Download Virus Structure and Method of Invasion

Virus Structure and Method
of Invasion
• http://videos.howstuffworks.com/tlc/29852understanding-viruses-video.htm
• Viruses called bacteriophages
– Can infect and set in motion a genetic
takeover of bacteria, such as Escherichia
coli
Figure 18.1
0.5 m
• Bacteriophages, also called phages
– Have the most complex capsids found
among viruses
Head
DNA
Tail
sheath
Tail
fiber
80
Figure 18.4d
225 nm
50 nm
(d) Bacteriophage T4
• Recall that bacteria are prokaryotes
– With cells much smaller and more simply
organized than those of eukaryotes
• Viruses Are smaller and simpler still
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Figure 18.2
• Obligate parasites --A virus has a
genome but can reproduce only
within a host cell
• Reproduction is their only true
characteristic of being alive
• Specific to type of cells they
target
-poliomylelitis virus attacks nerve
cells
-hepatitis virus attacks liver cells
Viral Structure
• Not a cell!
• Hijacks biochemical machinery of host
cell to carry out processes necessary to
reproduce
• Obligate intracellular parasite
Viral Genomes
• Viral genomes may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA
• Classes
of
animal
viruses
Table 18.1
Capsids and Envelopes
• A capsid
– Is the protein shell that encloses the viral genome
– Can have various structures
Capsomere
of capsid
RNA
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18  250 mm
20 nm
50 nm
Figure 18.4a, b (a) Tobacco mosaic virus (b) Adenoviruses
• Some viruses have envelopes
– Which are membranous coverings derived
from the membrane
of the host cell
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
Figure 18.4c
50 nm
(c) Influenza viruses
Bacteriophage
Invasion of a cell by a virus
• Virus can lie dormant for many years until it
comes into contact with a suitable host cell
• Binds with molecules on surface of host cell
• Herpes
-whole virus enters cell
• Bacteriophage
-viral DNA injected via hollow tail
Alteration of Cell Instruction
• Virus takes control of cell machinery
• Depends on host for
-ATP
-supply of free nucleotides
• Virus suppresses cell’s normal nucleic
acid replication and protein synthesis
• Manufactures many identical copies of
viral nucleic acid and protein coats
The Lytic Cycle
• The lytic cycle
– Is a phage reproductive cycle that
culminates in the death of the host
– Produces new phages and digests the host’s
cell wall, releasing the progeny viruses
• 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.
Figure 18.6
Head
Tails
Tail fibers
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 Lysogenic Cycle
• The lysogenic cycle
– Replicates the phage genome without
destroying the host
• Temperate phages
– Are capable of using both the lytic and
lysogenic cycles of reproduction
• The lytic and lysogenic cycles of phage
, 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
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle
is induced
Figure 18.7
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
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.
Retrovirus
• Contains RNA
• No DNA to transcribe into mRNA
• Reverse transcriptase injected into cell
by virus with RNA
-enzyme reverses normal transcription
-Produces viral DNA from RNA
-Virus uses DNA to replicate
Some video clips
• Bacteriophage entering a cell
• HIV replication
• http://www.youtube.com/watch?v=HhhRQ
4t95OI
Emerging Viruses
• Emerging viruses
– Are those that appear suddenly or suddenly
come to the attention of medical scientists
• Severe acute respiratory syndrome (SARS)
– Recently appeared in China
(b) The SARS-causing agent is a coronavirus
(a) Young ballet students in Hong Kong
like this one (colorized TEM), so named for the
wear face masks to protect themselves
“corona” of glycoprotein spikes protruding from
from the virus causing SARS.
the envelope.
Figure 18.11 A, B
• Outbreaks of “new” viral diseases in humans
– Are usually caused by existing viruses that
expand their host territory
Viral Diseases in Plants
• More than 2,000 types of viral diseases of
plants are known
• Common symptoms of viral infection include
– Spots on leaves and fruits, stunted growth, and
damaged flowers or roots
Figure 18.12
• Plant viruses spread disease in two major
modes
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from
a parent
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 proteins that cause brain diseases in
mammals
– Propagate by converting normal proteins into
the prion version
Prion
Original
prion
Many prions
Normal
protein
Figure 18.13
New
prion
• Concept 18.3: Rapid reproduction, mutation,
and genetic recombination contribute to the
genetic diversity of bacteria
• Bacteria allow researchers
– To investigate molecular genetics in the
simplest true organisms
The Bacterial Genome and Its
Replication
• The bacterial chromosome
– Is usually a circular DNA molecule with few
associated proteins
• In addition to the chromosome
– Many bacteria have plasmids, smaller circular
DNA molecules that can replicate independently
of the bacterial chromosome
Operons: The Basic Concept
• In bacteria, genes are often clustered into
operons, composed of
– An operator, an “on-off” switch
– A promoter
– Genes for metabolic enzymes
• Bacterial cells divide by binary fission
– Which is preceded by replication of the bacterial
chromosome
Replication
fork
Origin of
replication
Termination
of replication
Figure 18.14
•
Mutation and Genetic
Recombination as Sources of
Genetic
Variation
Since bacteria can reproduce rapidly
– New mutations can quickly increase a
population’s genetic diversity
• An operon
– Is usually turned “on”
– Can be switched off by a protein called a
repressor
• The trp operon: regulated synthesis of
repressible enzymes
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD
trpC
trpE
trpR
trpB
trpA
Operator
Regulatory
gene
mRNA
5
3
RNA
polymerase
Start codon
Stop codon
mRNA 5
E
Protein
Inactive
repressor
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the
promoter and transcribes the operon’s genes.
Figure 18.21a
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Figure 18.21b
Repressible and Inducible Operons: Two
Types of Negative Gene Regulation
• In a repressible operon
– Binding of a specific repressor protein to the
operator shuts off transcription
• In an inducible operon
– Binding of an inducer to an innately inactive
repressor inactivates the repressor and turns on
transcription
• The lac operon: regulated synthesis of
inducible enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
3
mRNA
Protein
No
RNA
made
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
lac operon
DNA
lacl
lacz
3
mRNA
5
lacA
RNA
polymerase
mRNA
mRNA 55'
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
• Inducible enzymes
– Usually function in catabolic pathways
• Repressible enzymes
– Usually function in anabolic pathways
• Regulation of both the trp and lac operons
– Involves the negative control of genes, because
the operons are switched off by the active form
of the repressor protein
Positive Gene Regulation
• Some operons are also subject to positive
control
– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)
• In E. coli, when glucose, a preferred food
source, is scarce
– The lac operon is activated by the binding of a
regulatory protein, catabolite activator protein
(CAP)
Promoter
DNA
lacl
lacZ
CAP-binding site
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
Active
and transcribe
CAP
Inactive lac
repressor
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.
If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces
Figure 18.23a
large amounts of mRNA for the lactose pathway.
• When glucose levels in an E. coli cell
increase
– CAP detaches from the lac operon, turning it off
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA
polymerase
can’t bind
Inactive
CAP
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.
When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Figure 18.23b