Viruses and Bacteriophages

General Characteristics of Viruses

Viruses are acellular infectious agents that require a host cell for replication. They are structurally simple but biologically diverse, and are distinct from prokaryotic and eukaryotic cells.

• Genetic Material: Viruses contain either DNA or RNA as their genetic material, but never both.

• Structure: The viral genome is enclosed in a protein coat called a capsid. Some viruses have an additional lipid envelope derived from the host cell membrane.

• Obligate Intracellular Parasites: Viruses cannot reproduce outside a host cell.

• Host Range: Each virus has a specific range of hosts it can infect, determined by host cell receptors and viral surface proteins.

Example: The influenza virus infects respiratory tract cells in humans, while bacteriophages infect bacterial cells.

Structure of a Bacteriophage

Bacteriophages are viruses that infect bacteria. They have a complex structure with specialized components for attachment and genome delivery.

• Capsid: The protein shell that encloses the viral genome.

• Tail Sheath and Tail Fibers: Structures used for attachment to the bacterial cell and injection of viral DNA.

• Baseplate with Pins: Facilitates attachment to the host cell surface.

Viral Replication Cycles

Lytic Cycle

The lytic cycle results in the destruction of the host cell and the release of new viral particles.

• Attachment: Virus binds to specific receptors on the host cell surface.

• Penetration: Viral DNA is injected into the host cell.

• Biosynthesis: Viral genes are expressed, and viral components are synthesized.

• Assembly: New virions are assembled from synthesized components.

• Release: Host cell lyses, releasing new viruses.

Example: T4 bacteriophage infecting Escherichia coli.

Lysogenic Cycle

In the lysogenic cycle, the viral genome integrates into the host genome and replicates along with it without causing immediate lysis.

• Prophage: The integrated viral DNA in the host genome.

• Lysogeny: The state in which the viral genome is maintained in the host without causing lysis.

• Induction: The process by which a prophage exits the host genome and enters the lytic cycle, often triggered by environmental stress (e.g., UV light).

Example: Lambda phage in E. coli.

Comparison of Lytic and Lysogenic Cycles

Stages of Viral Replication

Viral Classification and Laboratory Methods

Classification Criteria

• Genetic Material: DNA or RNA, single- or double-stranded

• Capsid Morphology: Helical, icosahedral, or complex

• Presence of Envelope: Enveloped or non-enveloped

• Host Range

Note: Biochemical tests are not used to classify viruses.

Laboratory Cultivation

• Continuous Cell Lines: Derived from transformed cells, can be maintained indefinitely.

• Primary Cell Lines: Derived directly from tissues, have a limited lifespan.

Prions

Definition and Discovery

Prions are infectious proteins that lack nucleic acids. They cause neurodegenerative diseases in humans and animals.

• Origin of Term: "Prion" is derived from "proteinaceous infectious particles." Discovered by Stanley Prusiner in 1982.

• Diseases: Mad cow disease (bovine spongiform encephalopathy), kuru, Creutzfeldt-Jakob disease.

• Unique Feature: Prions lack nucleic acid, distinguishing them from viruses and other infectious agents.

Key Terms and Concepts

• Prophage: Lysogenic viral DNA integrated into the host genome.

• Induction: Activation of a prophage to enter the lytic cycle, often triggered by stress (e.g., UV light).

• Plaque: A clear area on a bacterial lawn caused by viral lysis of host cells.

• Glycoprotein Spikes: Not all viruses have glycoprotein spikes on their capsids; these are more common in enveloped viruses.

• Latent Virus: A virus that remains dormant within the host cell and can reactivate later (e.g., shingles).

Summary Table: Virus, Bacteriophage, and Prion Features

Additional Info

• Shingles: An example of reactivation of a latent virus (varicella-zoster virus).

• Viral Envelope Acquisition: Occurs during the release stage, when the virus buds from the host cell membrane.

• Unique Viral Features: Viruses may contain an RNA genome, a feature not found in bacteria.