Study Guide: Viruses and Prions in Microbiology

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 classification and replication.

• Viruses are composed of protein only: This is incorrect; most viruses contain both protein and nucleic acid.

• Viruses lack genetic material: Incorrect; all viruses contain genetic material (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: Some viruses have a lipid envelope in addition to the protein capsid.

Key Terms:

• Capsid: The protein shell enclosing the viral genome.

• Envelope: A lipid membrane derived from the host cell, present in some viruses.

• Genome: The genetic material of the virus, either DNA or RNA.

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 functions:

• Protection: Shields the viral genome from environmental damage.

• Recognition: Facilitates attachment to host cells via specific interactions.

• Replication: Not a direct function of the outermost layer, but essential for the viral life cycle.

Example: The envelope of influenza virus contains glycoproteins that recognize and bind to host cell receptors.

Classification of Viruses

Characteristics Used for Classification

Viruses are classified based on several key characteristics:

• Type of host: Animal, plant, or bacterial viruses.

• 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: Less commonly used.

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: Important for bacteriophages.

• Interactions between viral and cellular surface molecules: Key determinant; viral proteins must bind to specific host receptors.

• Particular genes shared with the infected cell: Less relevant.

• Presence of an envelope: Influences entry and exit mechanisms.

Viral Morphology

Shape of a Virion

The shape of a virion is primarily determined by:

• The capsid: The protein shell gives the virus its shape (e.g., icosahedral, helical).

• Envelope: May alter the overall appearance but not the basic shape.

• Genome segments: Do not determine shape.

Viral Envelope

Composition and Function

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

• Contains cellular phospholipids and viral proteins

• Does not contain only host cell proteins or only viral proteins

• Does not contain sterols exclusively

Viral Genomes

Types and Examples

Viral genomes vary in structure and composition:

• Single linear dsRNA

• Multiple pieces of linear dsDNA

• Single linear ssRNA

• Single circular ssRNA

• Multiple pieces of linear ssRNA

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

Definition of a Virion

What is a Virion?

A virion is the complete, infectious form of a virus outside a host cell.

• Extracellular virus genome

• Virus particle outside a cell

• Empty capsid: Not a virion.

• Virus genome integrated into host cell genome: Not a virion.

Viral Susceptibility to Damage

Types of Infectious Particles

Some viral particles are more susceptible to environmental damage:

• Enveloped viruses: More easily damaged due to the fragile lipid envelope.

• Naked capsid viruses: More resistant to environmental stress.

• Prions: Proteinaceous infectious agents, highly resistant.

• Bacteriophages: Infect bacteria, variable resistance.

Stages of the Lytic Replication Cycle

Order of Stages

The lytic cycle involves several distinct stages:

1. Attachment: Virus binds to host cell.

2. Entry: Viral genome enters the host cell.

3. Synthesis: Viral components are produced.

4. Assembly: New virions are assembled.

5. Release: Virions exit the host cell.

Viral Replication

Similarity to Cellular Processes

Replication of most dsDNA viruses is similar to normal cellular DNA replication processes.

• dsDNA viruses: Use host cell machinery for replication.

• ssRNA viruses: Require unique replication strategies.

Viral Entry Mechanisms

Attachment and Entry

Viruses enter host cells via several mechanisms:

• Membrane fusion: Common for enveloped viruses (e.g., herpesviruses).

• Direct penetration: Used by some non-enveloped viruses.

• Endocytosis: Virus is engulfed by the host cell.

Reverse Transcriptase and Viral Replication

Associated Viruses

Reverse transcriptase is an enzyme that synthesizes DNA from an RNA template.

• Retroviruses: e.g., HIV, use reverse transcriptase.

• dsRNA and ssRNA viruses: Generally do not use reverse transcriptase, except retroviruses.

Viral Genomes Acting as mRNA

Direct Translation

Some viral genomes can act directly as mRNA:

• Retroviruses: RNA genome is reverse transcribed to DNA.

• ssRNA viruses: Positive-sense ssRNA can be directly translated.

RNA-dependent RNA Transcriptase

Replication of Animal Viruses

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

• ssRNA viruses: Negative-sense require transcriptase to produce mRNA.

• Retroviruses: Use reverse transcriptase.

Release of Naked Capsid Viruses

Mechanisms of Release

Naked capsid viruses are commonly released by lysis, which destroys the host cell.

• Lysis: Host cell bursts, releasing virions.

• Budding: More common for enveloped viruses.

Viruses and Cancer

Oncogenic Viruses

Some viruses can cause cancer by interrupting regulatory sequences of repressor genes:

• Retroviruses: Known to be oncogenic.

• dsRNA and ssRNA viruses: Less commonly associated with cancer.

Prions: Unique Infectious Agents

Properties of Prions

Prions are infectious proteins that differ from other agents:

• Act at slow velocities

• Cannot reproduce outside a cell

• Resistant to incineration

• Do not contain nucleic acids

Infectious Particles Without Protein Structure

Viroids and Prions

Some infectious particles lack protein in their structure:

• Viroids: Small, circular RNA molecules without protein coat.

• Prions: Proteinaceous, lack nucleic acid.

Continuous Cell Cultures

Disadvantages

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

• Genetic differences from original source

• Viruses may not reliably infect them

• Limited number of generations

• Expense of preparation

Summary Table: Comparison of Infectious Agents

Agent	Genetic Material	Protein Structure	Envelope	Replication
Virus	DNA or RNA	Capsid (protein)	Some have envelope	Requires host cell
Viroid	RNA	None	None	Requires host cell (plants)
Prion	None	Protein only	None	Converts normal proteins
Bacteriophage	DNA or RNA	Capsid (protein)	Some have envelope	Requires bacterial host