Viruses, Viroids, and Prions: Tiny but Deadly (Chapter 13 Study Notes)

Study Guide - Smart Notes

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

Viruses, Viroids, and Prions: Tiny but Deadly

Introduction

This chapter explores three types of acellular infectious agents—viruses, viroids, and prions—which, despite their small size, can cause significant diseases in humans, animals, and plants. Understanding their structure, replication, and impact is essential for microbiology students.

Viruses

What are Viruses?

Viruses are infectious agents that are much smaller and simpler than even bacteria. They are composed of one or a few types of biomolecules, typically nucleic acids (DNA or RNA) surrounded by a protein coat (capsid). Viruses cannot reproduce independently and do not grow or respond to the environment outside a host cell. However, they can cause many human diseases by hijacking the host cell's metabolic machinery to increase their population.

Structure: Consist of genetic material (DNA or RNA) and a protein coat; some have an additional lipid envelope.
Replication: Require a host cell to replicate; direct the synthesis and assembly of new virus particles within the cell.
Diseases: Responsible for illnesses such as influenza, HIV/AIDS, and COVID-19.

Definition: A virus is a non-cellular infectious agent that can only replicate inside living cells.

Are Viruses Alive?

Viruses occupy a unique position in biology. Outside of host cells, they do not exhibit characteristics of life such as metabolism or growth. Within cells, they direct cellular processes to produce new viruses. Thus, they are considered biological entities at the edge of life.

Outside cells: Inert particles, not alive.
Inside cells: Active, directing synthesis and assembly of viral components.

Viral Replication: The Central Dogma and Strategies

Viruses must accomplish two main tasks to replicate:

Copy their genetic material (genome).
Make the proteins needed to form the capsid and other structures.

The process depends on the type of nucleic acid in the viral genome. The Central Dogma of Biology (DNA → RNA → Protein) guides these principles, but viruses use diverse strategies to achieve replication.

Messenger RNA (mRNA): The template for protein synthesis in cells and viruses.
Genome Types: Viruses may have DNA or RNA genomes, which can be single-stranded (ss) or double-stranded (ds).

Types of Viral Genomes

ssDNA: Single-stranded DNA
dsDNA: Double-stranded DNA
ssRNA: Single-stranded RNA (can be positive-sense [+] or negative-sense [−])
dsRNA: Double-stranded RNA

Replication Strategies

dsDNA Viruses: Replicate their genome in the host cell nucleus; viral proteins are made in the cytoplasm.
ssRNA (+) Viruses: Genome acts as mRNA and is directly translated into proteins.
ssRNA (−) Viruses: Genome cannot be directly translated; must be converted to (+) RNA by viral RNA polymerase.
Retroviruses: Use reverse transcriptase to convert RNA genome into DNA, which integrates into the host genome.

Equation:

Additional info: Some viruses carry their own polymerases or transcriptases to facilitate replication, especially RNA viruses.

Viral Release and Spread

Lytic Release: Non-enveloped viruses often lyse the host cell, causing cell death and acute infections.
Budding: Enveloped viruses exit the cell by budding through the cytoplasmic membrane, acquiring a lipid envelope and evading the immune system.

Viruses and Cancer (Oncogenes)

Oncogenes and Viral-Induced Cancer

Some viruses can cause cancer by interfering with normal cell growth controls. Oncogenes are genes that, when mutated or overexpressed, can lead to uncontrolled cell division and tumor formation.

Mechanisms: Viruses may carry oncogenes or activate host oncogenes, leading to cancer.
Examples: Human papillomavirus (HPV) can cause cervical cancer by producing proteins (E6 and E7) that disrupt cell cycle regulation.
Other Cancers: Epstein-Barr virus (Burkitt's lymphoma), Hepatitis B virus (liver cancer), Kaposi's sarcoma-associated herpesvirus.

Additional info: Cancer risk increases with exposure to carcinogens (e.g., smoking, radiation) and viral infection.

Growing Viruses in the Laboratory

Methods for Culturing Viruses

Viruses cannot be grown outside of living cells. Scientists use several methods to culture viruses:

Bacterial Cultures: Bacteriophages are grown in bacterial cells.
Animal Cell Cultures: Viruses infecting animals are grown in animal cells or fertilized eggs.
Embryonated Eggs: Used for vaccine production; viruses are injected into specific sites within the egg.
Tissue Culture: Cells isolated from tissues and grown in flasks or dishes; allows for controlled study of viral replication.

Additional info: Ethical and technical issues arise when using animals or plants for viral culture.

Viroids

Characteristics of Viroids

Viroids are extremely small, circular pieces of RNA that infect plants. They lack a protein coat (capsid) and do not code for proteins. Viroids cause several plant diseases, such as potato spindle tuber and cucumber pale fruit.

Genome: Small, circular RNA (as few as 250 nucleotides).
Infectivity: Infectious in plants; spread through mechanical damage or contaminated tools.
Diseases: Potato spindle tuber, cucumber pale fruit, and others.

Prions

Characteristics of Prions

Prions are infectious proteins that cause neurodegenerative diseases in animals and humans. A prion is a misfolded form of a normal cellular protein (PrP) that can induce other normal proteins to misfold, leading to disease.

Structure: Abnormal prion proteins have a high content of β-pleated sheets.
Transmission: Spread by ingestion, contact with infected tissue, or inherited mutations.
Diseases: Creutzfeldt-Jakob disease (humans), bovine spongiform encephalopathy (BSE, "mad cow disease"), scrapie (sheep).
Resistance: Prions are highly resistant to heat and chemicals; destroyed only by extreme conditions.

Additional info: Prion diseases are rare but always fatal, causing progressive neurological decline.

Comparison Table: Viruses, Viroids, and Prions

Agent	Genetic Material	Protein Coat	Infects	Diseases
Virus	DNA or RNA	Present	Animals, plants, bacteria	Influenza, HIV, COVID-19, etc.
Viroid	RNA (circular, small)	Absent	Plants	Potato spindle tuber, cucumber pale fruit
Prion	None (protein only)	Absent	Animals, humans	Creutzfeldt-Jakob disease, BSE

Summary

Viruses, viroids, and prions are acellular infectious agents with unique structures and replication strategies.
Viruses require host cells for replication and can cause a wide range of diseases, including cancer.
Viroids infect plants and consist only of small, circular RNA molecules.
Prions are infectious proteins that cause fatal neurodegenerative diseases.