Viruses, Viroids, and Prions: Structure, Classification, and Comparison with Bacteria

Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from cellular life forms such as bacteria. They possess distinct structural and functional properties that define their role as obligate intracellular parasites.

• Obligate Intracellular Parasites: Viruses require living host cells to multiply, as they cannot reproduce independently.

• Genetic Material: Viruses contain either DNA or RNA as their genetic material, but never both.

• Protein Coat: The genetic material is enclosed within a protein coat called a capsid.

• Lack of Ribosomes: Viruses do not possess ribosomes and therefore cannot synthesize proteins independently.

• No ATP-Generating Mechanism: Viruses lack the metabolic machinery to generate ATP, relying entirely on the host cell for energy.

• Additional info: Some bacteria, such as Chlamydia and Rickettsia, also require living host cells to multiply, but they are cellular organisms and differ fundamentally from viruses.

Comparison: Viruses vs. Bacteria

The following table summarizes the key differences between typical bacteria, Rickettsias/Chlamydias, and viruses:

Characteristic	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	Yes/No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Additional info: This table highlights that viruses lack many features of cellular life, such as plasma membranes, ribosomes, and metabolic pathways, making them fundamentally different from bacteria.

Host Range of Viruses

The host range of a virus refers to the spectrum of host cells it can infect. This range is typically narrow, with most viruses infecting only specific types of cells within one host species.

• Specificity: Determined by the presence of specific attachment sites on the host cell and compatible cellular factors.

• Examples:

  • HIV infects CD4+ T cells.

  • Hepatitis C infects hepatocytes via LDL receptors.

• Bacteriophages: Viruses that infect bacteria are called bacteriophages or "phages" for short.

• Size Range: Viruses range from 20 nm to 1000 nm in length, making them much smaller than most bacteria.

Additional info: The specificity of viral infection is a key factor in disease transmission and pathogenesis. Bacteriophages are important tools in molecular biology and biotechnology.