Transcript video slide - Foothill Technology High School

Viruses: a kind of “borrowed life”
HIV infected T-cell
Viral Structure
• not cells
• small infectious particles w/
– DNA or RNA enclosed in a protein coat
(capsid)
– in some cases, a membranous envelope
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Viral Genomes
• genomes may be 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
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
Membranous Envelopes
• help viruses infect hosts
• surround the capsids of influenza viruses and
many other viruses found in animals
• derived from the host cell’s membrane
– contain a combination of viral and host cell
molecules
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-7
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
Bacteriophages
• AKA phages
• are viruses that infect bacteria
• most complex capsids found among viruses
– w/ elongated capsid head that encloses
their DNA
– A protein tail piece attaches to host and
injects the phage DNA inside
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Viral Reproduction
• obligate intracellular parasites
– can reproduce only within a host cell
• Each virus has a host range
– a limited number of host species that it can
infect
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
VIRUS
1 Entry and
DNA
uncoating
Capsid
3 Transcription
and manufacture
of capsid proteins
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
• Viruses use the
host’s cellular
machinery to
reproduce itself
• enzymes,
• ribosomes,
• tRNAs,
• amino acids,
• ATP,
• and other
molecules
4 Self-assembly of
new virus particles
and their exit from
the cell
Two Reproductive Mechanisms
1. the lytic cycle
2. lysogenic cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Lytic Cycle
• A reproductive cycle that kills the host cell by
lysing it or bursting it open with new viral
progeny
• Bacteriophages and other viruses can
reproduce in this way
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-5-1
1 Attachment
Fig. 19-5-2
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Fig. 19-5-3
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
3 Synthesis of viral
genomes and
proteins
Fig. 19-5-4
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
Fig. 19-5-5
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
• At the start, this cycle replicates viral genome
without destroying the host
• The viral DNA molecule is inserted into the
host cell’s chromosome
– This integrated viral and host DNA is known
as a prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Lysogenic Cycle
• An environmental signal can trigger the virus
genome to exit prophage and switch to the lytic
mode
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-6
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.
Viral Classification
• two key variables used to classify viruses
– DNA or RNA?
– Single-stranded or double-stranded?
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RNA Viruses (Retroviruses)
• Retroviruses use reverse transcriptase to
copy their RNA genome into DNA
– Ex. HIV
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
The viral DNA that
is permanently
integrated into the
host genome is
called a provirus
(unlike
prophage)
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
mRNA
HIV
Membrane of
white blood cell
0.25 µm
HIV entering a cell
New HIV leaving a cell
Viral Diseases
• Viruses damage or kill cells and tissues:
– reproduction
– release of hydrolytic enzymes from
lysosomes
• Some viruses cause infected cells to produce
toxins that lead to disease symptoms
• Others have envelope proteins that are toxic
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Vaccines
• derivatives of pathogens that stimulate the
immune system to mount defenses against the
actual 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Emerging Viruses
• appear suddenly and come to the attention of
scientists
• Outbreaks of “new” viral diseases in humans
are usually caused by existing viruses that
expand their host territory
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Flu epidemics are caused by new strains of
influenza virus to which people have little
immunity
• New viral diseases can emerge when viruses
spread from animals to humans
• 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
• These strains can cause pandemics, global
epidemics
• The “avian flu” is a virus that recently appeared
in humans and originated in wild birds
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