Characterizing and Classifying Viruses, Viroids, and Prions

Overview

This chapter explores the fundamental properties, structures, and replication strategies of viruses, viroids, and prions. It also examines their roles in disease, laboratory cultivation, and their unique biological characteristics compared to other microorganisms.

Virion Structure and Classification

Basic Structure of Virions

A virion is a complete, infectious virus particle. Virions can be enveloped or nonenveloped (naked), and their structure determines how they interact with host cells.

• Capsid: The protein shell surrounding the viral genome, composed of subunits called capsomeres.

• Nucleocapsid: The combined structure of the viral genome and capsid.

• Envelope: A lipid membrane derived from the host cell, present in enveloped viruses. Contains glycoproteins and spikes for host cell recognition.

• Shapes: Viruses may have helical (rod-like), polyhedral (many-sided), or complex shapes.

• Genetic Material: Can be DNA or RNA, single-stranded (ss) or double-stranded (ds), and may be linear or circular.

Example: Influenza virus is an enveloped virus with a helical nucleocapsid and surface spikes.

Bacteriophages

Bacteriophages are viruses that infect bacteria. They have complex structures and are important in molecular biology research.

Viral Replication and Host Interaction

Entry and Replication of Animal Viruses

Animal viruses use several mechanisms to enter host cells and replicate:

• Direct Penetration: The viral genome is injected directly into the host cell.

• Endocytosis: The host cell engulfs the virus in a vesicle.

• Membrane Fusion: The viral envelope fuses with the host cell membrane, releasing the nucleocapsid.

• Uncoating: The viral capsid is removed, releasing the genome into the host cell.

Replication strategies differ based on the type of viral genome:

• Positive-sense ssRNA viruses: Genome acts as mRNA and is directly translated by host ribosomes.

• dsDNA viruses: DNA is transcribed to mRNA, then translated to proteins.

• Retroviruses: Use reverse transcriptase to convert RNA into DNA, which integrates into the host genome as a provirus.

New viruses are assembled in the host cell and released by:

• Budding: Enveloped viruses acquire their envelope from the host cell membrane during release.

• Exocytosis: Viruses are expelled in vesicles.

• Cell lysis: Nonenveloped viruses often cause the host cell to burst, releasing virions.

Most viruses exhibit host specificity, infecting only certain species or cell types due to specific receptor interactions.

Persistent and Latent Infections

• Latent viruses: Remain dormant in host cells and can reactivate later.

• Persistent infections: Virus is continuously present, often at low levels.

Obligate Intracellular Parasitism and Laboratory Cultivation

Obligate Intracellular Parasites

Viruses are obligate intracellular parasites, meaning they require living host cells to replicate. They cannot grow on standard nutrient media like bacteria.

Methods of Virus Cultivation

• Continuous cell culture: Immortalized cell lines that can be maintained indefinitely.

• Diploid cell culture: Derived from normal tissue, limited lifespan.

• Bacteriophage cultivation: Grown in bacterial cultures.

These methods differ from bacterial cultivation, which uses nutrient agar or broth.

Viral Pathogenesis and Disease

Tissue Destruction and Disease

Viruses can cause disease by destroying host cells, disrupting tissue function, or triggering immune responses. Some viruses are associated with neoplasia (uncontrolled cell growth) and cancer.

• Example: Human papillomavirus (HPV) is linked to cervical cancer.

Viroids and Prions

Viroids

Viroids are small, circular RNA molecules that infect plants. They lack a protein coat and do not encode proteins, but can cause significant agricultural diseases.

Prions

Prions are infectious proteins that cause neurodegenerative diseases known as spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease, mad cow disease). Prions propagate by inducing misfolding of normal proteins in the host.

• Stanley Prusiner discovered prions and their role in disease.

Key Vocabulary Table

Notable Scientists

• Dmitri Ivanowsky: Early researcher in virology, contributed to the discovery of viruses.

• Stanley Prusiner: Discovered prions and their role in disease.

Summary Table: Virus Replication Strategies

Additional info: The above notes expand on the provided objectives and vocabulary, offering definitions, examples, and context for key concepts in viral biology, viroids, and prions.