Virus Structure and Classification
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Direct answer
Virus structure and classification rest on two features: the capsid, a protein shell with icosahedral, helical or complex symmetry that encloses the genome, and the nucleic acid type. Some virions add a lipid envelope acquired from the host cell during budding. The Baltimore scheme sorts all viruses into seven classes by genome type and mode of mRNA synthesis, and that scheme is the one exams test most often.
What you must remember
- Icosahedral symmetry: The capsid has 20 triangular faces, 12 vertices and 5-fold axes, as seen in adenovirus, herpesvirus and poliovirus.
- Helical symmetry: The genome lies inside a coiled protein helix, as in influenza and rabies virus, and rabies virus appears bullet-shaped.
- Complex symmetry: Poxviruses and bacteriophages do not fit a single pattern, so they are classed as complex, with bacteriophage T4 having an icosahedral head and a helical tail.
- Envelope tests: Enveloped viruses lose infectivity on exposure to ether and detergents, whereas naked viruses resist them, so ether sensitivity is the classic bench test.
- Envelope origin: The envelope is a lipid bilayer taken from the host membrane during budding, and it carries glycoprotein spikes that mediate attachment and fusion.
- Nucleic acid types: Genomes are DNA or RNA, single or double stranded, linear or circular, and RNA genomes may be segmented, as in influenza and rotavirus.
- Baltimore classes: Group I is dsDNA, II ssDNA, III dsRNA, IV positive-sense ssRNA, V negative-sense ssRNA, VI ssRNA with reverse transcription (retroviruses) and VII dsDNA with reverse transcription (hepadnaviruses).
Where the exam tries to catch you
Most errors come from mixing up envelope, symmetry and Baltimore group as if they were one property.
- Trap: Assuming every segmented RNA virus is negative-sense Correct: Rotavirus is segmented dsRNA in Group III, while influenza is segmented negative-sense ssRNA in Group V, so segmentation alone does not fix the class.
- Trap: Treating an envelope as proof of an RNA genome Correct: Herpesviruses and poxviruses are both enveloped but carry double-stranded DNA, so the envelope tells you nothing about the nucleic acid.
- Trap: Calling bacteriophages naked and icosahedral Correct: Bacteriophage T4 has a complex structure with an icosahedral head and a helical tail, which is why it is classed separately.
Frequently asked questions
Which Baltimore group contains retroviruses?
Retroviruses belong to Baltimore Group VI, which holds single-stranded positive-sense RNA viruses that copy their genome into DNA using reverse transcriptase. Hepadnaviruses such as hepatitis B virus belong to a different class, Group VII.
Why is ether sensitivity used to test for an envelope?
Ether sensitivity is used because enveloped viruses lose infectivity when their lipid bilayer is dissolved, whereas naked viruses with protein capsids resist it. The test is quick and cheap, but it does not reveal whether the genome is DNA or RNA.
Why is rabies virus described as bullet-shaped?
Rabies virus has a helical nucleocapsid wrapped in an envelope, and the envelope gives the virion its bullet shape. Its genome is single-stranded negative-sense RNA, which places rabies virus in Baltimore Group V.
Which virus families have segmented genomes and why does it matter?
Orthomyxoviruses such as influenza and reoviruses such as rotavirus have segmented genomes. Segmentation allows reassortment when two strains infect the same cell, and this is the basis of antigenic shift in influenza.
What do adenovirus and herpesvirus have in common structurally?
Adenovirus and herpesvirus both have icosahedral capsids and double-stranded DNA genomes. Herpesvirus is enveloped, whereas adenovirus is a naked virus, so the two are separated by the envelope test.