Virology Basics

Definition and Properties of Viruses

• Viruses are the smallest infectious agents, containing one kind of nucleic acid (RNA or DNA) in their genome, usually as a single molecule.
• Viruses are obligatory intracellular parasites.

Properties of Viruses

• Very small in size, requiring an electron microscope to be seen (except for poxvirus).
• Can pass through bacterial filters.
• Obligate intracellular parasites, replicating only inside host cells due to lack of machinery for replication.
• Cannot be grown on artificial culture media, only on living cells (tissue culture, embryonated eggs, or living animals).
• Genome is either DNA or RNA, never both.

Structure of Viruses

• The virus particle (virion) consists of a nucleic acid core called the "virus genome" surrounded by a protein coat called the capsid.
• Viral nucleic acid:
  • Contains a single kind of nucleic acid (RNA or DNA) that encodes genetic information for replication.
  • May be single-stranded or double-stranded.
  • Most RNA animal viruses have single-stranded RNA genomes, while most DNA animal viruses have double-stranded DNA genomes.
• Functions of viral nucleic acid:
  • Essential infectious component of the virion.
  • Encodes genetic information for replication.
  • Carries heritable characteristics of the virus.

Capsid

• Protein coat surrounding the nucleic acid, made of tiny protein subunits called "capsomeres".
• Nucleic acid surrounded by its protein coat is called the nucleocapsid.
• Functions:
  • Protects nucleic acid from damage.
  • Facilitates attachment to host cell during infection.
  • Source of viral antigens.

Symmetry of Virus Capsids

• Arrangement of capsomers determines virus symmetry, which may be:
  • Icosahedral or cubical symmetry (e.g., herpesviruses and adenoviruses).
  • Helical symmetry (e.g., influenza virus).
  • Complex symmetry (e.g., poxvirus and bacteriophages).

Envelope

• Lipoprotein membrane composed of lipid from the host cell membrane and virus-specific protein.
• Frequently contains glycoproteins in the form of spike-like projections that attach to host cell receptors.
• Matrix protein mediates interaction between capsid proteins and the envelope.

Morphology

• Most animal viruses are spherical.
• Poxviruses are brick-shaped.
• Influenza virus is spherical or filamentous.
• Bacteriophages are spermatozoa-like.

Metabolism

• Viruses are obligatory intracellular parasites.
• Contain no enzymes associated with energy metabolism, relying on host cells for replication.