what does it mean to be alive?. are the tiny viruses infecting this e. coli cell alive? 0.5 mm
TRANSCRIPT
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 PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures byErin Barley
Kathleen Fitzpatrick
Viruses
Chapter 19
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
Extracted sapfrom tobaccoplant withtobacco mosaicdisease
RESULTS
Passed sapthrough aporcelain filterknown to trapbacteria
Healthy plantsbecame infected
Rubbed filteredsap on healthytobacco plants
1 2 3
4
Structure of Viruses
• Viruses are not cells, they are infectious particles
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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
Capsomereof capsid
RNA CapsomereDNA
Glycoprotein Glycoproteins
Membranousenvelope RNA
CapsidHead
DNA
Tailsheath
Tailfiber
18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)
20 nm 50 nm 50 nm 50 nm(a)Tobacco
mosaic virus(b) Adenoviruses (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…
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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.
© 2011 Pearson Education, Inc.
Animation: Simplified Viral Reproductive Cycle
Right-click slide / select “Play”
VIRUS
2
1
3
4
Entry anduncoating
Replication
Transcriptionand manufacture ofcapsid proteins
Self-assembly ofnew virus particlesand their exit fromthe cell
DNA
Capsid
HOSTCELL
Viral DNA
ViralDNA
mRNA
Capsidproteins
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.
© 2011 Pearson Education, Inc.
Animation: Phage T4 Lytic Cycle
Right-click slide / select “Play”
Figure 19.5-1
Attachment1
Figure 19.5-2
Attachment
2
1
Entry of phageDNA anddegradation of host DNA
Figure 19.5-3
Attachment
2
1
3
Entry of phageDNA anddegradation of host DNA
Synthesis ofviral genomesand proteins
Figure 19.5-4
Attachment
2
1
43
Entry of phageDNA anddegradation of host DNA
Synthesis ofviral genomesand proteins
Assembly
Phage assembly
Head Tail Tailfibers
Figure 19.5-5
Attachment
2
1
5
43
Entry of phageDNA anddegradation of host DNA
Release
Synthesis ofviral genomesand proteins
Assembly
Phage assembly
Head Tail Tailfibers
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.
© 2011 Pearson Education, Inc.
Animation: Phage Lambda Lysogenic and Lytic Cycles
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• 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
New phage DNA and proteinsare synthesized and assembledinto phages.
The cell lyses, releasing phages.
Phage
PhageDNA
The phageinjects its DNA.
Bacterialchromosome
Lytic cycle
lytic cycleis induced
or
Phage DNAcircularizes.
Certain factorsdetermine whether
lysogenic cycleis entered
Lysogenic cycle
Prophage
Daughter cellwith prophage
Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.
Cell divisionsproduce apopulation ofbacteria infectedwith the prophage.
The bacterium reproduces,copying the prophage andtransmitting it to daughtercells.
Phage DNA integrates intothe bacterial chromosome,becoming a prophage.
Capsid
RNA
Envelope (withglycoproteins)
Capsid and viral genomeenter the cell
HOST CELL
Viral genome(RNA)Template
mRNA
ERCapsidproteins
Copy ofgenome(RNA)
New virus
Glyco-proteins
The replicative cycle of an enveloped RNA 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
Reversetranscriptase HIV
Viral envelope
Capsid
RNA (twoidenticalstrands)
HOSTCELL
Viral RNAReversetranscriptase
RNA-DNAhybrid
DNA
NUCLEUSProvirus
ChromosomalDNA
RNA genomefor thenext viralgeneration
mRNA
New virus
HIV
Membraneof whiteblood cell
0.25 m
HIV entering a cell
New HIV leaving a cell
HIV: a retrovirus
© 2011 Pearson Education, Inc.
Animation: HIV Reproductive Cycle
Right-click slide / select “Play”
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
(c) 1918 flu pandemic
2009 pandemicscreening
(b)2009 pandemic H1N1influenza A virus
(a)
1 m
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 Creutzfeldt-
Jakob disease in humans are all caused by prions
© 2011 Pearson Education, Inc.
Figure 19.11
Prion
Normal protein
Originalprion
Newprion
Aggregatesof prions
Test your understanding
Test Your Understanding, question 8
Time Time
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Test Your Understanding, question 6