Download chapt15_lecture_anim - OCC

Chapter 15
Viruses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The flu pandemic of 1918 killed more people in the United
States than World Wars I and II, the Korean War, and the
Vietnam War combined.
Learning Outcomes
• Explain how viruses spread through a population
and cause disease.
• Understand why many biologists do not consider
viruses to be alive.
• Describe the general structure of viruses.
• Identify how viruses are categorized.
• Describe the major stages of viral replication in a
host cell.
• Contrast the difference between the lytic cycle
versus lysogenic cycle of viruses.
Learning Outcomes
• Explain how viruses cause disease in animals
• Give examples of various virally mediated
human diseases
• Explain how prions and viroids cause disease.
• Explain how viruses cause disease in plants
15.1 Viruses Are Infectious Particles
of Genetic Information and Protein
A. Viruses Are Smaller and Simpler Than
Cells
• 80nm in diameter
– 12x smaller than bacterium
• All viruses have
– Genetic information – DNA or RNA
– Protein coat – capsid
• Some may have an envelope
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.Tobacco mosaic virus
(filamentous)
b.T-even bacteriophage
(spaceship)
RNA
c. Adenovirus
(icosahedral)
Capsid
protein
75 nm
50 nm
TEM (false color)
TEM (false color)
100 nm
TEM (false color)
e. Poxvirus
(oval,enveloped)
d. Herpesvirus
(icosahedral, enveloped)
Envelope
Capsid
200 nm
TEM (false color)
300 nm
TEM (false color)
a: © Dr. O. Bradfute/Peter Arnold/Photolibrary; b: © Eye of Science/Photo Researchers; c: © E.O.S./ Gelderblom/Photo Researchers;
d: © NIAID/Peter Arnold/Photolibrary; e: © George Musil/Visuals Unlimited/Getty Images
Figure 15.1 Viruses of Many Shapes and Sizes.
15.1 Viruses Are Infectious Particles
of Genetic Information and Protein
B. Virus’s Host Range Consists of the
Organisms It Infects
• Virus can enter only a cell that has a
specific receptor on its surface
• Virtually all species of animals, fungi,
plants, protists, and bacteria get viral
infections
• Reservoir of a virus is the site where it
exists in nature
15.1 Viruses Are Infectious Particles
of Genetic Information and Protein
C. Are Viruses Alive?
• Most biologists do not consider a virus to
be alive
– Does not metabolize, respond to stimuli, or
reproduce on its own
• Do have genetic material – do mutate
– Natural selection
• Not part of a taxonomic hierarchy
15.2 Viral Replication Occurs
in Five Stages
1.
2.
3.
4.
5.
•
Attachment
Penetration
Synthesis
Assembly
Release
Amount of time varies
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Protein coat
Viral DNA
Virus
1
Receptor
Host cell
1 Attachment:
Virus binds cell surface receptor.
Viral
DNA
2
Transcription
2 Penetration:
Viral nucleic acid is released inside
host cell.
RNA
3
Replication
3
Translation
3 Synthesis:
Host cell manufactures viral nucleic
acids and proteins.
Coat proteins
and other
proteins
4
Viral DNA
4
Assembly:
New viruses are assembled from
newly synthesized coat proteins,
enzymes, and nucleic acids.
5 Release:
New viruses leave the host cell.
5
Figure 15.2 Viral Replication.
15.3 Cell Death May Be
Immediate or Delayed
A. Some Viruses Kill Cells Immediately
• Lytic infection
– Virus enters cell, immediately replicates and
lyses host
– “Phage therapy” for bacterial infections
• Unlikely to acquire resistance
• Targeted to specific bacteria
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Virus infects new host cell.
Daughter cells carry
Integrated viral DNA.
Host DNA
Lysis occurs; new
viruses released
from host cell.
Viral DNA
Viral DNA released in host cell.
(A) LYTIC PATHWAY
New viruses
assemble
in host cell.
(B) LYSOGENIC PATHWAY
Chromosome
replicates as cell
divides.
Viral proteins Viral DNA
Host cell produces viral
proteins and viral DNA.
Viral DNA incorporated in
host chromosome as prophage.
Figure 15.3 Lysis and Lysogeny.
15.3 Cell Death May Be
Immediate or Delayed
B. Viral DNA Can “Hide” in a Cell
• Lysogenic infection
– Genetic material of a virus is replicated along
with the host cell’s chromosome
• Prophage
– Host not immediately destroyed
– Can switch to lytic pathway
15.4 Effects of a Viral Infection
May Be Mild or Severe
A. Symptoms Result from Cell Death and
the Immune Response
• Influenza virus causes flu
– Dead and damaged cells in the airway cause
the respiratory symptoms of influenza,
including cough and sore throat
– Fever and body aches caused by cytokines
released by immune system
15.4 Effects of a Viral Infection
May Be Mild or Severe
B. Some Animal Viruses Linger for Years
• Latent infection does not produce disease
symptoms
– Viral genetic information is inside the cell
• Herpes simplex virus type I
– Cold sores on lips
– Stressed cells release viruses that infect other cells
– Cold cores are localized death of these cells
15.4 Effects of a Viral Infection
May Be Mild or Severe
B. Some Animal Viruses Linger for Years
• HIV
– Retrovirus – uses reverse transcriptase
– RNA genome
– Infects helper T cells
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Loss of these cells leads to AIDS
Glycoprotein
Reverse transcriptase
RNA
Protein coat
Viral envelope
50 nm
• Human papillomavirus causes cervical
cancer by signaling host cell to divide
continuously
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glycoprotein
1 Virus binds receptors on cell
membrane and enters cell. Enzymes
remove viral protein coat.
Reverse transcriptase
RNA
Protein coat
Viral envelope
2 Reverse transcriptase catalyzes
formation of DNA complementary
to viral RNA.
Host cell
50 nm
3 New DNA strand serves as a
template for complementary
DNA strand.
1
Reverse transcriptase
Viral proteins and RNA
Viral RNA
2
Chromosomal
DNA
5 Viral genes transcribed to RNA.
Some RNA will be packaged
into new viruses.
6
RNA-DNA
hybrid
7
3
Viral DNA
4
6 Viral mRNA translated into HIV
proteins at ribosomes in cytoplasm.
7 Protein coats surround viral
RNA and enzymes.
5
DNA
8 New viruses bud from host cell.
8
Figure 15.4 Replication of HIV.
4 Double-stranded DNA is incorporated
into host cell’s genome.
15.4 Effects of a Viral Infection
May Be Mild or Severe
C. Drugs and Vaccines Help Fight Viral
Infections
• Some antiviral drugs interfere with
enzymes or other proteins that are unique
to viruses
• Few medicines inhibit viruses without
killing infected host cells
• Complicated by genetic variability of
viruses
15.4 Effects of a Viral Infection
May Be Mild or Severe
C. Drugs and Vaccines Help
Fight Viral Infections
• Vaccination teaches
immune system to
recognize virus
• Childhood vaccinations
greatly reduced or
eliminated many viral
Figure 15.5 Viruses from Eggs.
infections
• Fertilized chicken eggs used
to produce viruses
15.5 Viruses Cause Diseases In
Plants
• First virus ever discovered was tobacco
mosaic virus
• Most spread by plant-eating insects
• Spread through plasmodesmata
• Can use posttranscriptional gene silencing
to fight off infections
Figure 15.6 Sick Plants.
Clicker Question
• What organisms DO NOT get viral
infections?
A.Animals (including humans)
B.Plants
C.Bacteria
D.Fungi
E.All of the above CAN get viral infections
15.6 Viroids and Prions Are Other
Noncellular Infectious Agents
A. A Viroid Is an
Infectious RNA
Molecule
• Naked RNA
• Does not encode
proteins
• Interferes with ability
to produce proteins
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Viroid
(circular RNA)
Theodore Diener/USDA Plant Virology Laboratory
Figure 15.7 Viroids Infect Plants.
15.6 Viroids and Prions Are Other
Noncellular Infectious Agents
B. A Prion Is an Infectious Protein
• “Proteinaceous infectious
particle”
• PrP – normal cellular protein
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
– Multiple shapes
– One abnormal and can cause
disease
• Contact with abnormal forms
causes normal form to switch
LM 60 µm
Normal
Diseased
(cow): © Pixtal/age fotostock (RF); (tissue): © Ralph Eagle Jr./Photo Researchers;
(normal prion): © James King–Holmes/Inst. of Animal Health/Photo Researchers; (diseased prion): © BSIP/Photo Researchers
Figure 15.8 Prion Disease.
15.6 Viroids and Prions Are Other
Noncellular Infectious Agents
B. A Prion Is an Infectious Protein
• Abnormal form causes brain cells to die
– Spongiform encephalopathies
– “Mad cow disease” (BSE)
– Human forms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Kuru – cannibalism
• Creutzfeld-Jakob disease (CJD)
• Fatal familial insomnia
LM 60 µm
(cow): © Pixtal/age fotostock (RF); (tissue): © Ralph Eagle Jr./Photo Researchers
Figure 15.8 Prion Disease.
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.c
15.7 Investigating Life: Scientific
Detectives Follow HIV’s Trail
• Ancestor of HIV may be virus called
simian immunodeficiency virus (SIV)
• 5 questions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1.
2.
3.
4.
5.
Are genomes of HIV and SIV similar?
Do viral genes share sequences?
Is SIV common enough to spread?
Do HIV and SIV occur in same places?
Is their a transmission pathway?
15.7 Investigating Life: Scientific
Detectives Follow HIV’s Trail
• SIVsm identified as
source of HIV-2
• Origin of HIV-1
unknown
– SIVcpz met many
criterion
– Only #3 a question
• Later study found
rates in wild at 18%
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SIVcpzANT
Eastern
chimpanzee
U455
LAI
HIV-1 (group M)
ELI
YBF30
HIV-1 (group N)
SIVcpzGAB2
SIVcpzGAB1
SIVcpzUS
ANT70
MVP5180
Central
chimpanzee
HIV-1 (group O)
Figure 15.9 HIV-1 from SIV.
15.7 Mastering Concepts
How could researchers use a
similar strategy to study the origin
of a new influenza virus?