Viral Structure
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Outside the host cell, viruses exist as particles known as virions, consisting essentially of the virus genome, packaged in a 'capsid,' and a lipid component, which contains proteins that aid entry into the host cell. Viruses can be classified as either:
- Naked- these contain only RNA/DNA and a protein coat
- Enveloped- as well as the RNA/DNA and protein coat, these viruses have a lipid-based membrane
Virus genome: Composed of either RNA or DNA- identification of viruses can be established by testing susceptibility to a ribonuclease or deoxyribonuclease
- Each nucleic acid is either single stranded or double stranded
- Nucleic acids can be either linear (free 3' and 5' ends) or circular (strands are covalently closed)
- Viral genomes can range from 1.7 kilobases (e.g. Porcine circovirus) to over 1200 kilobases (e.g. Mimivirus)
- The genes of some viruses are segmented, coded by two or more nucleic acid fragments, e.g. influenza virus
Viral proteins: Proteins that make up virions are usually structural proteins, used for:
- Protection of the genome
- Attachment to host cell
- Fusion of envelope to host cell membrane
- Non-structural proteins:
- enzymes, e.g. reverse transcriptase
- primers
- transcription factors
Capsids
This is the protein coat that surrounds the genome, and is responsible not only for genome protection but also recognition and attachment to the host cell
- Form symmetrical structures, e.g. helix, icosahedron, rod, cone
Membranes:
- Most viruses have a lipid membrane. If present at the surface, the membrane is known as the 'envelope'. Several viruses keep the lipid membrane within themselves
- Often associated with proteins such as glycoproteins- these often aid the fusion process