Viruses and Their Replication: Structure, Genomes, and Infection Cycles

Viruses: Definition and Fundamental Properties

What is a Virus?

Viruses are unique genetic elements that require a living host cell for replication. Unlike cellular organisms, viruses are not considered living because they lack independent metabolic activity and cannot reproduce outside a host.

• Definition: A virus is a genetic element that can replicate only inside a living cell (host cell).

• Genome: Viruses possess their own genome, which encodes functions necessary for replication.

• Non-cellular Nature: Viruses are not cells and are not classified as living organisms.

• Dependence on Host: Viruses rely on the host cell for energy, metabolic intermediates, and protein synthesis.

• Infection: Replication occurs only after entry into a suitable host cell, a process termed infection.

Viral Genomes: Diversity and Organization

Types and Characteristics of Viral Genomes

Viral genomes exhibit remarkable diversity in their chemical composition, structure, and replication strategies.

• Nucleic Acid Type: Viral genomes may be composed of DNA or RNA.

• Strandedness: Genomes can be single-stranded (ss) or double-stranded (ds).

• Sense: Single-stranded RNA (ssRNA) viruses may be positive-sense (+) or negative-sense (−).

• Shape: Genomes may be linear or circular.

• Unusual Genomes: Some viruses use both DNA and RNA as genetic material at different life cycle stages.

• Genome Size: Viral genomes are typically much smaller than those of cellular organisms; the smallest viruses contain less than 2000 base pairs and only two genes.

Example: Genome Types in Viruses

• ssDNA viruses (e.g., Parvoviruses)

• dsDNA viruses (e.g., Herpesviruses)

• ssRNA viruses (e.g., Poliovirus, Influenza virus)

• dsRNA viruses (e.g., Rotavirus)

• Retroviruses (ssRNA-RT, e.g., HIV)

Classification of Viruses by Host

Host Range and Virus Types

Viruses are classified based on the type of host they infect, reflecting their ecological and evolutionary diversity.

• Bacterial viruses: Bacteriophages

• Archaeal viruses

• Protozoan viruses

• Fungi viruses: Mycoviruses

• Animal viruses

• Plant viruses

Types of Viral Infection

Lytic vs. Lysogenic Infection

Viruses employ different strategies to propagate within their hosts, primarily through lytic or lysogenic cycles.

• Lytic Infection:

  • Virus replicates and destroys the host cell.

  • Redirects host metabolism to support viral replication and assembly of new virions.

  • Host cell is lysed to release new virions, which can infect additional cells.

• Lysogenic Infection:

  • Viral genome integrates into the host genome and is replicated along with it.

  • Host cell is not destroyed immediately.

  • Genetically alters the host, which may acquire new properties.

Example: Bacteriophage Lambda

• Can undergo both lytic and lysogenic cycles in Escherichia coli.

• Lysogenic conversion can confer new traits to the bacterial host.

Summary Table: Viral Genome Types

The following table summarizes the main types of viral genomes and representative examples:

Genome Type	Example Virus	Host
ssDNA	Parvovirus	Animals
dsDNA	Herpesvirus	Humans
ssRNA (+)	Poliovirus	Humans
ssRNA (−)	Influenza virus	Humans
dsRNA	Rotavirus	Humans
ssRNA-RT	HIV (Retrovirus)	Humans

Additional info: The above notes expand on the brief points in the slides, providing definitions, examples, and context for key concepts in virology relevant to a college-level microbiology course.