Viruses, Viroids, and Prions Lecture
Viruses, Viroids, and Prions
Introduction
Subject: Viruses, Viroids, and Prions
Author: Dr. Frank Sauer
Text: © Dr. Sauer 2015-2020
Images: © 2016 Pearson Education, Inc.
Definitions
Virus:
Obligatory intracellular parasites that can only replicate within host cells.
Very small and filterable, sharing characteristics with small bacteria (e.g., rickettsias).
Entity Virus Definition:
Contains a single type of nucleic acid, either DNA or RNA.
Surrounded by a protein coat, which may be enveloped in lipids, proteins, and carbohydrates.
Multiplies inside a living cell by using host metabolic machinery due to a lack of its own enzymes.
Causes synthesis of specialized structures for transferring viral nucleic acid to other cells.
Virus Comparison
Comparison of Viruses and Bacteria:
Feature
Bacteria
Rickettsias
Chlamydias
Viruses
Intracellular Parasite
No
Yes
Yes
Yes
Plasma Membrane
Yes
Yes
Yes
No
Binary Fission
Yes
Yes
Yes
No
Capable of Passing
No
No/Yes
Yes
Yes
Bacteriological Filters
Yes
Yes
No
No
Possess DNA and/or RNA
Yes
Yes
Yes
No
ATP-Generating Metabolism
Yes
Yes/No
Yes
No
Ribosomes
Yes
Yes
Yes
No
Sensitive to Antibiotics
Yes
Yes
Yes
No
Sensitive to Interferon
No
No
No
Yes
Host Range
Host Range:
The spectrum of host cells that a virus can infect.
Most viruses can only infect specific cell types of one specific host, while some can infect a wide range of hosts including animals, plants, protists, fungi, and bacteria.
Bacteriophage (Phage):
Viruses that infect bacteria.
The host range of a virus is influenced by:
Recognition and Attachment:
Virus binds to specific receptor sites on host cells.
Availability of Cellular Factors:
Factors in the host cell required for virus reproduction.
Examples of Receptors:
For bacteriophages: components in the cell wall, fimbriae, and flagella.
Specific interactions between virus and host cell form the basis for viral therapies, such as tumor-destroying (oncolytic) viruses used in cancer treatment.
Size and Classification of Viruses
Size of Viruses:
Ranges from 20 to 1000 nm.
Virion: Complete, fully developed infectious viral particle.
Classification: Based on nucleic acid (DNA or RNA), structural properties of the coat.
Types of Nucleic Acids:
DNA: double-stranded and single-stranded.
RNA: single-stranded and double-stranded.
Can be linear or circular genomes, with lengths varying from a few thousand to 25,000 nucleotides (or base pairs).
Table of Examples:
Size of Various Viruses:
Poliovirus: 24 nm
Rhinovirus: 30 nm
Adenovirus: 30 nm
Rabies Virus: 90 nm
Various other viruses: size comparisons available.
Structure of Viruses: Capsid and Envelope
Capsid:
The protein coat that protects viral nucleic acid, varying in structure based on nucleic acid type.
Capsomers: Protein subunits making up the capsid.
The arrangement of capsomers is unique to each virus type.
Envelope:
In some viruses, the capsid is covered by an envelope consisting of lipids, proteins, and carbohydrates.
Enveloped viruses acquire their envelope from the host cell's plasma membrane when they exit the cell.
Envelopes may contain proteins from both the virus and the host cell.
Spikes:
Protein-carbohydrate complexes projecting from the envelope's surface, aiding in cell attachment.
Used for identification; influenza viruses use spikes for hemagglutination, which binds red blood cells together.
Nonenveloped Virus:
Capsid without an envelope; protects nucleic acid from enzymes and aids in host cell attachment.
Immune Response to Viruses
Virus infections stimulate the host's immune system to produce antibodies, which bind to surface components of the virus, leading to its degradation.
However, some viruses evade this response by expressing mutant surface components (e.g., influenza virus).
General Morphology of Viruses
Helical Viruses:
Appear as long rods, either rigid or flexible (e.g., rabies and Ebola viruses).
Polyhedral Viruses:
Many viruses, particularly those infecting plants and animals, have a polyhedral shape, often forming an icosahedron (20 triangular faces, 12 corners) (e.g., poliovirus, adenovirus).
Enveloped Viruses:
Enveloped versions are spherical, either helical (e.g., influenza virus) or polyhedral (e.g., human herpes virus).
Complex Viruses:
Have specialized structures (e.g., bacteriophage with tails).
Virus Classification
Genus names end in -virus, family names in -viridae, order ends in -ales.
Viral species are groups of viruses that share the same genetic information and ecological niche (host range).
No specific epithets are used; descriptive common names like human immunodeficiency virus (HIV) are applied.
Strains are indicated by numbers (e.g., HIV-1).
Example Families of Viruses Affecting Humans:
Parvoviridae: Causes fifth disease (human parvovirus B19).
Adenoviridae: Causes respiratory infections.
Herpesviridae: Includes herpes simplex viruses causing cold sores, chickenpox, mononucleosis.
Hepadnaviridae: Involves hepatitis B virus.
Virus Isolation, Cultivation, and Identification
Bacteriophages:
Can be cultivated in liquid and solid media.
Plaques: Round clear spots in bacterial lawns caused by lysis of bacteria.
Animal Viruses:
Some require living animals for cultivation, while others can grow in embryonated eggs (used in vaccines).
Cell Culture:
Infection of cultured cells leads to cell deterioration known as cytopathic effects (CPE).
Types of cell lines used include primary cell lines, diploid cell lines from human embryos, and continuous/cancerous cell lines.
Viral Multiplication
General Process of Viral Multiplication:
Involves a few essential genes from the viral genome, including capsid proteins and enzymes for replication.
Viruses replicate and may produce thousands of similar viruses in a single host cell, often leading to host cell death.
One-Step Growth Curve:
The Eclipse Period refers to the time when viruses disappear from the medium due to uptake by host cells.
Multiplication of Bacteriophages
Lytic Cycle:
Ends with the lysis of the host cell, including the following steps:
Attachment: Virus binds to receptor on the host cell.
Penetration: Phage lysozyme breaks the cell wall; viral nucleic acid is injected.
Biosynthesis: Viral DNA is transcribed, leading to protein synthesis.
Maturation: Assembly of complete virions occurs.
Release: Lysis of the host cell releases new virions.
Lysogenic Cycle:
Host cell remains alive while the bacteriophage genome integrates into the host cell's genome.
Consequences of Lysogeny
Lysogenic Bacteria:
Are immune to reinfection by the same phage.
Phage Conversion: Helpful genes may be introduced (e.g., certain toxins).
Transduction:
Bacteriophage can transfer bacterial genes during infection, categorized as generalized or specialized transduction depending on the process involved.
Multiplication of Animal Viruses
Similar to Bacteriophages:
Animal viruses attach to plasma membrane proteins and glycoproteins, use receptor-mediated endocytosis, and undergo uncoating.
Specific enzymes are involved in biosynthesis and release patterns, typically through budding for enveloped viruses.
Cancer and Viruses
Viral infections can lead to cancer development; mechanisms can involve viral genetic material integrating into host DNA.
Specific oncogenic viruses include those in families such as Retroviridae and Hepadnaviridae (e.g., causing liver cancer).
Oncogenes can arise from alteration to genes inherited from animal cells or induced by mutagens.
Other Viral Infections
Latent Viral Infections:
Virus remains inactive within a host cell until activation (e.g., cold sores).
Persistent Viral Infections:
Gradually progress over time and are usually fatal (e.g., SSPE from Measles virus).
Plant Viruses
Plant viruses may lead to significant agricultural diseases and can spread through insect vectors.
Protective barriers like cell walls complicate their infection mechanisms.
Prions and Viroids
Prions:
Proteinaceous infectious particles causing diseases through the conversion of normal proteins into pathogenic forms (e.g., CJD).
Viroids:
Short, naked RNA molecules causing various plant diseases.
Summary of Viral Biology
Viral reproduction and complications arising from infections demonstrate complexities unique from bacteria.
Pathogenicity and therapeutic implications of viruses influence biological, medical, and agricultural fields.