Virology: Introduction and Virus Structure

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Questions and Answers

What is virology?

The study of viruses, complexes of nucleic acids and proteins that have the capacity for replication in animal, plant and bacterial cells.

What are the structural components of a virus?

Nucleic acid (DNA or RNA), a protein coat called a capsid, and in some cases, an envelope.

What is the function of the capsid of a virus?

It protects the nucleic acid, helps transport nucleic acid between cells, and provides specificity for attachment.

What is the difference between helical and polyhedral viruses?

Helical viruses have a long rod shape, while polyhedral viruses have a many-sided, often icosahedral shape. Signup and view all the answers

Viruses always contain both DNA and RNA.

False Signup and view all the answers

Where do viruses replicate?

In living cells, either eukaryotic or prokaryotic, as they are obligate intracellular parasites. Signup and view all the answers

What are viroids?

Small, single-stranded, circular RNAs that do not encode proteins and are associated with plant disease. Signup and view all the answers

Which of the following is NOT a characteristic of a virus?

They replicate independently of host cells Signup and view all the answers

Which of the following viruses is classified as a complex virus?

Bacteriophage Signup and view all the answers

What are the six basic stages of viral replication?

Attachment, penetration, uncoating, replication, assembly, and release. Signup and view all the answers

How do enveloped viruses typically exit the host cell?

By budding, acquiring a portion of the host cell's membrane as their envelope. Signup and view all the answers

Which of the following viruses is a DNA virus?

Hepatitis B virus Signup and view all the answers

Viral genomes can be either single-stranded or double-stranded.

True Signup and view all the answers

Which of the following viruses is associated with plant disease?

Viroid Signup and view all the answers

What is the difference between early and late mRNAs in DNA viruses?

Early mRNAs encode for proteins necessary for viral replication, while late mRNAs encode for structural proteins. Signup and view all the answers

Which of the following is a type of RNA virus?

Retrovirus Signup and view all the answers

What are the two main criteria for classifying viruses?

The type of nucleic acid (DNA or RNA) and the morphology (shape) of the virus. Signup and view all the answers

Study Notes

Viruses: Introduction and Characteristics

Virology is the study of viruses, complex structures of nucleic acids and proteins capable of replicating in animal, plant, and bacterial cells.
Viruses are parasites that utilize host cell functions for replication, causing changes in the host cell, including altered antigenicity and metabolic processes.
Viral activity can lead to cell death.
Viral replication results in the manifestations of viral disease within the host.
Viruses infect all living organisms.
Humans regularly ingest and inhale billions of virus particles.
Viruses are components of human genetic material.

Virus Structure

Viruses are small, typically 0.01-0.3 µm in size, and remain infectious after passing through bacterial filters.
Viruses are fully dependent on living cells for replication—eukaryotic or prokaryotic cells—obligatory intracellular parasites.
Some viruses may possess complex enzymes (RNA or DNA polymerase) but cannot replicate their genome independently.
Viruses are acellular; they replicate by utilizing the cellular machinery of other organisms.

History of Virus Discovery

The term “virus” originated in the early 1790s, pre-germ theory, and referred to poisons.
Significant milestones (dates are approximate):
- 1798: Jenner's work on smallpox
- 1890: Iwanowski's discovery of viruses.
- 1911: Beijerinck's research.
1930s-1970s: Continued advancement of virology and development of techniques.
1984: Identification of the AIDS virus.
1995: Development of multiple drug treatments for HIV/AIDS (protease inhibitors and reverse transcriptase inhibitors)

Virus Size

Virus sizes vary.
Example virus sizes are given in nanometers (e.g., poliovirus - 30 nm, Ebola virus - 970 nm, bacteriophages - 24-225 nm).
Virus sizes are compared to bacteria and human cells.

Virus Structure: Composition

Capsid: The protein coat surrounding the viral genetic material.
- Capsid subunits are called capsomers; capsomer arrangement defines the virus's symmetry.
Viral envelope (not all viruses): Some viruses gain a lipoprotein coat ("budding").
Function of the capsid:
- Protection of Nucleic Acid.
- Transport of Nucleic Acid From Cell to Cell.
- Provides Specificity for Attachment.

Virus Morphology

Helical viruses: Resemble rods (e.g., rabies, Ebola).
Polyhedral viruses: Many-sided (e.g., poliovirus).
Complex viruses: More complex structure (e.g., bacteriophages).

Unconventional Agents

Viroids: Small, single-stranded circular RNA molecules that are not packaged and appear to cause plant diseases only.
Prions: Proteinaceous particles that affect the structure of other proteins, causing various neurological diseases (e.g., Creutzfeldt-Jakob Disease).

Viral Replication

Viral replication has six stages:
- Attachment: Viruses bind to host cell receptors.
- Penetration: Entry into the host cell by endocytosis or fusion.
- Uncoating: Viral capsid is degraded.
- Replication: Viral genetic material and proteins are produced.
- Assembly: Viral components assemble into new viruses.
- Release: Viruses exit the host cell (lysis or budding).

Viral Classification

Classification based on nucleic acid type (DNA or RNA):
RNA-containing viruses are classified into various groups (Picornaviruses, REOviruses, Bunyaviruses, retroviruses, and more)
DNA-containing viruses are categorized (Parvoviruses, HepaDNAviruses, Papovaviruses, Adenoviruses, Herpesviruses, and Poxviruses).
Classification also based on disease symptoms: generalized disease such as smallpox, or specific organ, as in nervous system, eye, lungs and more.
Additional classification based on the geometry of the viral capsid or morphological characteristics.

Virus Genomes

Viruses can have either DNA or RNA genomes but not both.
The nucleic acid can be single-stranded or double-stranded.
Different types of viral genomes correlate with different replication mechanisms.

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Description

This quiz covers the fundamental concepts of virology, including the characteristics and structure of viruses. Explore how viruses function as obligate intracellular parasites, their replication processes, and their impact on living organisms. Test your knowledge on viral infections and their implications for health.