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Questions and Answers
Viruses discovered in 1883 were identified as larger than bacteria.
Viruses discovered in 1883 were identified as larger than bacteria.
False
Dmitri Ivanovsky was the first to use filters to identify viruses.
Dmitri Ivanovsky was the first to use filters to identify viruses.
True
Viruses can replicate their genomes independently of living cells.
Viruses can replicate their genomes independently of living cells.
False
Capsomeres are the structural units of the protein coat in virus particles.
Capsomeres are the structural units of the protein coat in virus particles.
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Most viruses are unaffected by interferon.
Most viruses are unaffected by interferon.
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Viruses cannot be observed with a simple microscope but can be observed with an electron microscope.
Viruses cannot be observed with a simple microscope but can be observed with an electron microscope.
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Prions are the largest known infectious particles and are composed of both protein and nucleic acid.
Prions are the largest known infectious particles and are composed of both protein and nucleic acid.
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Viroids consist of circular double-stranded RNA molecules and have a protein coat.
Viroids consist of circular double-stranded RNA molecules and have a protein coat.
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The structure of viral nucleic acids includes only double-stranded RNA.
The structure of viral nucleic acids includes only double-stranded RNA.
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There are four types of symmetry structures in viruses: helical, cubical, binal, and complex.
There are four types of symmetry structures in viruses: helical, cubical, binal, and complex.
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Study Notes
Discovery and Early Studies of Viruses
- Adolf Mayer identified viruses in 1883 while researching mosaic tobacco leaves, noting their smaller size compared to bacteria.
- Dmitri Ivanovsky, in 1892, filtered the sap from infected tobacco leaves, proving the presence of infectious agents smaller than bacteria, and coined the term "virus," meaning "poison" in Latin.
Virology Overview
- Virology is the scientific study of viruses and viral diseases.
- Viruses are the simplest and smallest forms of life, capable of replicating only within living susceptible cells.
Structure and Composition of Viruses
- Viruses consist of:
- A nucleic acid genome composed of either DNA (Deoxyribonucleic acid) or RNA (Ribonucleic acid).
- A protein coat known as the capsid enclosing the genome.
- In some cases, a lipid membrane called an envelope.
Definitions
- Virion: A complete infectious virus particle composed of RNA or DNA core with a protein coat and possibly an external envelope.
- Capsid: The protein shell that encases the nucleic acid genome.
- Capsomeres: Morphological units seen in icosahedral viruses that cluster polypeptides.
- Nucleocapsid: The protein-nucleic acid complex representing the packaged form of the viral genome.
General Properties of Viruses
- Virus particles range from 20-500 nm in diameter.
- Obligatory intracellular microorganisms that replicate inside host cells.
- Lack organelles and rely on host cells for replication and metabolic functions.
- Antibiotics are ineffective against viruses, but they are sensitive to interferon.
- Cannot grow on artificial media; they need living cells for proliferation.
- Some viruses can cause latent infections.
- Only visible under an electron microscope.
Classification Basis for Viruses
- Classification is based on:
- Virion morphology: Size, shape, symmetry, peplomers, and membrane presence.
- Genome properties: Type (DNA or RNA), size, strand type (single or double).
- Replication methods and genome organization.
- Protein characteristics: Number, size, amino acid sequences.
- Antigenic properties: Reaction to various antisera.
- Physicochemical properties: Stability, molecular mass, buoyant density, susceptibility to agents.
- Biological properties: Natural host range and mode of transmission.
Viral Nucleic Acids
- Genetic information is organized as:
- Double-stranded DNA in many organisms.
- Single-stranded DNA in certain viruses.
- Single-stranded RNA in myxoviruses.
- Double-stranded RNA in reoviruses.
- Distinction between DNA and RNA can be made using DNAase and RNAase, using acridine orange stain for visualization.
Viroids and Prions
- Viroids: Composed of circular single-strand RNA, lack a protein coat or envelope, and cause diseases in plants, not in humans.
- Prions: The smallest known infectious particles, consisting solely of protein, causing fatal diseases like scrapie in sheep and kuru in humans.
Viral Symmetry and Structure
- Nucleocapsid structure results from the arrangement of viral nucleic acid with the capsid, leading to different symmetries:
- Helical symmetry
- Cubical symmetry
- Binal symmetry
- Complex symmetry
- Classification of viruses also considers characters of viral nucleic acid, including type, shape, molecular weight, and replication site.
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Description
Explore the discovery of viruses in plants, focusing on the groundbreaking research by Adolf Meyer and Dmitri Ivanovsky. This quiz covers the early identification of viruses and their impact on agriculture, particularly in tobacco. Test your knowledge of these pivotal moments in the study of infectious agents.
