Transcript Class_ch_09_lecture_presentation_PC.pptx
CH8 - PRACTICE QUESTIONS
Viral replication is A) independent of the host cell's DNA but dependent on the host cell's enzymes and metabolism.
B) independent of both the host cell's DNA and the host cell's enzymes and metabolism. C) dependent on the host cell's DNA and RNA.
D) dependent on the host cell's DNA, RNA, enzymes, and metabolism.
© 2015 Pearson Education, Inc.
CH8 - PRACTICE QUESTIONS
Viral replication is
A) independent of the host cell's DNA but dependent on the host cell's enzymes and metabolism.
B) independent of both the host cell's DNA and the host cell's enzymes and metabolism. C) dependent on the host cell's DNA and RNA.
D) dependent on the host cell's DNA, RNA, enzymes, and metabolism.
© 2015 Pearson Education, Inc.
CH7 - PRACTICE QUESTIONS
Enveloped viral membranes are generally ________ with associated virus-specific ________.
A) lipid bilayers / phospholipids B) protein bilayers / lipids C) glycolipid bilayers / phospholipids D) lipid bilayers / glycoproteins © 2015 Pearson Education, Inc.
CH7 - PRACTICE QUESTIONS
Enveloped viral membranes are generally ________ with associated virus-specific ________.
A) lipid bilayers / phospholipids B) protein bilayers / lipids C) glycolipid bilayers / phospholipids
D) lipid bilayers / glycoproteins
© 2015 Pearson Education, Inc.
PowerPoint ® Lecture Presentations prepared by John Zamora Middle Tennessee State University
C H A P T E R
9
Viral Genomes and Diversity
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9.1 Size and Structure of Viral Genomes
• Viral genome size (Figure 9.1) • Smallest circovirus: 1.75-kilobase single strand • Largest megavirus: 1.25-megabase pairs • Viral genomes (Figure 9.2) • Either DNA or RNA genomes • Some are circular, but most are linear © 2015 Pearson Education, Inc.
9.1 Size and Structure of Viral Genomes
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Figure 9.1
•
9.1 Size and Structure of Viral Genomes
Baltimore Classification (7 classes) • Based upon viral genome: • RNA vs DNA, single vs double stranded David Baltimore 1975 Nobel Prize Discovery of Retroviruses
Class I & VII dsDNA (
±
) virus Class II ssDNA (+) virus Class III dsRNA (
±
) virus Class IV ssRNA (+) virus Class V ssRNA ( –) virus Class VI ssRNA (+) retrovirus Transcription of the minus strand Synthesis of the minus strand dsDNA intermediate (replicative form) Transcription of the minus strand mRNA (+) Used directly as mRNA Transcription of the minus strand Reverse transcription dsDNA intermediate Transcription of the minus strand
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Figure 9.2
9.1 Size and Structure of Viral Genomes
Replication DNA Viruses Class I Class II Class VII
classical semiconservative classical semiconservative, discard ( –) strand transcription followed by reverse transcription
Transcription RNA Viruses Class III Class IV Class V Class VI
make ssRNA (+) and transcribe from this to give ssRNA ( –) complementary strand make ssRNA ( –) and transcribe from this to give ssRNA (+) genome make ssRNA (+) and transcribe from this to give ssRNA ( –) genome make ssRNA (+) genome by transcription of ( –) strand of dsDNA
Class I & VII dsDNA (
±
) virus Class II ssDNA (+) virus Class III dsRNA (
±
) virus Class IV ssRNA (+) virus Class V ssRNA ( –) virus Class VI ssRNA (+) retrovirus Transcription of the minus strand Synthesis of the minus strand dsDNA intermediate (replicative form) Transcription of the minus strand mRNA (+) Used directly as mRNA Transcription of the minus strand Reverse transcription dsDNA intermediate Transcription of the minus strand
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Figure 9.2
9.2 Viral Evolution
• Viruses may have arisen after cells • Remnants of cells that replicate with the help of a live cell • Viruses may have played a role in RNA to DNA transition © 2015 Pearson Education, Inc.
Figure 9.3
9.2 Viral Evolution
Although the Virus mediated RNA-DNA transition has holes, it could explain several key processes: 1.
Explains where DNA came from (RNA cells would become extinct).
2.
Explain the Archaea/Bacteria discrepancy: Evolution of DNA metabolism were different events 3.
Based upon “avoidance maneuvers” – viruses staying ahead of hosts © 2015 Pearson Education, Inc.
•
9.2 Viral Evolution
Viral phylogeny • Most viral genes from nature have unknown function (ORF) • This makes it difficult to construct phylogenetic tree Phylogeny of nucleocytoplasmic large DNA viruses using DNA metabolic enzymes © 2015 Pearson Education, Inc.
Figure 9.4
Refresher on Viral Diversity – Chapter 8
Figure 8.19
Figure 8.21
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9.3
Single Stranded DNA Bacteriophages: ɸX174 • Some bacteriophages contain single-stranded DNA genomes of the plus configuration • Transcription of the genome is preceded by synthesis of a complementary strand of DNA © 2015 Pearson Education, Inc.
9.3
Single Stranded DNA Bacteriophages: ɸX174 •
Bacteriophage ɸX174
• Contains a circular single stranded DNA genome inside an icosahedral virion • Very small genome with overlapping genes • Replication occurs via rolling circle replication © 2015 Pearson Education, Inc.
Figure 9.5
9.3
Single Stranded DNA Bacteriophages: ɸX174 rolling circle replication
+ − Cut site at origin ϕX174 replicative form 3′ end of strand Growing point Displaced strand 5 ′ Roll Growing point + 5 ′ 1.
Replicative form DNA is nicked by gene A protein.
2.
New plus strand begins synthesis.
3.
Continued extension of original plus strand with synthesis of new plus strand 6.
ϕX174 replicative form ready for new genome synthesis Roll + 4.
One revolution complete and one progeny virus genome made 5 ′ 5.
Cleavage and ligation by gene A protein + 7.
One ϕX174 genome of plus strand ssDNA
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Figure 9.6
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu •
Bacteriophage T7
:
Infects E. coli
• Icosahedral head and a very short tail • Genome always enters host cell in same orientation • Order of genes on the T7 chromosome influences regulation of virus replication • DNA replication employs T7 DNA polymerase and involves terminal repeats and the formation of concatemers © 2015 Pearson Education, Inc.
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu DNA replication employs T7 DNA polymerase and involves terminal repeats and the formation of concatemers © 2015 Pearson Education, Inc.
Figure 9.8
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu DNA replication employs T7 DNA polymerase and involves terminal repeats and the formation of concatemers © 2015 Pearson Education, Inc.
Figure 9.8
•
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu
Bacteriophage Mu
• Large virus with an icosahedral head, helical tail, and six tail fibers • "Mutator" phage: Induces mutations in host genome • Useful in bacterial genetics; Temperate phage • Invertible G region of genome determines host range • Genome is integrated into the host chromosome via a transposase © 2015 Pearson Education, Inc.
Figure 9.9a
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu
Insertion point A A G C A G C T T C G T C G Host DNA G C C G A C G G C T T C G T C Mu Staggered cuts in host DNA are made by transposase.
A G C A G C G T T G C A A Mu DNA is inserted at host DNA cut site.
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G C C G A A G C A G C G G C T T C G T C Mu 5-Base-pair duplication A G C A G C G T T T C G T C G C A A Repair of DNA leads to formation of 5-base-pair duplication.
Figure 9.9b
9.3
Double-Stranded DNA Bacteriophages: T7 and Mu •
Bacteriophage Mu
• In both lytic and lysogenic pathways, the genome is replicated as part of a larger DNA molecule • Lysogenic state requires sufficient amounts of a repressor protein to prevent transcription of integrated Mu DNA • Genome is packaged into the virion with short (5 bp) sequences of host DNA at either end © 2015 Pearson Education, Inc.
9.5 Viruses of
Archaea
• Most viruses that infect
Archaea
resemble those that infect enteric bacteria • Only double-stranded DNA viruses have been identified so far Spindle Shaped SSV1 -infects
Sulfolobus solfataricus
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Fillamentous SIFV -infects
Sulfolobus solfataricus
Figure 9.10
9.5 Viruses of
Archaea
• Most viruses that infect
Archaea
resemble those that infect enteric bacteria • Only double-stranded DNA viruses have been identified so far Spindle Shaped PAV1 -infects
Pyrococccus abyssi
ATV -infects
Acidianus convivator
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Figure 9.10
9.6 Uniquely Replicating DNA Animal Viruses
• Double-stranded DNA animal viruses that have unusual replication strategies •
Pox viruses
•
Adenoviruses
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9.6 Uniquely Replicating DNA Animal Viruses
•
Pox viruses
• DNA replicates in the cytoplasm Nucleocapsid © 2015 Pearson Education, Inc.
TEM of Smallpox
Figure 9.11
9.6 Uniquely Replicating DNA Animal Viruses
•
Adenoviruses
• Major group of icosahedral, linear, double-stranded DNA viruses • Cause mild respiratory infections in humans • DNA replicates in the nucleus • Replication requires protein primers and avoids synthesis of a lagging strand (Figure 9.12) © 2015 Pearson Education, Inc.
Figure 9.12
9.7 DNA Tumor Viruses
•
Polyomavirus SV40
• Induces tumors in animals • Nonenveloped virion with an icosahedral head • No enzymes in the virion; replicates in host nucleus • DNA is circular • Small genome, has overlapping genes © 2015 Pearson Education, Inc.
Figure 9.13a
9.7 DNA Tumor Viruses
•
Some polyomaviruses cause cancer
• In permissive host cells, virus infection results in the formation of new virions and the lysis of the host cell • In nonpermissive host cells, the virus DNA becomes integrated into host DNA (analogous to a prophage), genetically altering cells in the process © 2015 Pearson Education, Inc.
9.7 DNA Tumor Viruses
SV40 © 2015 Pearson Education, Inc.
Figure 9.13b
9.7 DNA Tumor Viruses
•
Herpesviruses
• Large group of viruses that cause diseases in humans and animals • Able to remain latent for extended periods of time • An important group causes clinical forms of cancer • Example: Epstein –Barr virus • Infection follows attachment of virions to specific cell receptors © 2015 Pearson Education, Inc.
9.7 DNA Tumor Viruses
Three classes of mRNA are produced • Immediate early : encodes five regulatory proteins • Delayed early : encodes DNA replication proteins • Late : encodes structural proteins of the virus particle © 2015 Pearson Education, Inc.
Figure 9.14
9.8 Positive-Strand RNA Viruses
• Replication of positive-strand RNA viruses requires a negative-strand RNA intermediate from which new positive strands are synthesized © 2015 Pearson Education, Inc.
9.8 Positive-Strand RNA Viruses
•
Phage MS2
• An example of a bacterial RNA virus that infects
E. coli
by attaching to pilus MS2 pilus © 2015 Pearson Education, Inc.
Figure 9.15a
9.8 Positive-Strand RNA Viruses
•
Phage MS2
• Possesses a small genome that is directly translated by a combination of host and viral enzymes • Possesses overlapping genes
Lysis protein 5′ Maturation protein Coat Replicase 3′ 1 130 1308 Genetic map of MS2 1335 1724 1761
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3395 3569 Figure 9.15b
9.8 Positive-Strand RNA Viruses
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Figure 9.15c
9.8 Positive-Strand RNA Viruses
•
Poliovirus
• Small virus • Host RNA and protein synthesis are inhibited when poliovirus replication begins Polio victim Egypt 18th Dynasty © 2015 Pearson Education, Inc.
Figure 9.16a
9.8 Positive-Strand RNA Viruses
•
Poliovirus
• Small virus • Viral RNA is translated directly, producing a single long, giant protein (
polyprotein
) that undergoes self-cleavage to generate ~20 smaller proteins necessary for nucleic acid replication and virus assembly (Figure 9.16c) • Contains the VPg protein which facilitates mRNA binding to host ribosomes © 2015 Pearson Education, Inc.
5′ + AAAA 3′ Synthesis of the new plus strand 3′ – 5′ VPg Poliovirus genome + Synthesis of the minus strand AAAA Poly(A) 3′ Translation Polyprotein 5′ Proteases cleave the polyprotein.
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Structural coat proteins Proteases RNA replicase Figure 9.16c
9.8 Positive-Strand RNA Viruses
•
Coronaviruses
• Larger virus • Cause respiratory infections, including SARS, in humans and other animals • SARS = Severe Acute Respiratory Syndrome • Within weeks, SARS spread from Hong Kong to infect individuals in 37 countries in early 2003.
• Between November 2002 and July 2003, an eventual 8,096 cases and 774 deaths reported in multiple countries with the majority of cases in Hong Kong © 2015 Pearson Education, Inc.
OUTBREAK!!!
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9.8 Positive-Strand RNA Viruses
•
Coronaviruses
• Virions: • are enveloped • Contain club-shaped glycoprotein spikes on their surfaces • Produces monocistronic mRNA • Replicate in cytoplasm © 2015 Pearson Education, Inc.
Figure 9.17a
9.8 Positive-Strand RNA Viruses
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Figure 9.17b
9.9 Negative-Strand RNA Animal Viruses
•
Negative-strand RNA viruses
• Negative-strand RNAs are complementary to the mRNA • They are copied into mRNA by an enzyme present in the virion • Only those that infect
Eukarya
are known © 2015 Pearson Education, Inc.
9.9 Negative-Strand RNA Animal Viruses
•
Rhabdoviruses
• Include viruses that cause: • Rabies in animals and humans • Vesicular stomatitis in cattle, pigs, and horses • Enveloped viruses © 2015 Pearson Education, Inc.
Figure 9.18a
9.9 Negative-Strand RNA Animal Viruses
•
Rhabdoviruses
(cont'd) • RNA of rhabdoviruses is transcribed in the host cytoplasm into two distinct classes (Figure 9.18b): 1. Series of mRNAs encoding the structural genes of the virus 2. Positive-strand RNA that is a copy of the complete viral genome © 2015 Pearson Education, Inc.
Transcription by viral RNA polymerase – Strand parental RNA RNA polymer ase mRNAs (+ sense) Translation using host enzymes RNA polymer ase Proteins + Strand RNA – Strand genomic RNA Viral envelope proteins added as virions bud through the host cytoplasmic membrane – Assembly Virions bud through the host cytoplasmic membrane.
Envelope – Progeny virus –
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Figure 9.18b
•
9.9 Negative-Strand RNA Animal Viruses
Influenza
• Enveloped, pleomorphic virus Segmented genome • Surface proteins interact with host cell surface •
Hemagglutinin
causes clumping of red blood cells •
Neuraminidase
breaks down sialic acid component of host cytoplasmic membrane © 2015 Pearson Education, Inc.
Figure 9.19
9.9 Negative-Strand RNA Animal Viruses
Hemagglutinin
> vaccine Seasonal Flu Vaccine © 2015 Pearson Education, Inc.
9.9 Negative-Strand RNA Animal Viruses
Neuraminidase
> Drug Inhibitors Neuraminic Acid Tamiflu © 2015 Pearson Education, Inc.
9.9 Negative-Strand RNA Animal Viruses
• Processes that help influenza elude the host immune system •
Antigenic shift
• Portions of the RNA genome from two genetically distinct strains of virus infecting the same cell are reassorted • Generates virions that express a unique set of surface proteins •
Antigenic drift
• Structure of neuraminidase and hemagglutinin proteins are subtly altered © 2015 Pearson Education, Inc.
•
9.10 Double-Stranded RNA Viruses
Reoviruses
• Nonenveloped nucleocapsid with a double shell of icosahedral symmetry • Virions contain virus-encoded enzymes necessary to synthesize mRNA and the new RNA genomes © 2015 Pearson Education, Inc.
Figure 9.20a and b
9.10 Double-Stranded RNA Viruses
•
Reoviruses
(cont'd) • Genome segmented into 10 –12 molecules of linear double-stranded RNA • Replication occurs exclusively in host cytoplasm within the nucleocapsid (Figure 9.20c) © 2015 Pearson Education, Inc.
9.10 Double-Stranded RNA Viruses
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Figure 9.20c
9.11 Viruses That Use Reverse Transcriptase
•
Retroviruses
(RNA viruses) and
hepadnaviruses
(DNA viruses) use reverse transcriptase for replication © 2015 Pearson Education, Inc.
•
9.11 Viruses That Use Reverse Transcriptase
Reverse Transcriptase
has three sequential activities: (1) RNA-dependent DNA polymerase, (2) RNAse-H activity, and (3) DNA-dependent activity © 2015 Pearson Education, Inc.
Figure 9.21
9.11 Viruses That Use Reverse Transcriptase
•
Retroviruses
(cont'd) • Gene expression and protein processing are complex • All retroviruses have the three genes: •
gag
encodes several small viral structural proteins •
pol
is translated into a large polyprotein • The
env
product is processed into two distinct envelope proteins © 2015 Pearson Education, Inc.
9.11 Viruses That Use Reverse Transcriptase
Alternative read through stop codon required in low amounts © 2015 Pearson Education, Inc.
Translation and process of retrovirus proteins
Figure 9.22
9.11 Viruses That Use Reverse Transcriptase
•
Hepadnaviruses
• Virions small, irregular-shaped particles (e.g. hepatitis B) • Unusual genomes : Tiny, partially double-stranded • Viral replication occurs through an RNA intermediate © 2015 Pearson Education, Inc.
Figure 9.23a
9.11 Viruses That Use Reverse Transcriptase
• • • •
Hepadnaviruses
Reverse transcriptase functions as primer for syntenthesis Four mRNAs are produced The largest mRNA is slightly larger than the genome and used as template for genome synthesis © 2015 Pearson Education, Inc.
Figure 9.23b
•
9.12 Viroids
Viroids
: infectious RNA molecules that lack a protein coat • Smallest known pathogens • Do not encode proteins; completely dependent on host-encoded enzymes • Small, circular, ssRNA molecules • Cause a number of important plant diseases © 2015 Pearson Education, Inc.
Figure 9.25
9.12 Viroids
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Healthy tomato plant Infected with potato spindle tuber viroid
Figure 9.24
9.13 Prions
•
Prions
: infectious proteins whose extracellular form contains no nucleic acid • Known to cause disease in animals (transmissible spongiform encephalopathies) • Host cell contains gene (
PrnP
) that encodes native form of prion protein that is found in healthy animals • Prion misfolding results in neurological symptoms of disease (e.g., resistance to proteases, insolubility, and aggregation) © 2015 Pearson Education, Inc.
9.13 Prions
Reported
United Kingdom Canada
BSE
183,841 17
vCJD
176 2 The pathogenic form (PrP Sc ) is protease resistant, insoluble, and forms aggregates in neural cells © 2015 Pearson Education, Inc.
Section through human brain tissue
Figure 9.27