Transcript Viruses, Prions - De Anza College

13-b Viruses
pp. 387-415
*Animation: Viral Replication
The Microbiology Place
Viral Replication
(Viral Replication Quiz)
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Lytic Cycle
Lysogenic Cycle
DNA Viruses
RNA Viruses
Viruses
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The viral genome ‘directs’ the host’s
metabolic machinery in order to multiply
Virus contains only:
Nucleic acid DNA or RNA
genes for just a few things
1. Structural components, e.g.,
capsid proteins
2. Few enzymes, e.g.,
lysozyme, reverse
transcriptase, integrase,
protease
Virus needs the following
from the host cell:
tRNA
Ribosomes, amino acids,
nucleotides
All enzymes for:
1. Viral protein & enzyme
synthesis, catabolism
2. ATP energy production
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The basic mechanism of viral multiplication
is similar for all viruses
Bacterial virus
Lytic cycle
Lysogenic cycle
Bacteriophage (BF)
T-even BF
Lambda ()
Animal virus
DNA
RNA
Herpesviridae
Retroviridae
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Bacteriophage Multiplication
BF viruses exhibit 2 types of life cycles
• A lytic cycle
– Lyses & kills host cell
E. coli
– Virus name: T-even
bacteriophages (T2,
T4, T6)
• A lysogenic cycle
– Does not lyse or kill the
host cell
– Incorporates DNA into
host cell’s DNA
• Called a prophage
– Remains ‘inactive,’ or
latent
– Virus name: lambda ()
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T-even BF – Lytic Cycle Stages
Name
Process
1
2
Attachment
Penetration
Phage attaches to host cell using tail fibers
Phage lysozyme opens cell wall, tail sheath
contracts forcing tail core and DNA into cell
3
Biosynthesis Phage DNA directs production of viral
components by host cell
Maturation
Assembly of phage particles
Release
Phage lysozyme breaks cell wall (to ‘escape’)
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5
See Fig. 13.11
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1. Attachment
2. Penetration
3. Biosynthesis
4. Maturation
5. Release
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BF Lambda () – Lysogenic Cycle
May proceed through a lytic stage
But, also incorporate DNA into the host
cell’s DNA to begin a lysogenic cycle
–
The inserted DNA now called a prophage
•
During lysogeny, prophage remains latent
•
Bacterial host cell is called a lysogenic cell
–
Host cell is immune to re-infection by the same phage
–
But, not immune to infection by other phages
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Fig. 13.12 Lysogenic cycle of bacteriophage  in E. coli
• Incorporates DNA into host cell’s DNA, begins lysogenic cycle.
• Phage remains inactive.
• Host cells called lysogenic cells.
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Result of Lysogeny – Phage Conversion
Host cells exhibit new properties
Bacteria are able to produce toxins when they carry
a lysogenic phage (prophage)
Bacteria
Disease
Corynebacterium diphtheriae
Streptococci
Clostridium botulinum
Diphtheria
Scarlet fever
Botulism
Vibrio cholera
Cholera
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Overview: RNA Virus Multiplication
Name
Process
1
2
Attachment
Entry
Virus attach to cell membrane
By endocytosis or fusion
3
Uncoating
By viral or host enzymes, separates NA
from protein coat
4
5
6
Biosynthesis
Maturation
Release
Production of NA and proteins
NA and capsid proteins assemble
By budding (enveloped viruses) or rupture
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Attachment, Entry, Uncoating
Pinocytosis
Fusion
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Figure 13.14a, b
Release of Virus, ‘Budding’
Figure 13.20a, b
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RNA Virus Multiplication - Retroviridae
RNA-containing viruses
gp120
HIV
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HIV Attachment, Entry
HIV glycoprotein
Binds to CD4+ receptor
HIV glycoprotein, gp120
Viral RNA
CD4+ Receptor
Cell membrane
Virus fuses to
cell membrane
Viral RNA is
released into cell
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HIV ‘Budding’
• Viral proteins
• Implant in the cell
membrane
• Virus buds off
• Carries some
membrane
• Studded with proteins
• That recognize, infect
new cells
Drawing: Russell Kightley Media
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Capsid Envelope
Reverse Transcriptase
2 identical + RNA strands
8. Budding
1. Attachment
2. Entry
3. Uncoating
Host DNA
RT
7. Viral proteins
processed
Viral RNA
Viral DNA
4. DNA
synthesis
Provirus
Viral proteins
5. RNAv, into
nucleus,
integrates
6. Transcription
HIV Multiplication
Figure 13.19
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HIV Infection
Viral RNA transcribed into viral DNA (RT)
Viral DNA integrated into DNAc of host cell
1. Generates active infection
2. May not produce new HIV, remains ‘hidden’ as
a provirus
3. HIV produced by host cell may also remain as
latent viruses in vacuoles within the host cell
Can persist for decades
Ch. 19, p. 568
4. Provirus or latent virus within host cell is
sheltered from the immune system (AB’s)
5. Reverse transcriptase enzyme step has a high
mutation rate
–
–
–
Lack ‘proofreading’ capability
Mutations probably introduced at every position in the
HIV genome
Indicates the challenges to vaccine / drug
development
Ch. 19, p. 568
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Active HIV Infection
Ch. 19, p. 568-9
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Latent HIV Infection
Latent
Activated
Ch 19, p. 568-70; Figure 19.14 a, b
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Progression of HIV Infection
From initial HIV infection to AIDS takes
about 10 years (in US)
– Devastates the immune system
Many diseases associated with HIV infection
– Protozoa (Cryptosporidium, Toxoplasma)
– Viruses (CMV, Herpes simplex)
– Bacteria (Mycobacterium [TB])
– Fungi (Pneumocystis, Histoplasma, Candida)
– Cancers (Kaposi’s sarcoma)
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Latent & Persistent Viral Infections
• Latent
– Virus remains in
asymptomatic host
cell for long periods
– Herpesviruses:
cold sores,
shingles
• Persistent
– Disease processes
occurs over a long
period; generally is
fatal
– Morbillivirus
(measles virus):
Subacute sclerosing
panencephalitis
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Q:
1. This figure shows an enveloped virus. During
which phase of the life cycle is this envelope
acquired?
a) Penetration
b) Biosynthesis
c) Attachment
d) Release
2. Which of the following is not used for
classification of viruses?
a) Size
b) Disease symptoms
a) Nucleic acid
b) Method of replication
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Q:
1. The following steps occur during bacteriophage
replication. What is the second step?
a) Lysis
b) Penetration
c) Biosynthesis
d) Attachment
2. What is the name given to viral DNA
incorporated into a lysogenic cell?
a) Bacteriophage
b) Latent phage
c) Oncogenic virus
d) Prophage
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Q:
1. What is the name of the process that involves
both the release and the enveloping of the virus?
a) Lysogeny
b) Budding
c) Conjugation
d) Reverse transcription
2. During lysogeny, the phage remains latent.
a) True
b) False
3. Viruses range in size from 20 to 1000 nm in
length, and are easily seen using a compound
light microscope.
a) True
b) False
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Q:
1. HIV can evade the immune system by all of the
following means except which one?
a) Remaining in vacuoles
b) Forming a provirus
c) Destroying the host’s antibodies
d) Infecting by cell-cell fusion
2. HIV infects the CD8 T cells as well as CD4 cells.
a) True
b) False
3. Heterosexual sex is the most common form of
HIV transmission.
a) True
b) False
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Definitions
Budding: process which forms the envelop around the capsid release of the
enveloped virus through the plasma membrane of an animal cell
Lytic cycle: A mechanism of phage multiplication that results in host cell lysis
Lysogenic cycle: Stages in viral development that result in the incorporation of viral
DNA into host DNA
Lysozyme: A BF enzyme capable of hydrolyzing bacterial cell walls
Persistent viral infection: a disease process that occurs gradually over a long time
Phage: another name for bacteriophage, a virus that infects bacterial cells
Phage conversion: genetic change in the host cell resulting from infection by a BF
Pinocytosis: the engulfing of virus by infolding of the plasma membrane
Prophage: phage DNA inserted into the host cell’s DNA, ‘pops’ out of DNA to initiate
the lytic cycle
Provirus: viral DNA integrated into the host cell’s DNA, but never comes out of the
chromosome and is protected from the host’s immune system and
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antiviral drugs
Viral Infections – Swine Flu
HA: Hemagglutinin
NA: Neuraminidase
PA: RNA polymerase subunit
PB1: RNA polymerase subunit
PB2: RNA polymerase subunit
NP: Nucleoprotein
M: Matrix protein, M1, M2
NS: Non-structural proteins
CDC Influenza Laboratory, May 2009
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• Hemagglutinin
–
–
–
–
Responsible for binding the virus to the cell
Also cause RBC’s to clump together
At least 16 different HA antigens (H1-H16)
The first 3, H1, H2, H3 are found in human influenza
viruses
• Neuraminidase
– Catalyze hydrolysis of terminal sialic acid residues
from viruses and host cell receptors
– Nine influenza subtypes known (N1-N9)
– N1, N2, N3 known in humans
• Matrix proteins
– Structural proteins linking viral envelope to virus core
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• New strain contains genes from 4 different flu
viruses
• The CDC identified 4 component strains as:
– North American swine influenza
– Swine influenza typically found in Asia, Europe
– North American avian influenza
– Human influenza
• Human populations have not been vaccinated
or naturally immunized
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• Pigs are susceptible to influenza viruses that
also infect birds and humans
• Act as a ‘mixing vessel’ allowing reassortment
• Re-assortment: (swap genes)
– Viral genome has 8 independent pieces of RNA
– The 8 strands of RNA from different viruses can
mix together
– The reassorted strain will share properties of its
predecessors
– And form a new type of virus
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• New strain appears to be result of
reassortment of these viruses:
– Human influenza
– Swine influenza
• Genetic characterization:
– HA gene similar to swine flu viruses in the US
– NA & M genes similar to swine versions in EU
• The HA in the new virus comes from pigs
– But some of the other genes come from bird
and human
• The CDC calls the mixture ‘very unusual’
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