Viruses, Viroids, and Prions: Structure, Replication, and Pathogenicity

Viruses, Viroids, and Prions

Introduction to Viruses

Viruses are acellular infectious agents that require a living host cell to replicate. They are distinct from cellular life forms and possess unique structural and genetic features.

• Virion: A complete, infectious virus particle outside a host cell, consisting of nucleic acid and a protein coat.

• Host Range: The spectrum of host cells a virus can infect, determined by specific interactions between viral surface proteins and host cell receptors.

• Receptors: Molecules on the host cell surface that viruses recognize and bind to, dictating host specificity.

Basic Features of Viruses

• Capsid: The protein coat surrounding the viral nucleic acid, composed of subunits called capsomeres.

• Capsid Shapes:

  • Helical: Rod-shaped, with nucleic acid coiled inside (e.g., Tobacco mosaic virus).

  • Polyhedral: Many-sided, often icosahedral (e.g., Adenovirus).

  • Complex: Complicated structures, often with additional components (e.g., bacteriophages).

• Nucleic Acid: Can be DNA or RNA, single- or double-stranded, linear or circular.

• Envelope: A lipid membrane derived from the host cell, surrounding some viruses. Contains viral proteins and glycoprotein spikes.

  • Function: Helps viruses evade the host immune system and facilitates entry into host cells.

  • Enveloped Viruses: e.g., Influenza virus, HIV.

  • Naked Viruses: Lack an envelope; e.g., Poliovirus, Adenovirus.

• Spikes: Surface projections that aid in attachment to host cells and can be targets for immune responses.

Bacteriophages and Zoonotic Infections

• Bacteriophage: A virus that infects bacteria. Often used in molecular biology and genetic engineering.

• Zoonotic Infection: Diseases caused by viruses that can be transmitted from animals to humans. Examples include rabies, Ebola, and influenza.

Taxonomy of Viruses

Viruses are classified based on nucleic acid type, replication strategy, morphology, and host range.

• Viral Species: A group of viruses sharing the same genetic information and ecological niche.

• Subspecies: Designated by numbers or letters (e.g., Herpes simplex virus 1 and 2).

Growth and Cultivation of Viruses

• Challenges: Viruses require living cells for replication, making them difficult to grow in artificial media.

• Methods:

  • In Animals: Used for studying pathogenesis and immune responses.

  • In Embryonated Eggs: Common for vaccine production (e.g., influenza vaccine).

  • In Cell Cultures: Most widely used; allows observation of cytopathic effects.

Viral Multiplication

Viruses must enter a host cell and use its machinery to replicate. The process differs between bacteriophages and animal viruses.

Lytic Cycle of Bacteriophages

• Attachment: Phage attaches to host cell.

• Penetration: Phage injects DNA into host.

• Biosynthesis: Phage DNA directs synthesis of viral components.

• Maturation: Assembly of phage particles.

• Release: Host cell lyses, releasing new phages.

Lysogenic Cycle

• Prophage: Phage DNA integrates into host genome and replicates with it.

• Outcomes: Host cell survives; phage can later enter lytic cycle.

Animal Virus Replication

• Attachment to host cell membrane.

• Entry by endocytosis or fusion.

• Uncoating of viral nucleic acid.

• Biosynthesis of viral components.

• Maturation and Release:

  • Budding: Virus acquires envelope from host membrane (enveloped viruses).

  • Lysis: Host cell bursts, releasing naked viruses.

Categories of Viral Infections

• Acute: Rapid onset, short duration (e.g., influenza).

• Latent: Virus remains dormant, can reactivate (e.g., herpes simplex).

• Persistent (Chronic): Continuous viral production (e.g., HIV, hepatitis B).

Replication of DNA and RNA Viruses

• DNA Viruses: Usually replicate in the host cell nucleus using host enzymes.

• RNA Viruses: Replicate in the cytoplasm. Require RNA-dependent RNA polymerase.

• Sense (+) RNA Viruses: Genome acts as mRNA for protein synthesis.

• Antisense (-) RNA Viruses: Genome is complementary to mRNA; must be transcribed to (+) sense RNA first.

• Retroviruses: Use reverse transcriptase to convert RNA to DNA, which integrates into the host genome as a provirus.

  • Replication pathway: $\text{RNA} \rightarrow \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$

Viruses and Cancer

• Oncogene: A gene that can transform a cell into a tumor cell.

• Oncogenic Virus: Virus capable of inducing tumors (e.g., Human papillomavirus, Epstein-Barr virus).

• Provirus Formation: Integration of viral DNA can disrupt normal cell regulation, leading to uncontrolled growth.

• Mechanisms: Activation of oncogenes, inhibition of tumor suppressor genes, or chronic inflammation.

• Examples: Cervical cancer (HPV), liver cancer (Hepatitis B and C viruses).

Influenza Virus

• Transmission: Respiratory droplets.

• Key Proteins:

  • Hemagglutinin (HA): Binds to host cell receptors.

  • Neuraminidase (NA): Facilitates release of new virions.

• Health Consequences: Can cause severe respiratory illness, complications in vulnerable populations.

• Annual Vaccination: Required due to antigenic drift and shift, leading to new viral strains.

Ebola Virus

• Relevance: Causes severe hemorrhagic fever with high mortality rates.

• Transmission: Direct contact with bodily fluids of infected individuals or animals.

Viroids

• Definition: Infectious, short, circular RNA molecules without a protein coat.

• Function: Cause plant diseases by interfering with gene expression.

Prions

• Definition: Infectious proteins that lack nucleic acids.

• Examples: Creutzfeldt-Jakob disease, mad cow disease (bovine spongiform encephalopathy).

• Mechanism: Prions induce abnormal folding of normal proteins in the brain, leading to neurodegeneration.

Feature	Virus	Viroid	Prion
Genetic Material	DNA or RNA	RNA (short, circular)	None
Protein Coat	Present (capsid)	Absent	Absent
Infects	Animals, plants, bacteria	Plants	Animals (especially nervous tissue)
Replication	Requires host cell machinery	Host RNA polymerase	Induces misfolding of host proteins

Additional info: Some explanations and examples have been expanded for clarity and completeness, such as the mechanisms of viral oncogenesis and the replication strategies of RNA viruses.