scene from the 1918 influenza pandemic.. scene from the 2003 sars scare
TRANSCRIPT
Scene from the 1918 influenza pandemic.
Scene from the 2003 SARS Scare
Comparing the size of a virus, a bacterium, and an animal cell
0.25 μm
Virus
Animalcell
Bacterium
Animal cell nucleus
4
T4 bacteriophage infecting an E. coli cell
0.5 μm
Infection by tobacco mosaic virus (TMV)
Viral infection of plants
Viral structure
18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm
20 nm 50 nm 50 nm 50 nm
(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
RNA
RNACapsomereof capsid
DNACapsomere
Glycoprotein Glycoprotein
Membranousenvelope
CapsidDNA
Head
Tail fiber
Tail sheath
Classes of
Animal Viruses
A simplified viral
reproductive cycle
VIRUS
Capsid proteins
mRNA
Viral DNA
HOST CELL
Viral DNA
Entry into cell anduncoating of DNA
Replication Transcription
DNA
Capsid
Self-assembly of new virus particles and their
exit from cell
The lytic cycle of phage T4, a virulent phageAttachment. The T4 phage uses
its tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.
Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,
injecting the phage DNA intothe cell and leaving an empty
capsid outside. The cell’sDNA is hydrolyzed.
Synthesis of viral genomes and proteins. The phage DNA
directs production of phageproteins and copies of the phagegenome by host enzymes, using
components within the cell.
Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,
and tail fibers. The phage genome ispackaged inside the capsid as the head forms.
Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100
to 200 phage particles.
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4 3
5
Phage assembly
Head Tails Tail fibers
The lytic and lysogenic cycles of phage λ, a temperate phage
Many cell divisions produce a large
population of bacteria infected with the
prophage.
The bacterium reproducesnormally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome,
becoming a prophage.
New phage DNA and proteins are synthesized
and assembled into phages.
Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle.
Certain factorsdetermine whether
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
The cell lyses, releasing phages.
Lytic cycleis induced
Lysogenic cycleis entered
Lysogenic cycleLytic cycle
or Prophage
Bacterialchromosome
Phage
PhageDNA
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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.
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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?
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
Capsid and viral genomeenter cell
2
The viral genome (red)functions as a template for
synthesis of complementary RNA strands (pink) by a viral
enzyme.
3
New virus8
RNA
Capsid
Envelope (withglycoproteins)
Glycoproteins on the viral envelope bind to specific receptor molecules
(not shown) on the host cell, promoting viral entry into the cell.
1
New copies of viralgenome RNA are made
using complementary RNAstrands as templates.
4
Vesicles transportenvelope glycoproteins to
the plasma membrane.
6
A capsid assemblesaround each viralgenome molecule.
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Complementary RNAstrands also function as mRNA,
which is translated into bothcapsid proteins (in the cytosol)and glycoproteins for the viral
envelope (in the ER).
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HOST CELL
Viral genome (RNA)
Template
Capsidproteins
Glyco-proteins
mRNA
Copy ofgenome (RNA)
ER
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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.
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Class VI: Retroviruses
The structure of HIV, the retrovirus that causes AIDS
Reversetranscriptase
Viral envelope
Capsid
Glycoprotein
RNA(two identical
strands)
Why are RNAViruses
More deadly than DNA viruses?
The reproductive cycle of HIV, a retrovirus
Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.
7
The viral proteins include capsid proteins and reverse transcriptase
(made in the cytosol) and envelope glycoproteins (made in the ER).
6
The double-stranded DNA is incorporated
as a provirus into the cell’s DNA.
4
Proviral genes are transcribed into RNA molecules, which serve as genomes for the
next viral generation and as mRNAs for translation into viral
proteins.
5
Reverse transcriptasecatalyzes the synthesis of
a second DNA strandcomplementary to the first.
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Reverse transcriptasecatalyzes the synthesis of aDNA strand complementary
to the viral RNA.
2
New viruses budoff from the host cell.9
Capsids areassembled aroundviral genomes and
reverse transcriptase molecules.
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mRNA
RNA genomefor the next
viral generation
Viral RNA
RNA-DNAhybrid
DNA
ChromosomalDNA
NUCLEUSProvirus
HOST CELL
Reverse transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV Membrane of white blood cell
The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.
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Virus videos
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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
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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
Model for how prions propagate
Prion
Normalprotein
Originalprion
Newprion
Many prions
Viruses and Cancer
Viral Defenses
Dengue Invasion Video
Review Questions
1. Viruses are not classified as prokaryotes because:
A. They contain membrane bound organellesB. They are multicellularC. They are unicellularD. They are acellularE. They lack genetic material
2. Which of the following can be found in all viruses:
A. RibosomesB. RNAC. DNAD. A Protein CoatE. Cell membrane
3. The virus HIV replicates using reverse transcriptase. Thus it can be inferred that the virus
A. Uses only DNAB. Uses only RNAC. Uses RNA as a template for DNAD. Uses DNA as a template for RNAE. Replicates continuously