Definition of viruses, structure of viruses.

Virion

extracellular, infectious, inactive form

Vegetative virus

replicative form (autonomous extrachromosomal element)

Structure of the virion

1. Protein coat (capsid)
2. Nucleic acid (DNA)
3. Envelope (peplon) - lipid membrane
4. Envelope proteins (peplomers)
5. Enzymes
6. Nucleic acid (RNA – retroviruses)

Classification – Based on Morphological Characteristics

Non-enveloped viruses
Enveloped viruses

3 Basic Structures of the Capsid

Icosahedral
Helical
Binary

Helical

• Paramyxoviridae - mumps

• Orthomyxoviridae - influenza

• Rhabdoviridae - rabies

• Filoviridae- ebola

Icosahedral

• Adenoviruses

• Polyomaviridae

Binary

Bacteriophages

Capsid

• Morphological units: Capsomeres
• Metastable structure

Capsid Functions

• Protection of the genome

• Recognition and “packaging” of the genome

• Interaction with cell membranes and acquisition of the envelope

• “Transport” of the genome: binding to receptors, membrane fusion, uncoating of the genome, and delivery to the appropriate location

Envelope - Peplon

• Outer envelope = phospholipid bilayer
• Peplomer = (glyco)protein embedded in the peplon
• The envelope originates from the infected host cell membrane (determines host-cell specificity).

Membrane sources

• Nuclear membrane – derived envelope: Herpesviruses
• Endoplasmic reticulum/Golgi–derived envelope: Coronaviruses, Bunyaviruses, Hepadnaviruses
• Plasma membrane – derived envelope: Orthomyxoviruses, Paramyxoviruses, Rhabdoviruses, Arboviruses, Retroviruses, Togaviruses