Fundamentals of Viruses and Prions: Structure, Replication, and Pathogenicity

Viruses: Composition and Structure

Key Components of Viruses

Viruses are acellular infectious agents composed of genetic material surrounded by a protective protein coat. Their structure is essential for infectivity and classification.

• Capsid: The protein shell that encloses the viral genome, providing protection and aiding in host cell attachment.

• Capsomere: Subunits that make up the capsid, often arranged in a repetitive pattern.

• Envelope: A lipid membrane derived from the host cell, present in some viruses, containing embedded viral proteins (spikes).

• Spikes: Glycoprotein projections on the surface of the envelope or capsid, facilitating attachment to host cells.

• Icosahedral: A common geometric shape of the capsid, consisting of 20 triangular faces.

• Helical: Capsid proteins arranged in a spiral around the nucleic acid.

• Virion: The complete, infectious virus particle.

Functions of the Capsid

The capsid serves several critical roles in the viral life cycle:

• Protection: Shields the viral nucleic acid from environmental damage and host enzymes.

• Attachment: Contains structures that recognize and bind to host cell receptors.

• Facilitation of Entry: Assists in delivering the viral genome into the host cell.

Viral Genomes

Types of Nucleic Acids in Viruses

Viruses are classified based on the type of nucleic acid they contain:

• RNA Viruses: Contain ribonucleic acid as their genetic material.

• DNA Viruses: Contain deoxyribonucleic acid as their genetic material.

Classification of Viruses

Structural and Envelope-Based Classification

Viruses can be classified by their structure and the presence or absence of an envelope:

• Enveloped Viruses: Possess a lipid membrane (envelope) surrounding the capsid.

• Non-enveloped Viruses: Lack an envelope; only the capsid surrounds the genome.

• Shape-Based Classification: Viruses may be icosahedral, helical, or complex in shape.

Bacteriophage Replication Cycles

Lytic Cycle of Bacteriophages

The lytic cycle is a process by which bacteriophages infect and destroy bacterial cells, producing new viral particles.

• Attachment: Phage binds to specific receptors on the bacterial surface.

• Entry: Phage injects its DNA into the host bacterium.

• Synthesis: Host DNA is degraded; phage proteins and genomes are synthesized.

• Assembly: New phage particles are assembled from synthesized components.

• Release: Host cell lyses, releasing new phages to infect other cells.

Lysogenic Cycle of Bacteriophages

The lysogenic cycle allows the phage genome to integrate into the host chromosome, remaining dormant until activation.

• Attachment: Phage attaches to host cell receptors.

• Entry: Phage injects DNA into the host cell.

• Integration: Phage DNA integrates into the host genome, forming a prophage.

• Replication: Prophage replicates along with host DNA during cell division.

• Excision: Prophage is excised by viral enzymes, initiating the lytic cycle.

Note: The lysogenic cycle does not immediately produce viruses and can lead to the lytic cycle under certain conditions.

Animal Virus Replication

Comparison with Bacteriophage Replication

Animal viruses and bacteriophages share similar replication steps but differ in entry and uncoating mechanisms.

• Entry: Animal viruses enter host cells via membrane fusion or endocytosis; bacteriophages inject their nucleic acid.

• Uncoating: Animal viruses must uncoat their capsid inside the host cell; bacteriophages leave their capsid outside.

• Lysogeny: Animal viruses may have latent phases similar to lysogeny in bacteriophages.

Example: Herpesviruses can remain latent in host cells, analogous to the lysogenic cycle.

Effects of Heat and Cold on Viruses

Enveloped vs. Non-enveloped Viruses

Physical conditions such as heat and cold affect viruses differently based on their structure.

• Enveloped Viruses: More sensitive to heat and freeze-thaw damage due to the fragile lipid envelope.

• Non-enveloped Viruses: More resistant to heat and cold; their protein capsid is more stable.

Virus Type	Heat Sensitivity	Cold Sensitivity
Enveloped	High (lipid envelope disrupted)	Moderate (freeze-thaw can damage envelope)
Non-enveloped	Low (protein capsid stable)	Low (capsid resists cold damage)

Viruses and Human Cancer

Oncogenic Viruses

Some viruses are associated with the development of human cancers.

• Prevalence: Viruses cause approximately 20-25% of human cancers.

• Mechanisms:

  • Some viruses promote oncogenes already present in the host.

  • Others interfere with tumor suppressor mechanisms.

• Examples: Human papillomavirus (HPV) and Epstein-Barr virus (EBV).

Prions and Prion Diseases

Nature and Pathogenicity of Prions

Prions are unique infectious agents composed solely of misfolded proteins, lacking nucleic acids.

• Definition: Prions are obligate intracellular parasites made of only misfolded proteins.

• Pathogenesis: Prion diseases result from misfolded prion proteins inducing misfolding in normal brain proteins, leading to neural tissue damage.

• Transmission: Accumulation of misfolded proteins causes transmissible spongiform encephalopathies (TSEs).

• Difference from Other Agents: Prions do not contain DNA or RNA and cause disease by altering protein shape, not by genetic material.

Example: Creutzfeldt-Jakob disease in humans.

*Additional info: Prion diseases are always fatal and currently have no effective treatment. They are resistant to standard sterilization procedures due to their proteinaceous nature.*