Transcript video slide - HCC Learning Web

Chapter 19
Viruses
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: A Borrowed Life
• Viruses lead “a kind of borrowed life” between
life-forms and chemicals
• Viruses are technically not alive; they need a
host to reproduce.
• Viruses are considered “vectors” – they bounce
from host to host, causing disease but also
transferring genetic material (process of
“transduction”)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Process of Transduction between
a virus and bacteria
0.5 µm
Concept 19.1: A virus consists of a nucleic acid
surrounded by a protein coat
• Viruses are not cells
• Viruses are very small infectious particles
consisting of nucleic acid enclosed in a protein
coat and, in some cases, a membranous
envelope
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Viral Genomes
• Viral genomes may consist of either
– Double- or single-stranded DNA, or
– Double- or single-stranded RNA
• Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus (more on
this later in the lecture)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Capsids and Envelopes
• A capsid is the protein shell that encloses the
viral genome; it is built from protein subunits
called capsomeres [explain reason]
• Some animal viruses have membranous
envelopes that help them infect hosts
• Viral envelopes, which are derived from the
host cell’s membrane, contain a combination of
viral and host cell molecules
• Viruses have different structures (next slide)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Glycoproteins
Glycoprotein
18  250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
virus
50 nm
Tail
fiber
80  225 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
Concept 19.2: Viruses reproduce only in host cells
• Viruses are obligate intracellular parasites,
which means they can reproduce only within a
host cell
• Each virus has a host range, a limited number
of host cells that it can infect. Phages (bacterial
viruses) are broad; animal viruses are specific
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
General Features of Viral Reproductive 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
Animation: Simplified Viral Reproductive Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Summary of Video
VIRUS
1 Entry and
DNA
uncoating
Capsid
Transcription
and manufacture
of capsid proteins
3
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
Reproductive Cycles of Phages
• Phages have two reproductive mechanisms:
the lytic cycle and the lysogenic cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Lytic Cycle
• The lytic cycle is a phage reproductive cycle
that culminates in the death of the host cell,
producing new phages by digesting the host’s
cell wall, releasing the progeny viruses
• A phage that reproduces only by the lytic cycle
is called “virulent” – (infective strain)
• Bacteria have defenses against phages,
including restriction enzymes that recognize
and cut up specific sequences of phage DNA
[explain why bacterial DNA is undigested]
Animation: Phage T4 Lytic Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Summary of Video
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
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 cell divides, it copies the phage DNA and
passes that copy on to daughter cells, until…
• An environmental signal triggers 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
Animation: Phage Lambda Lysogenic and Lytic Cycles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Summary of Video
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
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
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and proteins
are synthesized and
assembled into phages.
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Summary of Cycles
Phage
DNA
The phage attaches to a
host cell and injects its DNA
Bacterial
chromosome
Lytic cycle
• Virulent or Temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages
Prophage
Lysogenic cycle
• Temperate phage only
• Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the
chromosome and initiate a lytic cycle
Reproductive Cycles of Animal Viruses
• There are two key variables used to classify
viruses that infect animals:
– Is the genetic material DNA or RNA?
– Is the DNA or RNA single-stranded or doublestranded?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
DNA
(+) sense
RNA
(-) non-sense
special (+) sense
Viral Envelopes
• Many animal viruses have a membranous envelope
• Viral glycoproteins on the envelope bind to specific
receptor molecules on the surface of a host cell
• Viral envelopes are also formed, in part, from the
host cell’s plasma membrane as the virus exits
from the host cell (shear forces tear off a piece of
the plasma membrane)
[Explain why eukaryotic cells do not lyse] –
productive cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins
ER
Glycoproteins
Copy of
genome (RNA)
New virus
RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found in viruses that infect
animal cells
• Retroviruses use reverse transcriptase (a.k.a. RNA-dependent
DNA polymerase) to copy their RNA genome into cDNA, which is
then integrated into the host genome
• The viral DNA that is integrated into the host genome is called a
provirus. Unlike a prophage, a provirus remains a permanent
resident of the host cell
• The host’s RNA polymerase transcribes the proviral DNA into RNA,
which function as mRNA for synthesis of viral proteins and as
genomes for new virus particles released from the cell
• HIV (Human Immunodeficiency Virus) is the retrovirus that causes
AIDS (Acquired ImmunoDeficiency Syndrome)
Animation: HIV Reproductive Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Summary of Video
Viral envelope
Glycoprotein
Capsid
Reverse
transcriptase
HIV
RNA (two
identical
strands)
HIV
Membrane of
white blood cell
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
HOST
CELL
(T4 cell)
0.25 µm
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
New HIV leaving a cell
mRNA
Evolution of Viruses
• Since viruses can only reproduce within cells, they
probably evolved as bits of cellular nucleic acid
• Candidates for the source of viral genomes are
plasmids (independent circular DNA in bacteria
and yeasts) and transposons (small mobile DNA
segments within a chromosome)
• Plasmids, transposons, and viruses are all mobile
genetic elements (vectors)
• The process of DNA transfer between bacterium is
called “conjugation”
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Process of Conjugation between bacteria
Sex pilus
G-ma got the Nobel Prize…
Concept 19.3: Viruses, viroids, and prions are
formidable pathogens in animals and plants
• Diseases caused by viral infections affect
humans, agricultural crops, and livestock
worldwide
• Smaller, less complex entities called viroids
and prions also cause disease in plants and
animals, respectively
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Viral Diseases in Animals
• Viruses may damage or kill cells by causing the
release of hydrolytic enzymes from lysosomes,
which lyse the host cell (and allow the release
of progeny virus)
• Some viruses cause infected cells to produce
toxins that lead to disease symptoms. Others
have envelope proteins that are toxic to the
host, also causing uncomfortable disease
symptoms
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Vaccines are harmless derivatives of
pathogenic microbes that stimulate the immune
system to mount an enhanced defense against
the pathogen, preventing viral illness [explain
primary vs. secondary immune response]
• Viral infections cannot be treated by antibiotics.
Antibiotics target prokaryotes (i.e. bacteria…)
• Antiviral drugs can help to treat, not cure, viral
infections
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Emerging Viruses
• Emerging viruses are those that appear
suddenly and come to the attention of
scientists (due to the rapid onset of disease)
• Outbreaks of “new” viral diseases in humans
are usually caused by existing viruses that
expand their host (species infected) territory
• Viral strains that jump species can exchange
genetic information with other viruses to which
humans have no immunity
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Epidemics are caused by new strains of a virus to
which people have little or no immunity
• Viral diseases in a small isolated population can
emerge and become global, causing a pandemic, or
global epidemic. The H1N1 influenza virus caused the
1918 “Spanish Flu” pandemic, killing 40 million people
• The “avian flu” is a virus that recently appeared in
humans and originated in wild birds. The SARS
outbreak in Hong Kong was caused by the H5N1
influenza virus. This disease was quelled by the culling
of 1.5 million birds and the vaccination of the
remainder
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
(a) The 1918 flu pandemic
0.5 µm
(b) Influenza A
H5N1 virus
(c) Vaccinating ducks
Viral Diseases in Plants
• Most plant viruses possess an RNA genome
• More than 2,000 types of viral borne plant disease
are known, causing spots on leaves and fruits,
stunted growth, and damaged flowers or roots
• Plant viruses spread disease in two major modes:
– Horizontal transmission, entering through
damaged cell walls (caused by hoeing)
– Vertical transmission, inheriting the virus from a
parent (caused by reproduction)
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Viroids and Prions: The Simplest Infectious Agents
• Viroids are very 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 (how do you destroy a prion?)
• Prions propagate by catalyzing the conversion
of normal proteins to abnormal prions
• Scrapie in sheep, mad cow disease, and
Jakob-Creutzfeldt disease in humans are all
caused by prions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Conformational change induced by a prion
Prion
Normal
protein
Original
prion
New
prion
Aggregates
of prions
You should now be able to:
1. Explain how capsids and envelopes are formed
2. Distinguish between the lytic and lysogenic
reproductive cycles
3. Explain why viruses are obligate intracellular
parasites
4. Describe the reproductive cycle of an HIV
retrovirus
5. Describe three processes that lead to the
emergence of new diseases
6. Describe viroids and prions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings