Viruses, Viroids, and Prions: Structure, Classification, and Pathogenesis

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Chapter 13: Viruses, Viroids, and Prions

Distinctive Features of Viruses

Viruses are unique infectious agents that require living host cells to multiply. They differ fundamentally from cellular organisms in structure and replication.

Obligate intracellular parasites: Viruses cannot reproduce outside a host cell.
Genetic material: Contains either DNA or RNA, never both.
Protein coat: Called a capsid, made of capsomeres.
No ribosomes: Cannot synthesize proteins independently.
No ATP-generating mechanism: Rely on host cell for energy.

Comparison of Viruses and Bacteria

Viruses and bacteria differ in many structural and functional aspects.

Characteristic	Viruses	Bacteria
Cell type	Acellular	Prokaryotic
Genetic material	DNA or RNA	DNA
Ribosomes	Absent	Present
ATP generation	Absent	Present
Antibiotic sensitivity	No	Yes
Growth	Only in host cell	On artificial media

Host Range

The host range of a virus refers to the spectrum of host cells it can infect.

Determined by specific host attachment sites and cellular factors.
Bacteriophages: Infect bacteria; receptor sites are on cell wall, fimbriae, or flagella.
Animal viruses: Infect animal cells; receptor sites are typically on plasma membrane.

Virus Sizes

Viruses are much smaller than bacteria, typically ranging from 20 to 300 nm.

Can only be visualized with electron microscopy.
Examples: Poliovirus (~30 nm), Ebola virus (~970 nm).

Viral Structure

The structure of viruses is diverse, but all contain nucleic acid and a protein coat.

Virion: Complete, fully developed viral particle.
Nucleic acid: DNA or RNA, single or double-stranded, linear or circular.
Capsid: Protein coat made of capsomeres.
Envelope: Lipid, protein, and carbohydrate coating (in some viruses).
Spikes: Projections from outer surface, used for attachment.

Nucleic Acid

Viral genomes vary in type and complexity.

DNA or RNA, never both.
Single-stranded or double-stranded.
Linear, circular, or segmented.
Total amount of nucleic acid varies from a few thousand to 250,000 nucleotides.

Capsid and Envelope

Capsid: Protein coat composed of capsomeres.
Envelope: Lipid, protein, and carbohydrate layer derived from host cell membrane; may contain viral proteins and spikes.

General Morphology

Viruses are classified by their shape and structure.

Helical viruses: Hollow, cylindrical capsid (e.g., rabies, Ebola).
Polyhedral viruses: Many-sided, most are icosahedral (e.g., adenovirus, poliovirus).
Enveloped viruses: Spherical, surrounded by envelope.
Complex viruses: Complicated structures (e.g., bacteriophage).

Taxonomy of Viruses

Viruses are classified based on nucleic acid type and replication strategy (Baltimore classification).

Seven groups referred to as "taxa".
Genus names end in virus.
Family names end in viridae.
Order names end in ales.
Viral species: Group of viruses sharing the same genetic information and ecological niche.

Baltimore Classification System

This system classifies viruses based on their nucleic acid and how mRNA is produced.

Groups include dsDNA, ssDNA, dsRNA, ssRNA (+), ssRNA (-), retroviruses, and others.

Growing Viruses in the Laboratory

Bacteriophages: Grown in bacteria; plaques indicate viral growth.
Animal viruses: Grown in living animals, embryonated eggs, or cell cultures.
Cell cultures: Diploid cell lines (limited generations) and continuous cell lines (immortal, e.g., HeLa cells).

Viral Identification

Viruses are identified by several laboratory methods.

Cytopathic effects: Observable changes in host cells.
Serological tests: ELISA detects virus by antibody reaction.
Nucleic acid tests: PCR amplifies viral genetic material.

Viral Multiplication

Viruses must invade host cells and take over their metabolic machinery to multiply.

A single virion can give rise to thousands of progeny in one host cell.
One-step growth curve: Describes the rapid increase in viral particles after infection.

Multiplication of Bacteriophages

Bacteriophages multiply by two main mechanisms: lytic and lysogenic cycles.

Lytic cycle: Phage causes lysis and death of host cell.
Lysogenic cycle: Phage DNA incorporates into host DNA as a prophage; host cell survives and replicates prophage DNA.

T-Even (Tequatrovirus) Bacteriophages: The Lytic Cycle

Attachment: Phage attaches to host cell.
Penetration: Phage injects DNA into host.
Biosynthesis: Phage DNA and proteins are produced.
Maturation: New phage particles assembled.
Release: Host cell lyses, releasing new phages.

Bacteriophage Lambda (λ): The Lysogenic Cycle

Phage DNA integrates into host chromosome as a prophage.
Prophage is replicated with host DNA.
Specialized transduction can transfer specific bacterial genes.

Multiplication of Animal Viruses

Attachment: Virus binds to cell membrane.
Entry: By receptor-mediated endocytosis or fusion.
Uncoating: Viral nucleic acid released from capsid.
Biosynthesis: Production of viral nucleic acid and proteins.
Maturation: Assembly of viral components.
Release: By budding (enveloped viruses) or rupture (nonenveloped viruses).

The Biosynthesis of DNA Viruses

DNA viruses replicate their DNA in the nucleus using host enzymes and synthesize capsid proteins in the cytoplasm.

Adenoviridae: Double-stranded DNA, nonenveloped; respiratory infections, tumors.
Poxviridae: Double-stranded DNA, enveloped; smallpox, MPox.
Herpesviridae: Double-stranded DNA, enveloped; includes HSV, VZV, EBV, CMV, HHV-6, HHV-8.
Papovaviridae: Double-stranded DNA, nonenveloped; warts, cancer.
Hepadnaviridae: Double-stranded DNA, enveloped; hepatitis B virus, uses reverse transcriptase.

The Biosynthesis of RNA Viruses

RNA viruses replicate in the host cell cytoplasm using RNA-dependent RNA polymerase.

Positive-sense (+) RNA: Acts as mRNA for protein synthesis.
Negative-sense (-) RNA: Must be transcribed to + strand.
Double-stranded RNA: Both strands present.
Retroviruses: Use reverse transcriptase to produce DNA from RNA.

Major RNA Virus Families and Examples

Coronaviridae: SARS-CoV-2 (COVID-19), enveloped, single-stranded RNA.
Togaviridae: Alphavirus, transmitted by arthropods.
Rhabdoviridae: Lyssavirus (rabies).
Picornaviridae: Enterovirus (polio), rhinovirus (common cold), hepatitis A.
Reoviridae: Rotavirus (gastroenteritis).
Retroviridae: Lentivirus (HIV), Oncoviruses.

Viruses and Cancer

Some viruses can cause cancer by transforming normal cells into tumor cells.

Proto-oncogenes: Genes involved in normal cell growth; mutations can lead to cancer.
Oncogenic viruses: Integrate into host DNA and induce transformation.
DNA oncogenic viruses: Adenoviridae, Herpesviridae, Papovaviridae, Hepadnaviridae.
RNA oncogenic viruses: Retroviridae (HTLV-1, HTLV-2, FeLV).
Oncolytic viruses: Used to treat cancer by killing tumor cells or stimulating immune response.

Latent and Persistent Viral Infections

Some viruses can remain in the host for long periods, either as latent or persistent infections.

Latent viruses: Remain asymptomatic in host cell; may reactivate (e.g., herpesviruses).
Persistent infections: Gradually increase over time; often fatal (e.g., subacute sclerosing panencephalitis).

Virus	Type of Infection	Example Disease
Herpes simplex virus	Latent	Cold sores
Varicella-zoster virus	Latent	Shingles
Measles virus	Persistent	Subacute sclerosing panencephalitis

Plant Viruses and Viroids

Plant viruses and viroids are important pathogens in agriculture.

Plant viruses: Enter through wounds or insect vectors; cause economically significant diseases.
Viroids: Short pieces of naked RNA; cause diseases like potato spindle tuber disease.

Virus Family	Genome Type	Example Disease
Tobamovirus	ssRNA	Tobacco mosaic virus
Potyvirus	ssRNA	Potato virus Y
Geminivirus	ssDNA	Tomato yellow leaf curl virus

Prions

Prions are infectious proteins that cause neurodegenerative diseases.

Proteinaceous infectious particles: No nucleic acid.
Transmitted by ingestion, transplant, or surgical instruments.
Diseases include mad cow disease, Creutzfeldt-Jakob disease, fatal familial insomnia, sheep scrapie.
PrPc: Normal cellular prion protein.
PrPSc: Abnormal prion protein; accumulates in brain cells, forming plaques.
Disease is caused by conversion of PrPc to PrPSc.

Summary Table: Key Differences Between Viruses, Viroids, and Prions

Agent	Genetic Material	Protein Coat	Diseases
Virus	DNA or RNA	Present	Human, animal, plant diseases
Viroid	RNA	Absent	Plant diseases
Prion	None	Absent	Neurodegenerative diseases

Key Equations and Concepts

One-step growth curve:

Plaque-forming units (PFU):

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and key equations.