Viruses Overview and Structure

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Questions and Answers

Which characteristic is NOT used to classify viruses?

Shape of the capsid

Presence of a cell wall (correct)

Type of genetic material

Type of host that it infects

What is the primary role of the capsid in a virus?

To facilitate cell division

To protect the nucleic acid genome (correct)

To replicate the viral genome

To provide energy for replication

Which type of microscopy is required to visualize most viruses?

Fluorescence microscopy

Optical microscopy

Electron microscopy (correct)

Light microscopy

Viruses replicate by directing which process in a host cell?

Viral nucleic acid replication Signup and view all the answers

What component of a virus is made up of proteins called capsomeres?

Capsid Signup and view all the answers

Which of the following describes the size range of viruses?

20 nm to 400 nm Signup and view all the answers

What is NOT a function of capsid proteins?

Energy production for replication Signup and view all the answers

Which viral structural component may be present in some viruses but not in others?

Envelope Signup and view all the answers

Viruses depend on which of the following for protein and nucleic acid production?

Ribosomes and enzymes of the host cell Signup and view all the answers

Icosahedral symmetry refers to which aspect of a virus?

Shape of the viral capsid Signup and view all the answers

What type of viruses are described as tough and relatively resistant to inactivation by various agents?

Non-enveloped viruses Signup and view all the answers

How are non-enveloped viruses typically transmitted?

By fecal/oral route, fomites, or small droplets Signup and view all the answers

Which structures are characteristic of bacteriophages?

Nucleic acid and protein with a tail structure Signup and view all the answers

What is a requirement for all viruses regarding mRNA synthesis?

They need to synthesize positive (+) mRNA. Signup and view all the answers

What is the primary component that non-enveloped viruses are released from infected cells?

Lysis Signup and view all the answers

What geographic region is most commonly associated with the spread of non-enveloped viruses?

Areas with poor sanitation and hygiene Signup and view all the answers

Which statement accurately reflects the nature of viral genomes?

Understanding viral genomes is essential for classifying viruses. Signup and view all the answers

What does a positive (+) nucleic acid represent in viral genetics?

The gene coding sequence Signup and view all the answers

What are viruses primarily considered?

Acellular microbes or infectious particles Signup and view all the answers

Which of the following correctly describes the behavior of viruses outside and inside a host cell?

Behave as non-living structures outside and living organisms inside Signup and view all the answers

Which of the following components distinguishes viruses from living cells?

Possession of either DNA or RNA Signup and view all the answers

What is a key characteristic of animal viruses?

They require a host cell for replication Signup and view all the answers

What are viroids, satellites, and prions classified as?

Subviral entities Signup and view all the answers

Which structural component is critical for the classification of viruses?

Capsid structure Signup and view all the answers

What term describes viruses that have the potential to cause cancer?

Oncogenic viruses Signup and view all the answers

What defines the viral replication cycle?

It has six distinct stages Signup and view all the answers

What role does single-stranded positive-sense RNA play within the host cell?

It functions as messenger RNA (mRNA). Signup and view all the answers

Which of the following viruses has a segmented genome?

Orthomyxoviruses Signup and view all the answers

How are viruses commonly named after cytopathology?

By the types of cells they infect. Signup and view all the answers

What does the 'ds' in dsDNA stand for?

Doubled strands Signup and view all the answers

What defines viral tropism?

The specific range of host species a virus can infect. Signup and view all the answers

Which virus is named after its association with a specific disease?

Rift Valley Fever virus Signup and view all the answers

Which factor is NOT involved in determining viral tropism?

Environmental temperature of the host. Signup and view all the answers

Which classification system has been established by the International Committee on Taxonomy of Viruses (ICTV)?

Hierarchical taxonomy Signup and view all the answers

What role do viral glycoproteins play in viral infection?

They target specific host cell receptors. Signup and view all the answers

What type of nucleic acid represents the viral genome in DNA-based viruses?

Both dsDNA and ssDNA Signup and view all the answers

Which naming convention categorizes viruses based on their biochemical features?

Naming after viral features such as shape or sensitivity Signup and view all the answers

Which genus of viruses does the measles virus belong to?

Morbillivirus Signup and view all the answers

What is meant by 'permissivity' in the context of viral infections?

The ability of a host cell to support viral replication. Signup and view all the answers

Study Notes

Viruses

Acellular organisms, also known as infectious particles or acellular microbes.
Non-living outside a host cell, existing as virions.
Living inside a host cell, behaving as viruses.
Viruses that infect animals are known as animal viruses.
Distinguished from living cells by five properties:
- Possess either DNA or RNA, unlike living cells which have both.
- Unable to replicate independently.
- Don't divide via binary fission, mitosis, or meiosis.
- Lack genes and enzymes for energy production.
- Depend on host cell for protein and nucleic acid production.
Invisible by light microscopy, requiring an electron microscope for visualization.
Range in size from 20 nm to 400 nm.
Can infect any living organism including animals, plants, bacteria, fungi, and protists.

Virus Structure

Composed of:
- Genome: DNA or RNA
- Capsid: Protects the genome.
- Nucleocapsid: Genome + Capsid.
- Viral specific glycoproteins.
- Some may have an envelope.

Virus Capsid

Protects the nucleic acid genome.
Made of single or multiple subunits (capsomeres) held together by non-covalent bonds.
Capsomeres: Compactly folded proteins with identifiable domains.
Functions of capsid proteins:
- Define tissue or species-specific transmission.
- Interact with host receptors for cell entry.
- Packaging/assembling of the virus.
- Enzymatic functions.
- Viral and/or host gene regulation.
- Evade/block host immune system.

Icosahedral Capsid

Icosahedral symmetry: 20 triangular faces.
Non-enveloped (naked) viruses:
- Viral nucleocapsid is naked.
- Resistant to organic solvents, detergents, drying, acid, and heat.
- Transmission via fecal/oral route, fomites, or small droplets.
- Released from infected cells by lysis.

Bacteriophage

Bacterial viruses, also called phages.
Structure:
- Head: Nucleic acid and protein.
- Tail and contractive sheath.
- Tail fibers and tail pins.
- Base / end plate.

Viral Genome

Understanding the viral genome is key for understanding genome replication and propagation.
Genome replication: Directly linked to viral classification and phylogenetic relationship.
All viruses need mRNA (+) strand synthesis.
Positive (+) nucleic acid is the gene coding sequence.
Negative (-) nucleic acid is the complementary strand to the gene-coding mRNA (template strand).

Positive vs.Negative Strands

Key concept in virology.
Positive-sense RNA: Functions as messenger RNA (mRNA) within the host cell.
Negative-sense RNA: Serves as a template for the production of mRNA.

Viral Genome Types

DNA-based genomes: Always presented by a single molecule, either dsDNA or ssDNA (positive or negative).
RNA-based genomes: Can be single or multiple molecules.
Multiple RNA nucleic acids: Either segmented or diploid.
- dsRNA: Reoviruses (segmented genome).
- ssRNA:
  - Positive (+): Retroviruses (diploid genome).
  - Negative (–): Orthomyxoviruses (segmented genome).

Naming Viruses

Disease associated with the virus:
- Poxvirus (smallpox).
- Hepatitis viruses (affect the liver).
- Reoviruses (respiratory, enteric, orphan).
- Human immunodeficiency virus (immune system).
Cytopathology caused:
- Respiratory syncytial virus (syncytium - fusion of infected cells).
- Cytomegalovirus (Greek cyto - "cell", and megalo - "large").
Places or people:
- Rift Valley Fever virus (Rift valley, Kenya).
- Epstein-Barr virus (also known as HHV-4).

Naming Viruses (continued)

Site of isolation:
- Adenovirus (adenoids).
- Enterovirus (enteric).
- Rhinovirus (from Greek - rhinos means “nose”).
Viral biochemical features:
- Retrovirus (retro - "reverse").
- Picornavirus (pico - "small", RNA - rna).
- Togavirus (toga - "mantle").

Virus Classification (ICTV)

International Committee on Taxonomy of Viruses (ICTV) established a universal system in 1966.
Ranked taxons: Order/family/sub-family/genus/species.
Example:
- Order: Mononegavirales
- Family: Paramyxoviridae
- Subfamily: Paramyxovirinae
- Genus: Morbillivirus
- Species: Measles virus

Baltimore Classification (defined in 1971)

A classification system based on viral genome characteristics and replication cycle.
Groups viruses into seven classes based on:
- Type of nucleic acid (DNA or RNA).
- Whether it's double-stranded or single-stranded.
- Whether it's positive-sense or negative-sense.
- The replication cycle (e.g., DNA viruses use DNA polymerase, whereas RNA viruses use RNA polymerase).

Viral Hosts and Tropism

Viral hosts: Range of susceptible species that can be infected.
Viral tropism: Specific organ, tissue, cell type, or range of organs and tissues infected.
Factors determining tropism:
- Viral glycoproteins (VAP) on the outer coat that target receptors on host cells.
- Presence of transcription factors allowing viral gene expression.
- Presence of cell enzyme pathways to produce viral proteins ("permissivity").

Viral Hosts and Tropism (continued)

Receptors are key.
Example:
- Influenza A virus attaches to sialic acid residues on the surface of mucosal cells.
- Respiratory tract cells are rich in sialic acid, leading to infection in the respiratory tract.

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Description

Explore the fascinating world of viruses, the acellular organisms that exist either as infectious particles or within host cells. This quiz will cover their structure, properties, and the differences between animal viruses and living cells, as well as their method of replication. Test your knowledge on these unique entities that can infect all forms of life.