Viruses, Viroids, and Prions

Introduction and Historical Context

Viruses are submicroscopic infectious agents that differ fundamentally from other microorganisms in their structure, biology, and reproduction. The discovery of viruses was prompted by diseases too small to be seen with light microscopy, leading to the development of filtration techniques and the coining of the term virus (Latin for toxin or poison). The advent of electron microscopy enabled direct visualization and study of viruses, facilitating advances in biotechnology and therapy.

• Edward Jenner pioneered vaccination against smallpox, leading to its eradication.

• Martinus Beijerinck coined the term virus.

• Bacteriophages were discovered to infect bacteria.

Impact: Viruses cause diseases such as polio, influenza, AIDS, and rabies. Antibiotics are ineffective against viruses; antiviral therapies and vaccines are required.

Future: Advances in synthetic chemistry, genetic engineering, and computational screening are improving viral therapies and prevention.

General Structure and Classification of Viruses

Virus Structure

Viruses consist of genetic material (DNA or RNA) enclosed in a protein shell called the capsid. They lack the machinery for independent replication and are obligate intracellular parasites.

• Capsid: Protein shell coded by viral genome.

• Capsomeres: Structural subunits of the capsid, assembled from protomeres.

• Nucleocapsid: Association of capsid proteins with viral nucleic acid.

• Virion: Fully assembled, infectious viral particle.

• Envelope: Lipid membrane surrounding some viruses, derived from host cell membranes.

• Spikes: Glycoprotein projections aiding in host cell attachment.

Virus Classification

Virus classification is based on morphology, nucleic acid type, replication mode, host range, and disease type. Two main systems are used:

• ICTV System: Hierarchical taxonomy (Order, Family, Subfamily, Genus, Species).

• Baltimore Classification: Groups viruses by genome type and replication strategy (relationship to mRNA).

ICTV Hierarchy:

• Order (-virales)

• Family (-viridae)

• Subfamily (-virinae)

• Genus (-virus)

• Species (-virus)

Baltimore Groups:

• Group I: dsDNA viruses

• Group II: ssDNA viruses

• Group III: dsRNA viruses

• Group IV: (+) ssRNA viruses

• Group V: (−) ssRNA viruses

• Group VI: ssRNA-RT (reverse-transcribing RNA)

• Group VII: dsDNA-RT (reverse-transcribing DNA)

Virus Morphology

Viruses are classified morphologically as:

• Helical Viruses: Rod-shaped capsomeres forming a helical structure (e.g., Tobacco mosaic virus, influenza).

• Icosahedral Viruses: Spherical appearance, icosahedral symmetry (e.g., poliovirus, adenovirus).

• Enveloped Viruses: Surrounded by a lipid envelope, often with spikes (e.g., influenza, herpesvirus).

• Complex Viruses: Neither purely helical nor icosahedral; may have tails or complex outer walls (e.g., poxvirus, bacteriophage).

Size Comparison

Viruses range from 20 nm (parvovirus) to 450 nm (poxvirus). Most are smaller than bacteria, but some (e.g., mimivirus) approach bacterial size.

Genome Types

Viral genomes may be:

• DNA: dsDNA (Group I), ssDNA (Group II), or dsDNA-RT (Group VII).

• RNA: dsRNA (Group III), (+) ssRNA (Group IV), (−) ssRNA (Group V), or ssRNA-RT (Group VI).

• Linear or circular, segmented or non-segmented.

RNA viruses often mutate rapidly due to lack of proofreading by RNA polymerases.

Multiplication of Viruses

Bacteriophage Multiplication

Bacteriophages infect bacteria and multiply via two main cycles:

• Lytic Cycle: Results in host cell lysis and release of new phages.

• Lysogenic Cycle: Phage genome integrates into host DNA as a prophage, replicates silently until activation.

Stages of Lytic Cycle:

1. Adsorption: Phage attaches to bacterial cell via tail fibers.

2. Penetration: Injection of nucleic acid into host cell.

3. Replication: Host machinery synthesizes viral components.

4. Assembly: Viral parts assembled into mature virions.

5. Release: Host cell lyses, releasing new phages.

Lysogeny: Phage DNA integrates into host genome, may persist indefinitely or reactivate.

Animal Virus Multiplication

Animal viruses multiply in a similar but distinct process:

1. Adsorption: Virus attaches to specific host cell receptors.

2. Penetration: Entry via endocytosis or membrane fusion.

3. Uncoating: Release of viral genome into cytoplasm.

4. Replication: Synthesis of viral nucleic acids and proteins.

5. Assembly: Encapsidation of viral genome.

6. Release: Budding (enveloped viruses) or cell lysis (naked viruses).

Host range is determined by receptor specificity. Antibodies can block viral attachment, conferring immunity.

Effects of Viral Infections

Types of Viral Infections

• Abortive: No viral production in nonpermissive cells.

• Lytic (Cytocidal): Host cell death in permissive cells.

• Persistent: Chronic, latent, or slow infections without cell death.

• Transforming: Oncogenic transformation, leading to cancer.

Host Cell Damage

• Morphological (Cytopathic) Effects: Altered cell shape, detachment, lysis, membrane fusion, inclusion bodies, apoptosis.

• Physiological Effects: Changes in ion movement, secondary messengers, altered antigenic properties.

• Biochemical Effects: Inhibition or alteration of DNA, RNA, and protein synthesis.

• Genotoxic Effects: DNA damage, mutations, integration of viral DNA (oncogenic viruses).

Table: Cytopathic Effects of Selected Viruses

Major Groups of Viruses Infecting Humans

DNA Viruses

• Parvoviridae: B19 virus (erythema infectiosum)

• Papillomaviridae: Human papillomavirus (warts, cervical cancer)

• Polyomaviridae: JC and BK viruses (respiratory, kidney, brain infections)

• Adenoviridae: Respiratory, GI, conjunctiva, CNS, urinary infections

• Herpesviridae: HSV-1, HSV-2, VZV, EBV, CMV, HHV-6, HHV-7, HHV-8 (herpes, chickenpox, mononucleosis, Kaposi sarcoma)

• Poxviridae: Smallpox, molluscum contagiosum

• Hepadnaviridae: Hepatitis B

Table: Characteristics of Herpesviridae

RNA Viruses

• Picornaviridae: Poliovirus (polio), rhinovirus (common cold), hepatitis A

• Caliciviridae: Norovirus (gastroenteritis)

• Astroviridae: Astrovirus (gastroenteritis)

• Togaviridae: Sindbis, Ross River, Chikungunya, rubella

• Flaviviridae: Yellow fever, dengue, tick-borne encephalitis, hepatitis C

• Reoviridae: Rotavirus (GI infections), Colorado tick fever

• Orthomyxoviridae: Influenza A, B, C

• Paramyxoviridae: Measles, mumps, respiratory syncytial virus

• Rhabdoviridae: Rabies

• Bunyaviridae: Bunyamwera, La Crosse, hantavirus, Crimean-Congo hemorrhagic fever

• Coronaviridae: SARS-CoV, MERS-CoV, SARS-CoV-2 (COVID-19)

• Retroviridae: HIV, adult T-cell leukemia

Table: Hepatitis Viruses

Subviral Agents

Viroids

Viroids are small, circular, single-stranded RNA molecules (246–375 nucleotides) that infect plants. They lack a protein coat and do not encode proteins. Replication is independent of helper viruses.

Table: Classification of Viroids

Virusoids (Satellites)

Virusoids are circular, single-stranded RNAs (220–388 nucleotides) with ribozyme activity. They require a helper virus for replication and are generally associated with plant infections. The hepatitis delta virus (HDV) is a human virusoid requiring hepatitis B coinfection.

Prions

Prions are infectious proteins lacking nucleic acid. They cause neurodegenerative diseases by inducing abnormal folding of normal proteins, leading to transmissible spongiform encephalopathies (e.g., Creutzfeldt-Jacob disease, mad cow disease).

Table: Human Prion Diseases

Therapeutic and Biotechnological Applications

Phage Therapy

Lytic bacteriophages are used therapeutically to treat antibiotic-resistant bacterial infections. Phage therapy is highly specific and can be an alternative to antibiotics, especially for untreatable infections.

Gene Therapy and Vaccines

Viruses such as adenoviruses and alphaviruses are used as vectors in gene therapy and vaccine development.

Summary of Key Points

• Virus classification is based on morphology, genome type, replication, host, and disease.

• Viruses are host and tissue specific.

• Four main morphological types: helical, icosahedral, enveloped, complex.

• Viral genomes are DNA or RNA, single- or double-stranded.

• Replication uses host cell machinery; mechanisms differ between bacteriophages and animal viruses.

• Viral infections can be abortive, lytic, or persistent, causing various cellular changes.

• Subviral agents include viroids, virusoids, and prions.

Formulas and Equations

• Virus Mutation Rate (RNA viruses):

• Baltimore Classification (Genome to mRNA):

Additional info:

• Some context and explanations were expanded for clarity and completeness.

• Tables were reconstructed and summarized for main content.

• Scientific names and terms were italicized where appropriate.