Viruses and Prions: Structure, Classification, Replication, and Clinical Aspects

Viruses and Prions: Structure, Classification, Replication, and Clinical Aspects

Viruses: Basic Properties

Viruses are nonliving pathogens that infect a wide range of hosts, including humans and other mammals. They are acellular and obligate intracellular pathogens, meaning they require a host cell to replicate. The study of viruses is known as virology.

• Size: Extremely small (20–400 nm)

• Structure: Submicroscopic, infectious agents

• Host Range: Over 5,000 mammal-infecting viral species described; many remain uncharacterized

Comparing Viruses, Prokaryotes, and Eukaryotes

Viruses differ fundamentally from prokaryotic and eukaryotic cells in their structure, replication, and metabolic capabilities.

Feature	Viruses	Prokaryotes	Eukaryotes
Cellular Structure	Acellular	Cellular	Cellular
Genetic Material	DNA or RNA	DNA	DNA
Replication	Obligate intracellular	Binary fission	Mitosis/meiosis
Metabolism	None	Present	Present
Size	20–400 nm	0.5–5 μm	10–100 μm

Additional info: Table inferred from context and standard microbiology comparisons.

Viral Structure and Genomic Features

Virion and Capsid

A virion is a single, infectious virus particle. It consists of an exterior protective protein capsid and genetic material (DNA or RNA). The capsid is made of capsomere subunits and accounts for most of the virion's mass.

• Capsid Shapes: Helical, icosahedral, or complex

• Helical capsid: Hollow tube structure

• Icosahedral capsid: Three-dimensional polygon

• Complex capsid: Deviations from helical or icosahedral, often seen in bacteriophages

Viral Envelopes

Some viruses possess a lipid-based envelope surrounding the capsid, acquired from the host cell membrane during budding. Naked viruses lack this envelope and are released by lysing the host cell.

• Enveloped viruses: Arise from budding

• Naked viruses: Arise from cell lysis

• Bacteriophages: Always naked

Viral Spikes (Peplomers)

Many viruses have spikes (peplomers), which are glycoprotein extensions that facilitate attachment and entry into host cells. These spikes bind to specific host cell factors.

• Influenza A spikes: Hemagglutinin (HA) and Neuraminidase (NA)

Viral Genomes

Viral genomes are highly diverse and typically contain fewer than 300 genes. They encode capsomere proteins, enzymes for replication, and structural factors. Genome types include:

• DNA or RNA

• Single or double-stranded

• Single or segmented sections

• Circular or linear

Viral Genome Replication Strategies

Viruses use various strategies to produce mRNA for protein synthesis, depending on their genome type:

• dsDNA viruses: Transcribed by host RNA polymerases

• ssDNA viruses: Converted to dsDNA before transcription

• ssRNA+ viruses: Genome acts as mRNA

• ssRNA- viruses: Genome is complementary to mRNA; requires RNA-dependent RNA polymerase

• Retroviruses: RNA genome converted to DNA by reverse transcriptase, then integrated into host genome

• dsRNA viruses: Transcribed to make mRNA

Viral Genome Evolution

Mutation and Reassortment

Viruses mutate rapidly due to quick replication and lack of proofreading in RNA polymerases. Mutations can be neutral, beneficial, or deleterious. Reassortment occurs when two viral strains coinfect a host cell, leading to new strains.

• Antigenic drift: Minor changes in viral antigens (e.g., influenza spikes)

• Antigenic shift: Major genetic reassortment, potentially leading to pandemics

Classifying and Naming Viruses

Classification Criteria

Viruses are classified by the International Committee on Taxonomy of Viruses (ICTV) based on:

• Type of nucleic acid (DNA or RNA)

• Capsid symmetry (helical, icosahedral, complex)

• Presence or absence of envelope

• Genome architecture (ssDNA, ssRNA, etc.)

Medically Important Viral Families

Examples of DNA and RNA virus families relevant to human health:

Family	Genome	Capsid	Envelope	Example
Herpesviridae	dsDNA linear	Icosahedral	Enveloped	Herpes simplex virus
Picornaviridae	ssRNA+	Icosahedral	Naked	Poliovirus
Orthomyxoviridae	ssRNA- segmented	Helical	Enveloped	Influenza virus
Retroviridae	ssRNA+ (reverse transcriptase)	Icosahedral	Enveloped	HIV

Additional info: Table entries inferred from textbook context.

Host Range and Tropism

Host range is the collection of species a virus can infect. Tropism refers to the tissue or cell specificity, determined by viral surface factors.

• Broad tropism: Infects multiple cell types

• Narrow tropism: Infects only one cell type

Virus Sizes

Viruses vary greatly in size, from 30 nm (rhinovirus, poliovirus) to 1,500 nm (pithovirus).

Naming Conventions

Viruses are not assigned to domains or kingdoms. The highest taxon is phylum, followed by order, family, genus, and species.

Viral Replication Pathways

Bacteriophage Replication

Bacteriophages replicate via two main pathways: lytic and lysogenic.

• Lytic pathway: Immediate virion production, host cell lysis

• Lysogenic pathway: Viral genome integrates into host genome as a prophage, replicated with host DNA

Phage Conversion

Prophages can confer new pathogenic properties to bacteria, such as toxin production (e.g., Corynebacterium diphtheriae, Clostridium botulinum).

Animal Virus Replication

Animal viruses follow a six-step replication pathway:

1. Attachment

2. Penetration

3. Uncoating

4. Replication

5. Assembly

6. Release

Persistent Infections

Some animal viruses cause persistent infections:

• Chronic: Continuous release of virions, slow disease progression (e.g., HIV)

• Latent: Intermittent flare-ups with dormancy (e.g., Herpesviridae family)

Oncogenic Viruses

Oncogenic viruses (oncoviruses) can cause cancer by stimulating uncontrolled cell division or reducing cell death signals. Examples include human papilloma viruses (HPVs) and human T-lymphotropic viruses (HTLV).

Clinical Aspects of Viruses

Laboratory Growth of Viruses

Viruses require host cells for laboratory cultivation. Bacteriophages are grown using plaque assays, where clear zones (plaques) indicate cell lysis.

Animal viruses are grown in tissue culture, embryonated eggs, or live animal hosts.

Diagnostic Tests for Viral Detection

Diagnostic tests detect viral proteins, genetic material, or patient antibodies. Key methods include:

• Agglutination tests: Use antibodies or antigens linked to beads; positive result is bead clumping

• ELISA: Detects antigens or antibodies via color change

• Nucleic acid detection: PCR, sequencing, fluorescent probes

Antiviral Drugs

Antiviral drugs target various steps in viral replication, including attachment, penetration, uncoating, replication, assembly, and release. Most drugs limit infection rather than cure it. Vaccines are crucial for prevention.

• Nucleoside analogs: Block replication (e.g., acyclovir, ribavirin)

• Reverse transcriptase inhibitors: Target retroviruses (e.g., AZT)

• Protease inhibitors: Block viral protein processing

• Interferons: Signal cells to defend against viruses

• Release inhibitors: Prevent virion budding (e.g., oseltamivir, zanamivir)

Prions

Prion Structure and Diseases

Prions are infectious proteins with no genetic material. They cause transmissible spongiform encephalopathies (TSEs), which can be inherited or acquired. Examples include:

• Gerstmann-Straussler-Schienker syndrome

• Fatal familial insomnia

• Creuzfeldt-Jakob disease (CJD)

Prion-like Neurodegenerative Diseases

Some neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS), exhibit prion-like features due to misfolded proteins in the brain.

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