Download 7.6 Viruses

Scene from the 1918 influenza
pandemic.
Scene from the 2003 SARS
Scare
Comparing the size of a virus,
a bacterium, and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25  m
T4 bacteriophage infecting an E. coli cell
0.5 m
4
Infection by tobacco mosaic virus
(TMV)
Viral infection of plants
Viral structure
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
18 × 250 mm
20 nm
(a) Tobacco mosaic virus
Glycoprotein
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
50 nm
(b) Adenoviruses
(c) Influenza viruses
80 × 225 nm
50 nm
(d) Bacteriophage T4
Classes
of
Animal
Viruses
Entry into cell and
uncoating of DNA
DNA
VIRUS
Capsid
Transcription
Replication
HOST CELL
Viral DNA
A simplified viral
reproductive
cycle
mRNA
Viral DNA
Capsid
proteins
Self-assembly of new
virus particles and thei
exit from cell
The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
2
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5
Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4
Head
Tails
Tail fibers
Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
3
Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
The lytic and lysogenic cycles of phage  , a
temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle
is entered
Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Bacterial Defense
• What defense do bacteria have against phage
infection?
• Restriction enzymes
• What do restirciton enzymes do?
• They cut up DNA The bacterial DNA is
modified to protect it from the restriction
endonucleases.
12
Herpesvirus
• Consists of double stranded DNA
• Envelope derived from host cell nuclear
envelope not from plasma membrane
• It, therefore, reproduces within the nucleus
• May integrate its DNA as a provirus
• Tends to recur throughout lifetime of infected
individual.
• Why do you think your immune system never
removes this virus from your body?
14
SARS (severe acute respiratory syndrome), a recently
emerging viral disease
(a) Young ballet students in Hong Kong
wear face masks to protect themselves
from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus
like this one (colorized TEM), so named for the
“corona” of glycoprotein spikes protruding from
the envelope.
The reproductive cycle of an enveloped RNA virus
1 Glycoproteins on the viral envelope
Capsid
bind to specific receptor molecules
(not shown) on the host cell,
promoting viral entry into the cell.
RNA
Envelope (with
glycoproteins)
2 Capsid and viral genome
enter cell
HOST CELL
The viral genome (red)
functions as a template for
synthesis of complementary
RNA strands (pink) by a viral
enzyme.
3
Viral genome (RNA)
Template
5
Complementary
RNA
strands also function as mRNA,
which is translated into both
capsid proteins (in the cytosol)
and glycoproteins for the viral
envelope (in the ER).
mRNA
Capsid
proteins
ER
Glycoproteins
New copies of viral
genome RNA are made
using complementary RNA
strands as templates.
4
Copy of
genome (RNA)
Vesicles
6
transport
envelope glycoproteins to
the plasma membrane.
8 New virus
7 A capsid assembles
around each viral
genome molecule.
RNA Viruses
• Different classes of RNA viruses: single
stranded range from class IV to class VI
• Class IV: invades as mRNA, is ready for
translation
• Clas V: RNA serves as template for mRNA
synthesis
• RNA is transcribed and serves as template for
mRNA and genomic RNA
• Viral replicase enzyme is packages within
viral capsid.
19
Class VI: Retroviruses
20
The structure of HIV, the retrovirus that causes AIDS
Glycoprotein
Viral envelope
Capsid
Why are RNA
Reverse
Viruses
transcriptase
More
deadly than
DNA viruses?
RNA
(two identical
strands)
The reproductive cycle of HIV, a retrovirus
HIV
Membrane of
white blood cell
1 The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
2 Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
HOST CELL
3 Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
4 The double-stranded DNA
is incorporated
as a provirus into the cell’s
DNA.
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
Provirus
5
Proviral genes are
transcribed into RNA molecules,
which serve as genomes for the
next viral generation and as
mRNAs for translation into viral
proteins.
RNA genome
for the next
viral generation
mRNA
6 The viral proteins include capsid
proteins and reverse transcriptase
(made in the cytosol) and envelope
glycoproteins (made in the ER).
8
9
New HIV leaving a cell
New viruses bud
off from the host cell.
Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7
Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
Virus videos
Vaccines / Drugs
• What are vaccines and how do they work?
• Introduce body to harmless or weakened
strain of the virus, so that your immune
system learns to recognize the virus prior to
invasion
• Few drugs around to fight viruses, most
interfere with DNA, RNA or protein synthesis
24
Viroids and Prions
• Viroids are naked circular RNA that
infect plants
• Prions are proteins that infect cells
• Examples of prions seen in scrapies in
sheep, mad-cow disease, and
Creutzfeldt-Jakob disease in humans
• How can a prion spread infection?
• Altered versions of proteins that can
alter other proteins
25
Model for how prions
propagate
Prion
Original
prion
Many prions
Normal
protein
New
prion
Viruses and Cancer
Viral Defenses
Dengue Invasion Video
Review Questions
1. Viruses are not classified as prokaryotes
because:
A.
B.
C.
D.
E.
They contain membrane bound organelles
They are multicellular
They are unicellular
They are acellular
They lack genetic material
2. Which of the following can be found in all
viruses:
A.
B.
C.
D.
E.
Ribosomes
RNA
DNA
A Protein Coat
Cell membrane
3. The virus HIV replicates using reverse
transcriptase. Thus it can be inferred that the
virus
A. Uses only DNA
B. Uses only RNA
C. Uses RNA as a template for DNA
D. Uses DNA as a template for RNA
E. Replicates continuously