Introduction to Viruses, Viroids, and Prions: Structure, Classification, and Replication

Chapter 6: Introduction to Viruses, Viroids, and Prions

Overview

This chapter provides a foundational understanding of viruses, viroids, and prions, focusing on their structure, classification, and replication. These noncellular infectious agents play significant roles in microbiology and human health.

General Size of Viruses

Microscopic Nature

• Viruses are extremely small, with most measuring less than 0.2 μm (micrometers) in diameter.

• Due to their size, viruses require an electron microscope for visualization.

• Viruses are much smaller than bacteria and eukaryotic cells.

• Example: Poliovirus is about 30 nm, while influenza virus is about 100 nm.

General Structure of Viruses

Basic Components

• Viral particle (virion): The complete, infectious form of a virus outside a host cell.

• Covering:

  • Capsid: Protein coat that encloses and protects the viral nucleic acid.

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

• Central core:

  • Nucleic acid molecule(s): Either DNA or RNA, but never both.

  • Enzymes: Present in some viruses, aiding in replication or integration.

General Structure of Viruses: Capsids

Capsid and Nucleocapsid

• Capsids are protein coats that enclose and protect the viral nucleic acid.

• Each capsid is made of identical protein subunits called capsomers.

• The capsid together with the nucleic acid is called the nucleocapsid.

• Naked viruses have only the nucleocapsid, lacking an envelope.

Structural Capsid Types

Helical and Icosahedral Capsids

• Helical capsids: Capsomers form a continuous helix, creating a cylindrical nucleocapsid.

• Icosahedral capsids: Three-dimensional, symmetrical polygons with 20 sides and 12 corners.

• Both types can be naked (no envelope) or enveloped (surrounded by a lipid membrane).

• Example: Herpes simplex virus is an enveloped, icosahedral virus.

Viral Envelope

Envelope Structure and Function

• Common in animal viruses; acquired when the virus leaves the host cell.

• Composed of host cell membrane and viral proteins.

• Spikes (glycoproteins) on the envelope are essential for attachment to host cells.

Complex Viruses

Structural Complexity

• Some viruses, such as bacteriophages, have complex structures with polyhedral heads, helical tails, and attachment fibers.

• Example: T4 bacteriophage of E. coli.

Types of Viral Morphology

Classification by Structure

• Viruses are classified based on capsid shape (helical, icosahedral, complex) and presence or absence of an envelope.

Viral Nucleic Acids

Genomic Diversity

• Viral genome is either DNA or RNA, never both.

• DNA viruses: Usually double-stranded (ds), may be single-stranded (ss), circular or linear.

• RNA viruses: Usually single-stranded, may be double-stranded, may be segmented.

• Positive-sense RNA: Ready for immediate translation.

• Negative-sense RNA: Must be converted to proper form for translation.

Other Substances in Viruses

Viral Enzymes

• Reverse transcriptase: Synthesizes DNA from RNA (e.g., HIV).

• Polymerases: Synthesize DNA or RNA.

• Replicases: Copy RNA.

Replication Cycles in Animal Viruses

Phases of Multiplication

• Adsorption: Virus binds to specific molecules on host cell.

• Penetration: Genome enters host cell.

• Uncoating: Viral nucleic acid is released from capsid.

• Synthesis: Viral components are produced.

• Assembly: New viral particles are constructed.

• Release: Viruses exit by budding (enveloped) or cell lysis (naked/complex).

Viral Penetration and Uncoating

Entry Mechanisms

• Fusion: Envelope fuses with host membrane (enveloped viruses).

• Endocytosis: Virus is engulfed by host cell (enveloped or naked viruses).

Viral Release

Methods of Exit

• Budding/Exocytosis: Enveloped viruses bud from host membrane; cell is not immediately destroyed.

• Lysis/Rupturing: Nonenveloped and complex viruses released when cell dies and ruptures.

Persistent Infections and Latency

Chronic and Latent States

• Persistent infections: Cell is not immediately lysed; virus may remain for weeks or lifetime.

• Chronic latent state: Virus periodically reactivates (e.g., Herpes simplex, Herpes zoster).

Oncogenic Viruses

Viruses and Cancer

• Some viruses are oncogenic, capable of initiating tumors.

• Oncoviruses: Mammalian viruses that can cause cancer (e.g., papillomavirus and cervical cancer).

Replication Cycle in Bacteriophages

Bacterial Viruses (Phages)

• Multiplication is similar to animal viruses, but only nucleic acid enters the cytoplasm.

• Phage replication steps:

  1. Adsorption

  2. Penetration

  3. Replication

  4. Assembly

  5. Maturation

  6. Lysis & Release

• Lytic cycle: Complete viral infection, cell lysis, and release of virions.

• Lysogeny: Viral DNA integrates into host genome as a prophage; cell is not lysed.

Lysogeny: The Silent Virus Infection

Prophage and Lysogenic Conversion

• Temperate phages undergo adsorption and penetration but do not replicate immediately.

• Viral genome inserts into bacterial genome as an inactive prophage.

• Prophage is copied during cell division, spreading the viral genome.

• Lysogenic conversion: Prophage genes may cause production of toxins or enzymes (e.g., Corynebacterium diphtheriae, Vibrio cholerae, Clostridium botulinum).

Cultivating and Identifying Animal Viruses

Methods of Cultivation

• In vitro: Tissue cultures using cultured cells.

• In vivo: Embryonated eggs or live animal inoculation.

• Identification involves clinical observation, cell culture, antibody tests, and sometimes plaque assays.

Detection and Treatment of Animal Viral Infections

Challenges and Approaches

• Diagnosis is more difficult than for other agents; requires clinical, laboratory, and serological methods.

• Antiviral drugs can cause serious side effects.

Prions and Other Nonviral Infectious Particles

Prions

• Prions are misfolded proteins with no nucleic acid.

• Extremely resistant to sterilization; cause transmissible spongiform encephalopathies (neurodegenerative diseases).

• Examples: Scrapie in animals, Creutzfeldt-Jakob Syndrome (CJS) in humans.

Viroids

• Viroids are short pieces of RNA with no protein coat; identified only in plants.

Table: Comparison of Virus, Viroid, and Prion

Agent	Genetic Material	Protein Coat	Envelope	Host Range
Virus	DNA or RNA	Present (capsid)	May be present	Animals, plants, bacteria
Viroid	RNA	Absent	Absent	Plants
Prion	None	Absent	Absent	Animals (including humans)

Key Equations

• Viral size comparison (not a formula, but a reference): $\text{Typical virus size} < 0.2\ \mu m$

Additional info: Academic context and examples have been added to clarify and expand upon the original notes, ensuring completeness and self-contained study material.