Viruses: Introduction and Classification

Overview of Viruses, Viroids, and Prions

Viruses are acellular infectious agents that require host cells for replication. They are distinct from living organisms due to their inability to carry out independent metabolic processes.

• Virion: The complete, infectious form of a virus outside a host cell, consisting of a nucleic acid core surrounded by a proteinaceous capsid.

• Virus: Tiny acellular agent with either DNA or RNA as its genome; lacks cellular structure.

• Prion: Infectious protein particle lacking nucleic acids; causes disease by inducing abnormal folding of normal proteins (e.g., Creutzfeldt-Jakob disease).

• Viroid: Naked RNA molecule that infects plants; lacks a protein coat.

Are Viruses Alive?

Viruses are not considered living organisms because:

• They are acellular (not made of cells).

• Cannot reproduce independently.

• Do not grow or undergo division.

• Do not perform metabolism or synthesize proteins independently.

Structural Characteristics of Viruses

Basic Components

• Nucleic Acid: Either DNA or RNA, single or double stranded.

• Capsid: Protein coat made of subunits called capsomeres; provides protection and attachment to host cells.

• Envelope: Some viruses have a lipid bilayer envelope derived from host cell membranes, containing viral glycoproteins.

• Spikes: Protein projections from the capsid or envelope, involved in host cell recognition and attachment.

Virus Envelope

• Acquired from host cell during replication or release.

• Composed of phospholipids and viral proteins (glycoproteins).

• Envelope proteins and glycoproteins are crucial for host recognition and penetration.

Virus Capsid Types

• Helical Viruses: Hollow, cylindrical capsid (e.g., Rabies virus).

• Polyhedral Viruses: Capsids with geometrical sides (e.g., Adenovirus).

• Complex Viruses: Complicated structures, such as bacteriophages with additional components (e.g., tail fibers).

Enveloped Virus Structures

• Envelope Helical Viruses: Helical capsid surrounded by an envelope.

• Envelope Polyhedral Viruses: Polyhedral capsid surrounded by an envelope.

Virus Sizes

Viruses are much smaller than cells and organelles:

• Poliovirus: ~30 nm

• Bacteriophage T4: ~225 nm

• Smallpox virus: ~200 nm x 300 nm

• Red blood cell: ~10,000 nm diameter

Families of Viruses That Affect Humans

Classification by Genome Type

Bacteriophage and Animal Viral Structure Compared

Host Specificity

• Plant viruses infect plant cells.

• Bacteriophages infect bacterial cells.

• Animal viruses infect animal cells.

Animal Virus Replication

General Steps

1. Attachment: Virus binds specifically to host cell receptors via capsid proteins or envelope glycoproteins.

2. Penetration (Entry): Virus enters host cell by direct penetration, membrane fusion, or endocytosis.

3. Biosynthesis: Viral genome is replicated and viral proteins are synthesized using host cell machinery.

4. Assembly and Maturation: Viral components are assembled into new virions.

5. Release (Exit): New virions exit the cell by lysis, budding, or exocytosis.

Attachment

• Specific interaction between viral proteins (spikes, capsomeres) and host cell receptors.

• Determines host range and tissue tropism.

Penetration

• Direct Penetration: Naked viruses inject genome into host cell.

• Membrane Fusion: Enveloped viruses fuse with host membrane, releasing genome.

• Endocytosis: Host cell engulfs virus in a vesicle; viral genome is released inside cell.

Biosynthesis

• Viral nucleic acid and proteins are synthesized using host cell machinery.

• Central Dogma: DNA → mRNA → Protein

Key equations:

Examples of Biosynthesis

• Positive-sense ssRNA virus: Genome acts as mRNA (e.g., Rhinovirus).

• Negative-sense ssRNA virus: Requires RNA-dependent RNA polymerase to synthesize mRNA (e.g., Rabies virus).

• Double-stranded RNA virus: Unwinds and transcribes mRNA from dsRNA (e.g., Rotavirus).

Assembly, Maturation, and Release

• Lysis: Host cell bursts, releasing naked viruses.

• Budding: Enveloped viruses acquire envelope as they exit host cell.

• Exocytosis: Viruses are released in vesicles.

Virus Latency

• Some animal viruses remain dormant in host cells (latency).

• Viral DNA may integrate into host chromosome, becoming a permanent part (e.g., HIV, chickenpox, herpes).

RT-ssRNA Virus Replication Cycle

• Retroviruses (e.g., HIV) copy their RNA to DNA via reverse transcriptase.

• Viral DNA integrates into host genome, establishing latency.

Viral Diseases: Measles and Herpes

Measles Virus (Morbillivirus)

• Etiology: Measles virus (ssRNA, helical capsid, enveloped)

• Virulence Factors:

  • Hemagglutinin: attaches to host epithelial cell receptors

  • Fusion protein: causes host cells to fuse and form syncytia (giant cells)

• Clinical Features:

  • Initial symptoms: sore throat, dry cough, headache, conjunctivitis, swollen lymph nodes, fever

  • Koplik's spots: oral lesions

  • Maculopapular rash: spreads from head to trunk and extremities

• Transmission: Airborne droplets; virus enters via nose, mouth, or conjunctiva

• Diagnosis: Clinical signs (rash, Koplik's spots)

• Treatment: No specific antiviral; ribavirin may be used; antibiotics for secondary infections

• Prevention: Live attenuated vaccine (MMR)

• Epidemiology:

  • Highly contagious

  • Humans are the only host

  • Vaccination has reduced incidence; outbreaks occur in unvaccinated populations

Herpes Simplex Virus (HHV-1 and HHV-2)

• Etiology: Human Herpes Viruses 1 and 2 (dsDNA, icosahedral capsid, enveloped)

• Virulence Factors:

  • Envelope glycoproteins: adhesion and fusion, formation of syncytia

  • Inactivation of complement

  • Latent infection: continual shedding even without symptoms

• Clinical Features:

  • Lips: herpes labialis (cold sores)

  • Fingers: herpes whitlow

  • Conjunctiva: ocular herpes

  • Genitals: herpes genitalis

• Pathogenesis:

  • Initial infection: replication in epithelial cells, inflammation, cell death, vesicle formation

  • Latent infection: virus remains inactive in nerve cells, reactivates intermittently

• Diagnosis: Presence of characteristic lesions

• Prevention: Gloves for healthcare workers; avoid contact with lesions

• Epidemiology:

  • HHV-1: usually acquired in childhood, often asymptomatic

  • HHV-2: acquired in adolescence/adulthood, often via sexual activity

  • Both can be transmitted between oral and genital mucosa

  • Herpes infections in adults are rarely life-threatening; neonatal infections can be fatal

Summary Table: Virus Replication Steps

Additional info:

• Central Dogma of Molecular Biology is essential for understanding viral biosynthesis:

• Syncytia formation allows viruses to evade immune detection by spreading directly between cells.

• Vaccination remains the most effective method for preventing viral diseases such as measles.