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

Viruses and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from cellular life forms. They are obligate intracellular parasites, meaning they require living host cells to multiply. Viruses can infect all forms of life, including animals, plants, bacteria, and archaea.

• Obligate Intracellular Parasites: Viruses cannot reproduce or carry out metabolic processes outside a host cell.

• Genetic Material: Viruses contain either DNA or RNA, but never both. Their genomes can be single-stranded or double-stranded, linear or circular, and are much smaller than those of cells.

• Structure: Viruses lack ribosomes, cytoplasmic membrane, cytosol, and organelles. They do not generate ATP.

• States: Viruses exist in two states: extracellular (virion) and intracellular (nucleic acid only).

• Classification: Viruses are classified based on their genetic material, structure, and host range.

Example: The hepatitis B virus is a DNA virus that infects human liver cells.

Are Viruses Alive?

The debate over whether viruses are alive centers on their inability to carry out independent metabolism, growth, or reproduction. Some scientists consider them complex chemicals, while others view them as the simplest forms of life due to their ability to replicate and evolve within host cells.

Comparison of Viruses and Cells

Viruses differ from cellular organisms in several fundamental ways. The following table summarizes these differences:

Property	Viruses	Cells
Metabolism	Inert outside cell	Metabolize independently
Growth/Division	Do not grow or divide	Grow and divide
Cellular Structure	Acellular	Cellular
Genetic Material	DNA or RNA	Both DNA and RNA
Protein Synthesis	No ribosomes	Ribosomes present
Replication	Assembly-line using host	Self-replicating

Structure of Viruses

Viruses are composed of a nucleic acid core surrounded by a protein coat called a capsid. Some viruses also possess a lipid envelope derived from the host cell membrane, with glycoprotein spikes for host recognition.

• Virion: The complete, infectious viral particle.

• Capsid: Protein shell made of capsomeres.

• Envelope: Lipid, protein, and carbohydrate layer present in some viruses.

• Spikes: Surface projections for attachment to host cells.

Viral Morphology

Viruses exhibit several morphological types:

• Helical: Rod-shaped, with nucleic acid inside a hollow, cylindrical capsid.

• Polyhedral: Many-sided, often icosahedral.

• Enveloped: Surrounded by a lipid envelope.

• Complex: Complicated structures, such as bacteriophages with heads and tails.

Host Range and Specificity

The host range of a virus is determined by the presence of specific attachment sites on the host cell and compatible cellular factors. Most viruses infect only certain cell types within a single host species, but some are generalists.

Viral Taxonomy

Viruses are classified into families and genera based on genetic material, structure, and host range. Genus names end in -virus, and family names end in -viridae. Viral species are defined by shared genetic information and ecological niche.

• Example: Papillomavirus (genus), Papillomaviridae (family), Human papillomavirus (common name).

Isolation, Cultivation, and Identification of Viruses

Viruses must be grown in living cells. Methods include:

• Bacteriophages: Grown in bacterial cultures, detected by plaque formation on agar plates.

• Animal Viruses: Grown in living animals, embryonated eggs, or cell cultures. Continuous cell lines (e.g., HeLa) are commonly used for research.

• Identification: Based on cytopathic effects, serological tests (antibody reactions), and nucleic acid analysis (PCR).

Viral Replication Cycles

Viruses replicate by hijacking the host cell's machinery. The two main cycles in bacteriophages are the lytic and lysogenic cycles.

• Lytic Cycle: Virus replicates and lyses the host cell, releasing new virions.

• Lysogenic Cycle: Viral DNA integrates into the host genome as a prophage, replicating with the host cell until induced to enter the lytic cycle.

Multiplication of Animal Viruses

The replication of animal viruses involves several steps:

1. Attachment: Virus binds to host cell membrane.

2. Entry: By receptor-mediated endocytosis or fusion.

3. Uncoating: Viral genome is released inside the cell.

4. Biosynthesis: Production of viral nucleic acids and proteins.

5. Maturation: Assembly of new virions.

6. Release: By budding (enveloped viruses) or cell lysis.

Biosynthesis of DNA and RNA Viruses

• DNA Viruses: Replicate DNA in the nucleus, synthesize capsid in the cytoplasm.

• RNA Viruses: Replicate in the cytoplasm using RNA-dependent RNA polymerase. Positive-sense RNA acts as mRNA; negative-sense RNA must be transcribed to positive-sense.

• Retroviruses: Use reverse transcriptase to make DNA from RNA, which integrates into the host genome (e.g., HIV).

Viruses and Cancer

Some viruses can cause cancer by integrating oncogenes into the host genome or activating host protooncogenes. Oncogenic viruses include certain DNA viruses (e.g., HPV, HBV) and RNA retroviruses (e.g., HTLV-1, HTLV-2).

• Oncogenes: Genes that can transform normal cells into cancerous cells.

• Transformed Cells: Exhibit abnormal growth and may express tumor-specific antigens.

Latent and Persistent Viral Infections

Some viruses can remain dormant (latent) in host cells and reactivate later, while others cause persistent infections with continuous viral production.

• Latent Infections: Virus remains inactive for long periods (e.g., herpes simplex, varicella-zoster).

• Persistent Infections: Virus is continuously released (e.g., HIV, hepatitis B).

Disease	Type	Causative Virus
Cold sores	Latent	Herpes simplex 1 and 2
Shingles	Latent	Varicellovirus
Cervical cancer	Persistent	Human papillomavirus
HIV/AIDS	Persistent	HIV

Prions

Prions are infectious proteins that cause neurodegenerative diseases known as spongiform encephalopathies. They lack nucleic acids and are highly resistant to standard sterilization.

• Structure: Prions are misfolded forms of normal cellular proteins (PrP), with β-pleated sheets instead of α-helices.

• Transmission: By ingestion, transplantation, or contact with contaminated instruments.

• Diseases: Include Creutzfeldt-Jakob disease, mad cow disease (BSE), and kuru.

• Deactivation: Prions are destroyed by incineration or autoclaving in concentrated sodium hydroxide.