemic
Fig. 19-9
(a) The 1918 flu pandemic
0.5 µm
(b) Influenza A
H5N1 virus
(c) Vaccinating ducks
Plant viruses
• More than 2,000 types of viral diseases of
plants are known and cause spots on
leaves and fruits, stunted growth, and
damaged flowers or roots
• Most plant viruses have an RNA genome
• Plant viral disease can spread by vertical
transmission from parent plant or by
horizontal transmission from an external
source.
Fig. 19-10
Viroids and Prions: The Simplest
Infectious Agents
• Viroids are circular RNA molecules that
infect plants and disrupt their growth
• Prions are slow-acting, virtually
indestructible infectious misfolded proteins
that cause brain diseases in mammals
• Prions propagate by converting normal
proteins into the prion version
• Scrapie in sheep, mad cow disease, and
Creutzfeldt-Jakob disease in humans are all
caused by prions
Viroids in Plants
Fig. 19-11
Misfolding of proteins to form prions
Remember: Prion - Protein
Prion
Normal
protein
Original
prion
New
prion
Aggregates
of prions
Scrapie in sheep
How Prions Arise
Why is it hard to treat viroid
and prion infections?
• Due to their simple structure, it is
difficult to attack them without
attaching native cell proteins or RNA
Hybrid Viruses
hybrid viruses
avian-human flu
viral replication