Viruses, Viroids, and Prions: Structure, Replication, and Classification

Study Guide - Smart Notes

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Viruses, Viroids, and Prions

General Information

Viruses, viroids, and prions are infectious agents that differ fundamentally from cellular life forms. They are studied extensively in microbiology due to their unique biology and impact on health.

Viruses are non-cellular particles containing genetic material (DNA or RNA) encased in a protein coat called a capsid.
Viruses are obligate intracellular parasites, meaning they require living host cells to reproduce.
They do not possess cellular structures such as organelles and cannot carry out metabolism independently.
Viruses are not considered living organisms because they cannot reproduce or carry out metabolic processes outside a host cell.
Antiviral drugs are often needed to treat viral infections, as antibiotics are ineffective.

Viruses

Definition and Key Properties

A virus is a microscopic infectious agent with distinct characteristics:

Non-cellular: Lacks cellular structure.
Genome: Contains either DNA or RNA.
Capsid: Protein coat that encases the viral genome.
Obligate intracellular parasite: Requires host cells for replication.
Cannot reproduce or metabolize independently.

Example: The influenza virus contains an RNA genome and infects respiratory tract cells.

Host Range and Bacteriophages

Viruses exhibit specificity for their host cells, known as host range.

Host range: The spectrum of host cells a virus can infect. Limited by the virus's ability to recognize and bind to specific cell receptors.
Bacteriophages: Viruses that infect bacteria. Example: Bacteriophage T2 infects Escherichia coli.
Phages inject their genome into the host cell, leaving the capsid outside.

Example: Human immunodeficiency virus (HIV) infects only certain immune cells due to specific receptor recognition.

Transmission and Tissue Tropism

Viruses spread through various transmission routes and may target specific tissues.

Transmission: The process of reaching and infecting a new host. Routes include respiratory droplets, blood, sexual contact, etc.
Tissue tropism: The range of tissue types a virus can infect. Some viruses have broad tropism (e.g., Ebola), while others are narrow (e.g., rabies targets nervous tissue).

Example: HIV is transmitted via blood or sexual contact and targets immune cells.

Viral Genomes

Virion Structure and Genome Types

A virion is a complete, infectious viral particle in its extracellular phase.

Composed of a nucleic acid genome (DNA or RNA) and a protein capsid.
Genome may be double-stranded or single-stranded.
Some viruses have an envelope derived from host cell membranes.
Upon infection, the viral genome enters the host cell and may undergo uncoating or classical entry.

Example: Influenza virus has a segmented RNA genome and an envelope.

The Capsid and Envelope

Capsid Structure and Function

The capsid is the protein shell that encases the viral genome and determines the virus's shape.

Capsid structure and shape are determined by the viral genome and account for most of the virus's mass.
Capsids are composed of protein subunits called capsomeres.
Capsomeres may be identical or diverse, depending on the virus.

Example: Adenovirus has an icosahedral capsid made of multiple capsomeres.

Envelope and Spike Proteins

Some viruses possess an envelope, which is a lipid membrane derived from the host cell.

Envelope: Covers the capsid and is acquired from the host cell's plasma membrane during viral exit.
Envelopes may have spike proteins (glycoproteins) that aid in host cell recognition and attachment.
Viruses lacking an envelope are termed nonenveloped viruses.

Example: HIV and influenza viruses are enveloped and use spike proteins for cell entry.

Feature	Enveloped Virus	Nonenveloped Virus
Outer Layer	Lipid envelope from host cell	Protein capsid only
Spike Proteins	Present	Absent
Example	Influenza, HIV	Adenovirus, Poliovirus

Key Terms and Definitions

Capsid: Protein shell surrounding the viral genome.
Capsomere: Subunit of the capsid.
Envelope: Lipid membrane derived from host cell, present in some viruses.
Spike protein: Glycoprotein on the viral envelope involved in host cell attachment.
Host range: The spectrum of host cells a virus can infect.
Tissue tropism: Specific tissues or cell types a virus can infect.
Bacteriophage: Virus that infects bacteria.
Virion: Complete, infectious viral particle.

Formulas and Equations

Viral Replication Rate (generalized):

Summary Table: Virus Classification

Type	Genome	Envelope	Host Range
DNA Virus	DNA (ss or ds)	May be present	Animals, plants, bacteria
RNA Virus	RNA (ss or ds)	May be present	Animals, plants
Bacteriophage	DNA or RNA	Usually absent	Bacteria

Additional info: These notes are based on introductory microbiology slides and have been expanded for clarity and completeness. The study guide covers the essential structural and functional aspects of viruses, with emphasis on their classification, replication, and host interactions.