Virology Basics

Study Notes

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.

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.

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.

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.

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).

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.