Download T4 bacteriophage infecting an E. coli cell - Biology

T4 bacteriophage infecting an E. coli cell
0.5 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparing the size of a virus, a bacterium, and an
animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Infection by tobacco mosaic virus (TMV)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Viruses
•
Size: 20 to ~200nm
•
Genomes
•
–
dsDNA, ssDNA, dsRNA, ssDNA
–
Usually one linear or circular molecule
Capsids and envelopes
–
Capsid: protein coat enclosing viral genome
–
Protein subunits form into regular shapes
• Rods, icosahedrons, polyhedral head with a tail
–
Viral envelope
• Derived from host membrane, with embedded viral
proteins
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 18.4 Viral structure
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
Glycoprotein
18 × 250 mm
20 nm
(a) Tobacco mosaic virus
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
50 nm
(b) Adenoviruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(c) Influenza viruses
80 × 225 nm
50 nm
(d) Bacteriophage T4
Viral host range
• Viruses are obligate intracellular parasites
• Requires appropriate host cell for almost all metabolism
• Host range limited by
– Type of host
– Type of cell
• Egs.
– Measles, polio, smallpox; humans only
– West Nile virus; mosquitos, birds, humans
– Cold viruses; upper respiratory tract cells only
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
West Nile Virus
•
MATERIAL SAFETY DATA SHEET - INFECTIOUS SUBSTANCES
•
SECTION I - INFECTIOUS AGENT
•
NAME: West Nile Virus
•
SYNONYM OR CROSS REFERENCE: West Nile encephalitis virus, West Nile encephalitis, WN virus, WNV,
arbovirus, viral encephalitis
•
CHARACTERISTICS: single stranded, positive sense RNA; lipid-enveloped virion 50 nm diameter; family Flaviviridae,
genus Flavivirus (1), Japanese encephalitis antigenic complex which includes the Japanese encephalitis, Murray
Valley encephalitis, St. Louis encephalitis, and Kunjin viruses (2)
•
SECTION II - HEALTH HAZARD
•
PATHOGENICITY: The disease is characterized by the sudden onset of a febrile "flu-like" illness. Most infections are
mild to moderate and symptoms can include malaise, anorexia, nausea, vomiting, eye pain, headache, myalgia, rash
and lymphadenopathy (3). More severe infections result in aseptic meningitis or encephalitis and symptoms can
include meningismus, mental status changes, occasional seizures, and coma. The overall case fatality rate for WNV
ranges from 4% to 11% (3). Risk of severe neurologic disease after infection increases markedly among persons 50
years of age and older (3).
•
EPIDEMIOLOGY: It is indigenous to Africa, Asia (India and Indonesia), Australia and Europe. In recent years, local
epidemics have been reported in North America, Romania, Russia, southern France, Israel and the Cape Province of
South Africa (4). In temperate and subtropical zones, cases occur in summer or early fall when mosquitoes are most
abundant (4).
•
HOST RANGE: Mammal, reptile (5,6) and avian hosts (4). Mammals (including humans, horses and squirrels) are
considered incidental or dead-end hosts (1,4), however viraemia in avians can be sufficient for transmission of infection
(1).
•
INFECTIOUS DOSE: Unknown.
•
MODE OF TRANSMISSION: Spread by the bite of an infected mosquito. Evidence has also been found demonstrating
indirect transmission from person-to-person through blood transfusions (7) and organ donations (8). West Nile virus
can also be transmitted from an infected animal-to-person through punctures and cuts.
•
INCUBATION PERIOD: Usually 3-14 days, with symptoms lasting 3-6 days (3).
•
COMMUNICABILITY: Not transmitted directly from person-to-person.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Viral reproduction
• Genome enters host cell
• Host enzymes and components used to make
new viral parts
• Viral genome and capsid spontaneously selfassemble
• Completed, infectious viruses exit the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Simplified viral reproductive cycle
Entry into cell and
uncoating of DNA
DNA
VIRUS
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of new
virus particles and their
exit from cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lytic vs lysogenic
• Lytic cycle (virulent phage)
– Release of virus burst and kills host cell
• Lysogenic cycle (temperate phage)
– Viral DNA integrates into host genome
• 1 phage protein prevents transcription of
other phage genes
– Can be transmitted to daughter cells
– Can initiate lytic cycle in response to
environmental signal
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
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.
2 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 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.
Head
Tails
Tail fibers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 λ, a
temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and
proteins are synthesized
and assembled into phages.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Classes of Animal Viruses, FYI only
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Animal viruses
• Viral envelopes
– Derived from plasma membrane of previous
host cell
– Used to enter new host cell
– Viral proteins studded in envelope
• Bind to specific receptors on target cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 18.8 The reproductive cycle of an enveloped RNA virus
1 Glycoproteins on the viral envelope
bind to specific receptor molecules
(not shown) on the host cell,
promoting viral entry into the cell.
Capsid
RNA
Envelope (with
glycoproteins)
2 Capsid and viral genome
enter cell
HOST CELL
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).
3 The viral genome (red)
functions as a template for
synthesis of complementary
RNA strands (pink) by a viral
enzyme.
mRNA
Capsid
proteins
ER
Glycoproteins
Copy of
genome (RNA)
4 New copies of viral
genome RNA are made
using complementary RNA
strands as templates.
6 Vesicles transport
envelope glycoproteins to
the plasma membrane.
8 New virus
7 A capsid assembles
around each viral
genome molecule.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The structure of HIV, the retrovirus that causes AIDS
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
RNA
(two identical
strands)
Retroviruses
• RNA virus with a reverse transcriptase
– Enzyme transcribes RNA to DNA
• Viral DNA enters host DNA (provirus)
– Integrates and becomes permanent in DNA
• Releases new virus from host cell
– Eventually host cell dies from viral
reproduction effects or apoptosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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).
New HIV leaving a cell
9 New viruses bud
off from the host cell.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
8 Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7 Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
AIDS
• Infects helper T-cells (TH), a critical component
of immune system
• TH cells facilitate communication between
immune cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
AIDS
• Immune dysfunction leads to opportunistic
infections
• Viral infections usually controlled by vaccines
– Antibiotics are NOT effective
– There are some new antiviral medications
but….
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
SARS (severe acute respiratory syndrome), a
recently emerging viral disease (NOT on exam)
(a) Young ballet students in Hong Kong
wear face masks to protect themselves
from the virus causing SARS.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(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.
Model for how prions propagate (NOT on exam)
Prion
Original
prion
Many prions
Normal
protein
New
prion
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 18.15 Can a bacterial cell acquire genes
from another bacterial cell? (NOT on exam)
EXPERIMENT Researchers had two mutant strains, one that could make arginine but not tryptophan
(arg+ trp–) and one that could make tryptophan but not arginine (arg– trp+). Each mutant strain and a
mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples
from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal
medium), solidified with agar.
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg− trp+
RESULTS Only the samples from the mixed culture, contained cells that gave rise to colonies on
minimal medium, which lacks amino acids.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(NOT on exam)
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
No
colonies
(control)
Colonies
grew
No
colonies
(control)
CONCLUSION
Because only cells that can make both arginine and tryptophan (arg+ trp+ cells)
can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that
no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell
from the mixture that formed a colony on the minimal medium must have acquired one or more genes
from a cell of the other strain by genetic recombination.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bacterial conjugation (NOT on exam)
Sex pilus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1 µm