Fundamentals of Viruses and Prions: Structure, Classification, and Replication

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Viruses: Structure and Composition

Basic Properties of Viruses

Viruses are unique infectious agents that differ significantly from cellular organisms. Their structure and composition are essential for understanding their biology and classification.

Viruses are composed of protein only (incorrect; most viruses contain nucleic acids).
Viruses lack genetic material (incorrect; all viruses contain either DNA or RNA).
Viruses lack cytoplasm and organelles (correct; viruses do not have cellular structures).
Viruses have no protein structure (incorrect; the capsid is made of protein).
Viruses are composed of both protein and lipid (correct for enveloped viruses; the envelope contains lipids).

Key Terms:

Capsid: The protein shell surrounding the viral genome.
Envelope: A lipid membrane derived from the host cell, present in some viruses.

Viral Structure and Function

Functions of the Outermost Layer of a Virion

The outermost layer of a virion, which may be a capsid or envelope, serves several critical functions:

Protection: Shields the viral genome from environmental damage.
Recognition: Facilitates attachment to host cells via specific interactions.
Both protection and recognition are primary functions; replication is not a direct function of the outer layer.

Classification of Viruses

Criteria for Virus Classification

Viruses are classified based on several characteristics:

Type of host: The organism a virus infects (e.g., animal, plant, bacteria).
Type of nucleic acid: DNA or RNA, single-stranded or double-stranded.
Type of life cycle: Lytic or lysogenic cycles.
Size and number of chromosomes are not primary classification criteria.

Host Specificity

Determinants of Host Specificity

Host specificity refers to the range of hosts a virus can infect, determined by:

Differences in size between virus and host cell (not a major factor).
Presence or absence of a cell wall (relevant for bacteriophages).
Interactions between viral and cellular surface molecules (most important).
Particular genes shared with the infected cell (less common).
Presence of an envelope (affects entry mechanisms).

Viral Morphology

Shape of a Virion

The shape of a virion is primarily determined by:

The capsid: The protein shell that encases the viral genome.
Number of genome segments and envelope influence overall morphology but are not the primary determinants.

Viral Envelope

Composition and Function

The viral envelope is a lipid bilayer derived from the host cell membrane, containing:

Cellular phospholipids
Viral proteins
Sometimes host cell proteins
Sterols may be present in some cases.

Viral Genomes

Types and Examples

Viral genomes vary in structure:

Single linear dsDNA
Multiple pieces of linear dsDNA
Single linear dsRNA
Single circular ssRNA
Multiple pieces of linear ssRNA

Example: The genome of influenza virus is multiple pieces of linear ssRNA.

Definition of a Virion

Virion Structure

A virion is the complete, extracellular form of a virus, consisting of:

The viral genome (DNA or RNA)
The capsid (protein shell)
Sometimes an envelope (lipid membrane)

Viral Infectivity and Damage

Susceptibility to Damage

Different types of infectious particles vary in their susceptibility to environmental damage:

Enveloped viruses: Most susceptible due to the fragile lipid envelope.
Naked capsid viruses: More resistant to environmental stress.
Prions: Highly resistant to damage.
Bacteriophages: Vary in resistance.

Stages of the Lytic Replication Cycle

Order of Events

The lytic cycle involves several stages:

Attachment: Virus binds to host cell.
Assembly: New viral particles are assembled.
Synthesis: Viral genome and proteins are synthesized.

Correct order: Attachment, Synthesis, Assembly.

Viral Replication

Similarity to Cellular Processes

Replication of most dsDNA viruses is similar to normal cellular processes, as they use host cell machinery for DNA replication and transcription.

Viral Entry Mechanisms

Herpesvirus Entry

Attachment of herpesviruses results in membrane fusion, allowing the viral genome to enter the host cell.

Reverse Transcriptase and Viral Genomes

Associated Viruses

Reverse transcriptase is an enzyme associated with:

Retroviruses (e.g., HIV)
ssRNA viruses (some types)

The genome of retroviruses can act directly as mRNA.

Replication Requirements for Animal Viruses

RNA-dependent RNA Transcriptase

Some animal viruses require RNA-dependent RNA transcriptase for replication, especially:

dsRNA viruses
ssRNA viruses
Retroviruses

Release of Naked Capsid Viruses

Mechanism of Release

Naked capsid animal viruses are commonly released from infected cells by lysis, which destroys the host cell.

Viruses and Cancer

Oncogenic Potential

Some viruses can cause cancer by interrupting genetic regulatory sequences. Retroviruses are most commonly associated with cancer, while dsRNA viruses are less likely to be involved.

Prions: Unique Infectious Agents

Properties of Prions

Prions are infectious proteins that differ from other agents:

They act at slow rates.
Cannot reproduce outside a cell.
Can be destroyed by incineration.
Do not contain nucleic acids.

Viroids and Prions: Structure

Protein Content

Viroids and prions are unique among infectious particles:

Viroids: Small, circular RNA molecules without protein coats.
Prions: Infectious proteins without nucleic acids.

Continuous Cell Cultures

Disadvantages

Continuous cell cultures are used for viral research but have limitations:

May be genetically different from the original source animal.
Viruses may not reliably infect them.
Limited number of generations.
Expense of preparation.
Limited number of divisions.

Comparison Table: Types of Infectious Agents

Agent	Genetic Material	Protein Content	Envelope	Replication
Virus	DNA or RNA	Yes (capsid)	Sometimes	Requires host cell
Viroid	RNA	No	No	Requires host cell
Prion	No	Yes	No	Requires host cell
Bacteriophage	DNA or RNA	Yes	Rarely	Requires bacterial host