History of Viruses in Botany

Questions and Answers

Viruses discovered in 1883 were identified as larger than bacteria.

False

Dmitri Ivanovsky was the first to use filters to identify viruses.

True

Viruses can replicate their genomes independently of living cells.

False

Capsomeres are the structural units of the protein coat in virus particles.

True Signup and view all the answers

Most viruses are unaffected by interferon.

False Signup and view all the answers

Viruses cannot be observed with a simple microscope but can be observed with an electron microscope.

True Signup and view all the answers

Prions are the largest known infectious particles and are composed of both protein and nucleic acid.

False Signup and view all the answers

Viroids consist of circular double-stranded RNA molecules and have a protein coat.

False Signup and view all the answers

The structure of viral nucleic acids includes only double-stranded RNA.

False Signup and view all the answers

There are four types of symmetry structures in viruses: helical, cubical, binal, and complex.

True Signup and view all the answers

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.

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.