Characterization and Classification of Viruses, Viroids, and Prions

Characterization and Classification of Viruses, Viroids, and Prions

Characteristics of Viruses

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

• Minuscule and Acellular: Viruses are extremely small and lack cellular structure.

• Genetic Material: Contain either DNA or RNA, but never both.

• Infectious Agents: Cause infections in various organisms and are responsible for many diseases in the industrialized world.

• Metabolic Inactivity: Cannot carry out metabolic pathways, grow, or respond to the environment independently.

• Dependence on Host: Require host cell machinery for reproduction and metabolic processes.

• Lack of Cellular Components: No cytoplasmic membrane, cytosol, or organelles.

• States: Exist in both extracellular and intracellular forms.

States of Viruses

Viruses exhibit two distinct states during their life cycle: extracellular and intracellular.

• Extracellular State (Virion):

  • Composed of a protein coat (capsid) surrounding the nucleic acid.

  • The combination of nucleic acid and capsid is called the nucleocapsid.

  • Some virions possess a phospholipid envelope derived from the host cell membrane.

  • The outermost layer provides protection and recognition sites for host cells.

• Intracellular State:

  • The capsid is removed upon entry into the host cell.

  • The virus exists as naked nucleic acid within the host.

Genetic Material of Viruses

Viral genomes display remarkable diversity, which is a primary criterion for their classification.

• Genome Type: May be DNA or RNA, but never both in a single virus.

• Forms: Table 13.3

  • Double-stranded DNA (dsDNA)

  • Single-stranded DNA (ssDNA) travel to nucleus and to form a template they form a hair pin ex parvoviruses

  • Double-stranded RNA (dsRNA)

  • Single-stranded RNA (ssRNA): can either be positive sense (can act like mRNA, polymerase) or Negative sense (viral RNA cannot be directly translated, can not be mRNA)

  • Retroviruses : Are +single stranded RNA (this has a special enzyme - reverse transcriptase--- CDNA--- +ssRNA ---translation of viral protein of ribosome)

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

• Size: Viral genomes are much smaller than those of cellular organisms.

Key Terms

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

• Capsid: Protein shell that encases the viral genome.

• Nucleocapsid: The combined structure of the viral genome and capsid.

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

Example

Influenza virus is an enveloped virus with a segmented, single-stranded RNA genome. Its envelope contains glycoprotein spikes that facilitate attachment to host respiratory cells.

Additional info:

• Viruses are classified based on their genome type, capsid morphology, presence or absence of an envelope, and host range.

• Viruses do not fit the classical definition of living organisms due to their inability to reproduce or metabolize independently.