Chapter 6: Introduction to Viruses, Viroids, and Prions – Structure and Classification

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Introduction to Viruses, Viroids, and Prions

Overview

Viruses, viroids, and prions are noncellular infectious agents studied in microbiology due to their unique properties and roles in disease. This chapter introduces their general structure, classification, and biological significance.

Viruses are microscopic agents, most measuring less than 0.2 μm, and require electron microscopy for visualization.
Viroids and prions are even simpler infectious particles, affecting plants and animals respectively.

General Size of Viruses

Microscopic Nature

Viruses are among the smallest infectious agents, much smaller than bacteria and eukaryotic cells.

Most viruses are <0.2 μm in diameter.
Visualization requires an electron microscope.
Size comparison: Viruses are significantly smaller than bacterial cells and eukaryotic organelles.
Example: Poliovirus (~30 nm), Influenza virus (~100 nm), compared to E. coli bacterium (~2,000 nm).

General Structure of Viruses

Basic Components

All viruses share a basic structure, though some components may be absent in certain types.

Viral particle (virion): The complete, infectious form of the 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, never both.
- Enzymes: Present in some viruses, aiding in replication or integration.

General Structure of Viruses: Capsids

Capsid and Nucleocapsid

The capsid is a protein shell that protects the viral genome and determines the virus's shape.

Capsids are made of protein subunits called capsomers.
The nucleocapsid is the combination of the capsid and the enclosed nucleic acid.
Viruses without an envelope are called naked viruses.
Example: A naked nucleocapsid virus consists only of the capsid and nucleic acid, with no external envelope.

Capsid Structure Types

Helical and Icosahedral Capsids

Capsids can be classified based on their structural arrangement:

Helical capsids: Capsomers form a continuous helix, creating a cylindrical nucleocapsid.
Icosahedral capsids: Three-dimensional, symmetrical polygons with 20 sides and 12 corners.
Some viruses have an envelope surrounding the capsid, while others are naked.
Example: Herpes simplex virus is an enveloped virus with an icosahedral capsid.

Viral Envelope

Origin and Function

The envelope is a lipid membrane acquired from the host cell during viral release.

Common in animal viruses.
Contains spikes (glycoproteins) essential for attachment to host cells.
Envelope formation occurs as the virus buds from the host cell membrane.

Complex Viruses

Structural Diversity

Some viruses exhibit complex structures beyond simple helical or icosahedral forms.

Bacteriophages (viruses that infect bacteria) may have a polyhedral head, helical tail, and attachment fibers.
Example: T4 bacteriophage of E. coli has a complex structure for infecting bacterial cells.

Viral Genome: Nucleic Acids

Genetic Material

Viruses contain either DNA or RNA as their genetic material, but never both.

DNA viruses are usually double-stranded (ds), but may be single-stranded (ss); can be circular or linear.
RNA viruses are usually single-stranded, but may be double-stranded or segmented.
Positive-sense RNA (): Ready for immediate translation.
Negative-sense RNA (): Must be converted to a proper form for translation.

Other Viral Substances

Enzymes

Some viruses carry enzymes necessary for their replication and infection processes.

Reverse transcriptase: Synthesizes DNA from RNA (e.g., HIV).
Polymerases: Synthesize DNA or RNA.
Replicases: Copy RNA.

Multiplication Cycles in Animal Viruses

Phases of Viral Multiplication

Animal viruses undergo a series of steps to infect host cells and produce new virions.

Adsorption: Virus binds to specific molecules on the host cell.
Penetration: Genome enters the host cell.
Uncoating: Viral nucleic acid is released from the capsid.
Synthesis: Viral components are produced.
Assembly: New viral particles are constructed.
Release: Assembled viruses exit the cell by budding (enveloped viruses) or cell lysis (naked viruses).

Viral Penetration and Uncoating

Entry Mechanisms

Viruses enter host cells via two main mechanisms:

Fusion: Viral envelope fuses directly with the host membrane (enveloped viruses).
Endocytosis: Entire virus is engulfed by the host cell (enveloped or naked viruses).

Release of Viruses

Methods of Exit

Viruses are released from host cells by:

Budding or exocytosis: Nucleocapsid buds from the membrane, shedding viruses gradually; cell is not immediately destroyed.
Lysis or rupturing: Nonenveloped and complex viruses are released when the cell dies and ruptures.

Persistent Infections and Latency

Chronic and Latent States

Some viruses establish persistent infections, remaining in the host for extended periods.

Cells are not immediately lysed.
Viruses may reactivate periodically (chronic latent state).
Examples: Herpes simplex virus (cold sores, genital herpes), Herpes zoster virus (chickenpox, shingles).

Oncogenic Viruses

Virus-Induced Cancer

Certain viruses can cause transformation of host cells, leading to cancer.

Oncogenic viruses initiate tumors in mammals.
Example: Human papillomavirus (HPV) is associated with cervical cancer.

Bacteriophage Replication Cycle

Phage Multiplication

Bacteriophages infect bacteria and replicate through a cycle similar to animal viruses, with some differences.

Adsorption: Phage binds to specific molecules on the bacterial cell.
Penetration: Only the nucleic acid enters the cytoplasm.
Replication: Viral components are produced.
Assembly: Viral components are assembled.
Maturation: Completion of viral formation.
Lysis & Release: Host cell is lysed, releasing new phages.

Lysogeny: The Silent Virus Infection

Temperate Phages and Prophage State

Some bacteriophages enter a reversible state called lysogeny.

Temperate phages insert their genome into the bacterial chromosome, becoming a prophage.
The cell is not lysed; the prophage is copied during cell division.
Lysogeny allows the virus to spread without killing the host cell.
Lysogenic conversion: Prophage genes may cause the production of toxins or enzymes, leading to pathology (e.g., Corynebacterium diphtheriae, Vibrio cholerae, Clostridium botulinum).

Cultivating and Identifying Animal Viruses

Methods

Viruses require living cells for cultivation and identification.

In vitro: Tissue cultures using cultured cells.
In vivo: Use of bird embryos or live animal inoculation.
Identification involves cell culture, antibody tests, and clinical assessment.

Prions and Other Nonviral Infectious Particles

Prions

Prions are misfolded proteins that cause neurodegenerative diseases.

Contain no nucleic acid.
Extremely resistant to sterilization.
Cause transmissible spongiform encephalopathies (TSEs).
Examples: Creutzfeldt-Jakob Syndrome (CJS) in humans, various animal prion diseases.

Viroids

Viroids are short pieces of RNA without a protein coat, identified only in plants.

Cause plant diseases.

Summary Table: Virus Structure Components

Component	Description	Present in All Viruses?
Capsid	Protein coat protecting nucleic acid	Yes
Envelope	Lipid membrane from host cell	No
Nucleic Acid	DNA or RNA (never both)	Yes
Enzymes	Assist in replication/integration	No

Key Equations and Terms

Positive-sense RNA:
Negative-sense RNA:
Reverse transcriptase reaction:

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.