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Questions and Answers
What is the initial step in the assembly of the replication cycle for viruses?
Which step of the assembly cycle requires proteolytic cleavage?
What is primarily involved in the assembly of RNA viruses?
In the assembly of adenoviruses, what assists in the formation of the hexon trimer?
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Which type of viruses undergo lysis of infected cells during egress?
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What is the main characteristic of the Flavivirus-like supergroup in viruses?
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How do enveloped viruses typically exit the infected cell?
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What type of interaction is fundamental for the formation of structural units in viruses?
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What key discovery in 1954 emphasized the helical structure of the TMV particle?
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What did Beijerinck conclude about the cause of tobacco plant infection in 1898?
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Which part of the TMV genome serves as the first mRNA for translating replication-related enzymes?
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Which technique was used to quantify TMV by counting local lesions on plant leaves?
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Which component was found to be part of the tobacco mosaic virus (TMV) structure by Bawden and Pirie in 1936?
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What is the structural unit of the TMV capsid?
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Which virus was the first to be discovered in humans, and when was it discovered?
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What major conclusion was drawn from the Hershey and Chase experiments regarding T2 phage in 1952?
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During which stage of TMV movement does viral replication complexes form in association with the endoplasmic reticulum?
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Which of the following statements is true about the chemical composition of viruses?
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What happens to the TMV virion when the capsid is isolated without RNA?
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Which disease is caused by the first animal virus discovered, and what are its symptoms?
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What is the significance of the UAG codon in the TMV genome expression?
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What type of experiment did Delbruck and Ellis conduct in 1939?
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What role does the movement protein (MP) play in TMV's inter-cellular movement?
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What was the primary conclusion of Loeffler and Frosch's work in 1898?
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What is the primary function of immediate early (IE) genes in DNA viruses?
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Which group of viruses typically requires the conversion of their genome to dsDNA for transcription?
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In which cellular location do RNA viruses predominantly replicate?
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Which type of RNA virus can be directly translated by RNA-dependent RNA polymerase (RdRp)?
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What is the role of RNA polymerase in group 1: dsDNA viruses?
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Which of the following best describes retroviruses?
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What distinguishes pararetroviruses from other retroviruses?
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Which mechanism is employed by negative-strand RNA viruses before translation occurs?
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What type of viral genome do group 6 retroviruses generate after reverse transcription?
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What is a common characteristic of oncogenic DNA viruses?
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What role does the N-terminal region of the TMV movement protein (MP) play in plant viral infection?
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How do the replicase proteins in the viral replication complexes (VRCs) interact with actin filaments?
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In the recent model of TMV cell-to-cell movement, which component is primarily thought to facilitate this movement?
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What is the primary function of the internal ribosome entry site (IRES) in the poliovirus genome?
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Which of the following correctly describes the structure of the poliovirus genome?
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How does poliovirus disrupt the translation of host cellular mRNAs?
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What is the significance of the 5:1 ratio of 126kD to 183kD molecules in VRCs?
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What is the primary role of virus-induced gene silencing (VIGS) in plants?
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Which proteins are primarily involved in cap-dependent translation in eukaryotes?
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How does the protease 2A pro function within the poliovirus life cycle?
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Study Notes
The Discovery of Viruses
- Beijerinck (1898): Filterable agent in tobacco mosaic disease (TMV) was not a bacteria, proposing a "contagium vivum fluidum" (contagious living fluid).
- Stanley (1935): Crystallized TMV particles, proving a proteinaceous nature.
- Bawden and Pirie (1936): Revealed the presence of RNA (5%) in TMV particles.
- Helmut Ruska (1939): Used electron microscopy (EM) to visualize TMV for the first time.
- Loeffler and Frosch (1898): Found filterable agent responsible for foot-and-mouth disease (FMD) in cattle.
- Yellow fever virus: The first human virus discovered in 1901, causing fatal epidemics in tropical regions.
- Twort (1915): Observed bacteriophages, viruses which infect bacteria, during research on vaccinia virus.
- d'Herelle (1915): Independently discovered bacteriophages, describing them as "invisible antagonistic microbe."
- Delbruck and Ellis (1939): Developed the "one-step growth experiment," crucial for understanding bacteriophage growth.
- Luria and Delbruck (1940): Founded the "phage group," initiating research on bacteriophages and their role in bacterial genetics.
- Hershey and Chase (1952): Used radioactive labeling to show DNA, not protein, is the genetic material in bacteriophage T2.
Viral Structure and Composition
- Nucleic acids: Primarily DNA or RNA (never both), forming the viral genome.
- Proteins: Structural proteins make up the capsid, non-structural proteins have various functions.
- Lipids: Only found in enveloped viruses, derived from the cellular lipid bilayer.
- Carbohydrates: Present in glycoproteins and glycolipids, involved in cell recognition and attachment.
The Revised Baltimore Classification System
- Places viruses into seven groups based on the pathway leading to mRNA and protein synthesis.
- Group 1: dsDNA viruses (e.g., herpesviruses, adenoviruses).
- Group 2: (+)ssDNA viruses (e.g., parvoviruses).
- Group 3: dsRNA viruses (e.g., reoviruses, rotaviruses).
- Group 4: (+)ssRNA viruses (e.g., picornaviruses, coronaviruses).
- Group 5: (-)ssRNA viruses (e.g., influenza viruses, rabies virus).
- Group 6: (+)ssRNA (retrovirus) viruses (e.g., HIV, HTLV).
- Group 7: dsDNA (pararetroviruses) viruses (e.g., hepadnaviruses).
Viral Biosynthesis: Gene Expression and Replication
- Transcription: The process of producing mRNA from the viral genome (DNA or RNA).
- Reverse transcription: Conversion of RNA to cDNA, specific to retroviruses.
- Translation: Synthesis of proteins from mRNA, relying on host cell machinery.
- Viral genome replication: Production of new viral genomes.
Gene Expression in DNA Viruses
- Immediate early (IE) genes: Expressed early in infection, influencing cellular processes like entering S phase, inhibiting host defenses, and regulating expression of other viral genes.
- Early (E) genes: Encode enzymes and factors for genome replication.
- Late (L) genes: Produce structural proteins for assembly, some expressed very late.
Viral Replication in the Host Cell
- DNA viruses: Generally infect dividing cells or force quiescent cells into S phase.
- RNA viruses: Replication cycle entirely within the cytoplasm, requiring association with specific intracellular membranes.
(+)ssRNA Viruses
- Transcription and replication carried out by viral-encoded RNA-dependent RNA polymerase (RdRp).
- gRNA serves as both the genome and mRNA, directly translated into proteins.
- ORF1 typically encodes a 126kDa protein with methyltransferase (MTR) and helicase domains.
- ORF2 encodes RdRp, often as part of a larger 183kDa protein due to leaky stop codons.
(-)ssRNA Viruses
- Virus genome exists as a ribonucleoprotein (RNP) complex.
- mRNA must be transcribed from the (-)ssRNA genome before translation can occur.
- Influenza viruses: RNPs enter the nucleus for replication and transcription, often requiring splicing of transcripts.
Retroviruses and Pararetroviruses
- Packaged reverse transcriptase (RT) converts the (+)ssRNA genome into dsDNA.
- dsDNA integrates into host genome, establishing latency and persistent infections.
- Viral mRNA is produced from the integrated dsDNA using host RNA polymerase.
Viral Assembly and Egress
- Assembly of individual structural units: Formation of capsid/nucleocapsid subunits.
- Assembly of capsid shells: Enclosing the viral genome.
- Packaging of viral genome: Incorporation of the viral genome and other components.
- Acquisition of envelope (enveloped viruses): Budding from the cell membrane.
- Exit from infected cell: Through lysis (naked viruses) or budding (enveloped viruses).
- Maturation: Post-assembly modifications, often involving proteolytic cleavage.
TMV: A Model (+)ssRNA Virus
- Transmission: Mechanically transmitted, stable in dead plant debris, persists for decades.
- Disease: Causes necrotic local lesions in infected plants.
- Key discoveries: First filterable virus (Beijerinck), crystallization (Stanley), EM imaging (Ruska), self-assembly (Fraenkel-Conrat), RNA structure (Franklin).
- Genome structure: Small (+)ssRNA genome, gRNA serves as mRNA, 5’ end cap, 3’ end tRNA-like structure, 3’ UTR with complex secondary/tertiary structure.
- Reconstitution experiment: Proved RNA as genetic material, demonstrating infectivity with RNA alone.
TMV movement within an infected plant
- Intracellular movement: Replicated viral RNA associates with endoplasmic reticulum (ER), forming viral replication complexes (VRCs).
- Intercellular movement: VRCs move via plasmodesmata with the help of movement protein (MP), expanding infection to neighboring cells.
- Long-distance movement: Virions enter sieve elements of the phloem, rapidly spreading throughout the plant, causing systemic infection.
VRCs and TMV Replication
- Composition: Replicase proteins, MP, gRNA, ER membrane.
- Helicase activity: Helicase domains within replicase proteins unwind dsRNA.
- 126kDa and 183kDa proteins: Required for VRC formation, with a 5:1 ratio.
VIGS (Virus-Induced Gene Silencing)
- Utilizes TMV as a vector to deliver silencing signals for specific genes in plants.
- Engineered TMV produces mRNAs for target genes, triggering RNA silencing pathways.
- Example: Silencing of PSY and PDS (chlorophyll biosynthesis genes) leads to altered pigmentation.
Poliovirus and the Picornaviridae Family
- Defining features: No envelope, icosahedral symmetry, oral-fecal transmission, gRNA as only mRNA translated into a single polyprotein.
- IRES: Internal ribosome entry site, facilitating translation initiation.
- Causes: Poliomyelitis, common cold, HepA, heart infections, diabetes, foot-and-mouth disease.
- Genome structure: (+)ssRNA genome, VPg (virion protein-genome linked) at 5’ end, 5’ UTR, single large ORF, 3’ UTR, polyA tail.
- Polyprotein: Cleaved by proteases into 11-12 functional proteins for replication.
- Shutdown of host translation: Poliovirus infection inhibits cap-dependent translation of cellular mRNAs.
- Mechanism: Cleavage of elF-4G by 2Apro (polio) or L protease (FMDV), preventing formation of the preinitiation complex at the cap.
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Description
Explore the early discoveries that led to our understanding of viruses, from Beijerinck's identification of filterable agents to the visualization of tobacco mosaic virus with electron microscopy. This quiz covers key figures and their contributions to virology, including bacteriophages and the first human virus. Test your knowledge on significant milestones in the discovery of viruses.
