capsid

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WHAT DOES IT
MEAN TO BE
ALIVE?
Are the tiny viruses infecting this E. coli cell alive?
0.5 mm
“I’D LIKE TO INFECT YOU WITH MY DNA”
(VIRAL REPRODUCTION)
;)
VIRAL VIDEO LINKS
NPR podcast : how viruses invade your body
SciShow: Top 5 Deadliest Disease!!
LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 19
Viruses
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Concept 19.1: A virus consists of a nucleic
acid surrounded by a protein coat
• Viruses were detected indirectly long before they
were actually seen
© 2011 Pearson Education, Inc.
The Discovery of Viruses: Scientific Inquiry
• Tobacco mosaic disease stunts growth of tobacco
plants and gives their leaves a mosaic coloration
• In the late 1800s, researchers hypothesized that a
particle smaller than bacteria caused the disease
• In 1935, Wendell Stanley confirmed this
hypothesis by crystallizing the infectious particle,
now known as tobacco mosaic virus (TMV)
© 2011 Pearson Education, Inc.
Figure 19.2
RESULTS
3 Rubbed filtered
1 Extracted sap 2 Passed sap
through a
sap on healthy
from tobacco
porcelain filter
tobacco plants
plant with
known to trap
tobacco mosaic
bacteria
disease
4 Healthy plants
became infected
Structure of Viruses
• Viruses are not cells, they are infectious particles
© 2011 Pearson Education, Inc.
Capsids and Envelopes
• A capsid is the protein shell that encloses the viral
genome
• Capsids are built from protein subunits called
capsomeres
• A capsid can have various structures
© 2011 Pearson Education, Inc.
Examples of viral structure
Capsomere
RNA
DNA
Membranous
RNA
envelope
Capsid
Head
DNA
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
18  250 nm
20 nm
(a) Tobacco
mosaic virus
Glycoproteins
70–90 nm (diameter) 80–200 nm (diameter)
50 nm
(b) Adenoviruses
80  225 nm
50 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
Concept 19.2: Viruses replicate only in
host cells
• Viruses are obligate intracellular parasites,
which means they can replicate only within a host
cell using the cell’s parts, much like…
© 2011 Pearson Education, Inc.
General Features of Viral Replicative
Cycles
• Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins
• The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules
• Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
© 2011 Pearson Education, Inc.
Animation: Simplified Viral Reproductive Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
1 Entry and
uncoating
DNA
VIRUS
3 Transcription
and manufacture of
capsid proteins
Capsid
2 Replication
HOST
CELL
Viral DNA
mRNA
Viral
DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
A simplified viral replicative cycle.
Replicative Cycles of Phages
• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle (see
animations that follow)
© 2011 Pearson Education, Inc.
Animation: Phage T4 Lytic Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Figure 19.5-1
1 Attachment
Figure 19.5-2
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Figure 19.5-3
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
3 Synthesis of
viral genomes
and proteins
Figure 19.5-4
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
3 Synthesis of
viral genomes
and proteins
Figure 19.5-5
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
5 Release
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
3 Synthesis of
viral genomes
and proteins
The Lysogenic Cycle
• The lysogenic cycle replicates the phage
genome without destroying the host
• The viral DNA molecule is incorporated into the
host cell’s chromosome
• This integrated viral DNA is known as a prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
© 2011 Pearson Education, Inc.
Animation: Phage Lambda Lysogenic and Lytic Cycles
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
• An environmental signal can trigger the virus
genome to exit the bacterial chromosome and
switch to the lytic mode
• Phages that use both the lytic and lysogenic
cycles are called temperate phages
© 2011 Pearson Education, Inc.
Figure 19.6
Phage
DNA
Daughter cell
with prophage
The phage
injects its DNA.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lytic cycle
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
lytic cycle
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
or
lysogenic cycle
is entered
Prophage
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Capsid
Capsid and viral genome
enter the cell
RNA
Envelope (with
glycoproteins)
HOST CELL
Viral genome
(RNA)
Template
mRNA
ER
Capsid
proteins
Copy of
genome
(RNA)
Glycoproteins
The replicative cycle of an enveloped
RNA virus.
New virus
RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found in
viruses that infect animals
• Retroviruses use reverse transcriptase to copy
their RNA genome into DNA
• HIV (human immunodeficiency virus) is the
retrovirus that causes AIDS (acquired
immunodeficiency syndrome)
© 2011 Pearson Education, Inc.
Glycoprotein
Viral envelope
HIV
Capsid
Reverse
transcriptase HIV
RNA (two
identical
strands)
Membrane
of white
blood cell
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 m
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus
HIV: a retrovirus
New HIV leaving a cell
Animation: HIV Reproductive Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Concept 19.3: Viruses, viroids, and prions
are formidable pathogens in animals and
plants
© 2011 Pearson Education, Inc.
Why viruses cause disease in animals,
other than by lysis
• Viruses may damage or kill cells by causing the
release of hydrolytic enzymes from lysosomes
• Some viruses cause infected cells to produce
toxins that lead to disease symptoms
• Others have molecular components such as
envelope proteins that are toxic
© 2011 Pearson Education, Inc.
• Vaccines are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the harmful pathogen
– Vaccines can prevent certain viral illnesses
• Viral infections cannot be treated by antibiotics
• Antiviral drugs can help to treat, though not cure,
viral infections
© 2011 Pearson Education, Inc.
Emerging Viruses
• Mostly spread from animals or massively mutated
pre-existing viruses
– The 2009 flu pandemic was likely passed to humans
from pigs; for this reason it was originally called the
“swine flu”
© 2011 Pearson Education, Inc.
Figure 19.9
1 m
(a) 2009 pandemic H1N1 (b) 2009 pandemic
screening
influenza A virus
(c) 1918 flu pandemic
Viroids and Prions: The Simplest Infectious
Agents
• Viroids are small circular RNA molecules that
infect plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
– Prions propagate by converting normal proteins into the
prion version
– Scrapie in sheep, mad cow disease, and CreutzfeldtJakob disease in humans are all caused by prions
© 2011 Pearson Education, Inc.
Figure 19.11
Prion
Normal
protein
Original
prion
New
prion
Aggregates
of prions
Test your understanding
A
Time
Number of viruses
Number of bacteria
Test Your Understanding, question 8
B
Time
Test Your Understanding, question 6
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