Characterization and Classification of Viruses, Viroids, and Prions

Characteristics of Viruses

General Properties of Viruses

Viruses are unique infectious agents that differ significantly from cellular life forms. They are responsible for a wide range of diseases affecting humans, animals, plants, and bacteria.

• Minuscule, acellular, infectious agents containing either DNA or RNA as their genetic material.

• Cause infections in various organisms and are responsible for many diseases in industrialized societies.

• Cannot carry out metabolic pathways independently.

• Do not grow or respond to environmental stimuli.

• Cannot reproduce without a host cell; instead, they recruit the host cell's metabolic machinery to replicate.

• Lack cytoplasmic membrane, cytosol, and organelles.

• Exist in two states: extracellular and intracellular.

Extracellular and Intracellular States

• Extracellular state: The virus is called a virion. It consists of a protein coat (capsid) surrounding the nucleic acid. The combination of nucleic acid and capsid is termed the nucleocapsid. Some virions possess a phospholipid envelope derived from the host cell.

• The outermost layer of the virion provides protection and contains recognition sites for host cells.

• Intracellular state: The capsid is removed, and the virus exists solely as nucleic acid within the host cell.

Virion Structure

Virions are complete virus particles, consisting of nucleic acid and, in some cases, an envelope.

• Capsid: Protein shell that encases the viral genome.

• Envelope: Lipid membrane acquired from the host cell, present in some viruses.

Genetic Material of Viruses

Genome Diversity

Viruses exhibit greater diversity in their genetic material than cellular organisms, which is a key factor in their classification.

• Viral genomes may be composed of DNA or RNA, but never both.

• Types of viral genomes include:

  • Double-stranded DNA (dsDNA)

  • Single-stranded DNA (ssDNA)

  • Double-stranded RNA (dsRNA)

  • Single-stranded RNA (ssRNA)

• Genomes may be linear and segmented or single and circular.

• Viral genomes are much smaller than those of cells.

Key Terms and Definitions

• Virion: The complete, infectious form of a virus outside a host cell.

• Capsid: The protein shell that surrounds the viral nucleic acid.

• Nucleocapsid: The combined structure of nucleic acid and capsid.

• Envelope: A lipid membrane derived from the host cell, surrounding some virions.

• Genome: The complete set of genetic material in a virus.

Example: Classification Based on Genetic Material

• Influenza virus: Has a segmented, single-stranded RNA genome.

• Herpes simplex virus: Contains double-stranded DNA.

Table: Types of Viral Genomes

Type of Nucleic Acid	Strandedness	Example Virus
DNA	Double-stranded (dsDNA)	Herpes simplex virus
DNA	Single-stranded (ssDNA)	Parvovirus
RNA	Double-stranded (dsRNA)	Rotavirus
RNA	Single-stranded (ssRNA)	Influenza virus

Additional info: The above table provides examples of viruses classified by their genome type, which is a primary method used in virology.