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[[Image:Virus structure.jpg|thumb|right|150px|Virus Structure - B. Catchpole, RVC 2008]]
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'''[[Image:PapillomavirusCapsid.jpg|thumb|right|100px|Capsid of papillomavirus]]
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[[Category:Viruses]]
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'''[[Image:PapillomavirusCapsid.jpg|thumb|right|100px|Capsid of papillomavirus]]
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[[Category:Viruses]]