Viruses and Prions: Structure, Classification, and Replication

Viruses and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ fundamentally from bacteria and other microorganisms. They are obligate intracellular parasites, meaning they require living host cells to multiply. Viruses are acellular and lack the machinery for metabolic energy generation and protein synthesis, relying entirely on host cells for these functions.

• Contain DNA or RNA (never both)

• Protein coat (capsid) surrounds the nucleic acid

• No ribosomes or ATP-generating mechanisms

• Size: typically 20–400 nm; rod-shaped or spherical

• Viruses do not grow or divide; they are assembled from pre-formed components

Viruses vs. Bacteria

Viruses and bacteria are both infectious agents, but they differ in structure, replication, and cellular machinery. The following table summarizes key differences:

	Typical Bacteria	Rickettsias/Chlamydias	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass through Bacteriological Filters	No	No/Yes	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Host Range and Specificity

The host range of a virus refers to the spectrum of host cells it can infect. Most viruses infect only specific types of host cells, determined by the presence of specific attachment sites and cellular factors.

• Bacteriophages infect bacteria (e.g., T4 phage binds only to E. coli receptors)

• SARS-CoV-2 binds specifically to ACE2 receptors on human cells

Viral Structure

The virion is the complete, fully developed viral particle. Viruses can have either DNA or RNA as their genetic material, which may be single- or double-stranded, linear or circular. The protein coat, called the capsid, is made of subunits called capsomeres. Some viruses have an envelope (lipid, protein, and carbohydrate coating) and spikes (projections from the outer surface).

• Capsid/Nucleocapsid: Stable array of proteins, sometimes glycosylated

• Envelope: Usually derived from host cell membrane, contains virus-specific proteins (peplomers), acquired during budding

• Spikes: Aid in attachment to host cells

Virion vs. Virus

A virus exists in two phases: as a virion (infectious particle) outside the cell, and as a virus inside the cell after losing its virion structure. The virion is responsible for transmission, while the virus inside the cell directs replication.

Viral Genomes

Unlike host cells, which always have DNA genomes, viruses can have a variety of genome types:

• ssDNA (single-stranded DNA)

• dsDNA (double-stranded DNA)

• ssRNA (single-stranded RNA)

• dsRNA (double-stranded RNA)

Nucleocapsid Structure and Virus Morphology

Viruses exhibit several structural forms:

• Helical: e.g., Tobacco mosaic virus (TMV)

• Icosahedral: e.g., Human rhinovirus

• Complex: e.g., Variola virus, T4 bacteriophage (combines helical and icosahedral components)

Enveloped vs. Non-Enveloped Viruses

Enveloped viruses acquire their lipid bilayer from the host cell membrane during budding. This envelope contains virus-specific proteins (peplomers) and is essential for infectivity. Non-enveloped (naked) viruses lack this lipid layer.

Fundamental Properties of Viruses

All viruses share several key properties:

1. Have a nucleic acid genome (DNA or RNA, never both)

2. Virus particles break down and release their genomes inside the cell

3. Virus particles are assembled from pre-formed components, not by division

4. Depend on host cell ribosomes for protein synthesis

5. Depend on host cell for energy generation

Virus Replication and Infectivity

Viruses are only biologically active inside host cells. They recruit the host cell's biosynthetic machinery to produce new viruses, often resulting in the destruction (lysis) of the host cell. Some viruses cause productive infections, while others may transform host cells (tumor viruses).

• Virus numbers can expand explosively, producing thousands of copies per infected cell

• Replication involves attachment, penetration, synthesis, assembly, and release

Prions

General Characteristics of Prions

Prions are unique infectious agents composed solely of protein, with no nucleic acid component. The prion protein and its encoding gene are found in normal, uninfected cells. Prions are associated with both infectious and inherited diseases.

• Prion diseases include Creutzfeldt-Jakob disease in humans, scrapie in sheep, and bovine spongiform encephalopathy (BSE, "mad cow disease") in cattle

• Prions cause neurodegenerative diseases characterized by spongiform changes in the brain

Additional info: Prions propagate by inducing misfolding of normal cellular proteins, leading to disease. They are resistant to conventional methods of sterilization and are not affected by nucleic acid-targeting treatments.

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