Viral Structure and Replication
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Direct answer
Icosahedral, helical or complex — every capsid follows one of three symmetries, wrapped around a genome that may be DNA or RNA, single- or double-stranded, positive or negative sense, with or without a lipid envelope picked up when the virus buds through host membranes. Replication then runs the same six steps regardless of design: attachment to a receptor, penetration by endocytosis or fusion, uncoating, biosynthesis of early non-structural and late structural components, assembly, and release by lysis or budding. Positive-sense RNA like poliovirus acts directly as messenger RNA; negative-sense viruses such as influenza must carry their own RNA-dependent RNA polymerase; retroviruses reverse-transcribe — distinctions the exam tests through Baltimore-style genome questions.
What you must remember
- Size extremes: poxviruses are the largest animal viruses (roughly 300 nm, complex symmetry, visible under light microscope limit arguments); parvovirus is the smallest (~20 nm, single-stranded DNA).
- Naked versus enveloped: enveloped viruses are ether- and acid-sensitive and spread through secretions and blood; naked viruses (enterovirus, rotavirus, adenovirus, parvovirus, papillomavirus) resist ether, acid and detergents and survive the gut — a favourite matching question.
- Genome flags: double-stranded DNA — pox, herpes, adenovirus; single-stranded DNA — parvovirus; partially double-stranded circular DNA — hepatitis B; positive-sense single-stranded RNA — picornavirus, togavirus, coronavirus, flavivirus; negative-sense — orthomyxovirus, paramyxovirus, rhabdovirus, filovirus, bunyavirus; double-stranded RNA — reovirus and rotavirus; retrovirus — positive-sense RNA reverse-transcribed to DNA.
- Attachment specificity: influenza haemagglutinin to sialic acid; HIV gp120 to CD4 with CCR5 or CXCR4; EBV to CD21; rabies to nicotinic acetylcholine receptor at the neuromuscular junction.
- Penetration styles: receptor-mediated endocytosis (influenza) versus direct membrane fusion (HIV gp41, herpes) — antiviral drug targets follow.
- Eclipse period: from uncoating until the first detectable progeny, virus cannot be recovered from the cell — distinguish it from latent infection.
- Release: lysis for poliovirus and most naked viruses; budding for enveloped viruses, with influenza taking its envelope from the plasma membrane and herpes from nuclear membranes.
- Influenza quirks: segmented genome enabling reassortment (pandemic antigenic shift), transcription in the nucleus (unique among RNA viruses except orthomyxo and retro), and the M2 ion channel uncoating step once targeted by amantadine.
Following one virus through the cycle
Track influenza to see the whole machinery. Haemagglutinin spikes clasp sialic-acid receptors; endocytosis internalises the virion; acidification of the endosome opens the M2 ion channel, loosening the matrix and dumping eight RNA segments into the cytoplasm — the exact step amantadine used to block before resistance retired it. The segments enter the nucleus, where the viral polymerase transcribes messenger RNA capped by scavenged host fragments; proteins return to assemble new virions, and neuraminidase (the oseltamivir target) cleaves sialic acid so the budding virus can leave. Now swap in poliovirus: a positive-sense genome that translates immediately in the cytoplasm, lyses the cell — no envelope, no nucleus, no budding. Two viruses, two complete logics, and most structure questions are simply asking which logic fits.
How the exam frames it
Three recurring frames: matching genome type to virus family (know the lists cold); naked-versus-enveloped behaviour in the gut or on disinfection; and antigenic shift versus drift — shift is segment reassortment producing pandemics, drift is haemagglutinin point mutation producing annual epidemics. The eclipse period is asked as a definition, and "which RNA virus replicates in the nucleus" (influenza) remains the perennial viva twist.
Frequently asked questions
Which viruses possess icosahedral symmetry?
Most DNA viruses including adenovirus, papovavirus, parvovirus and picornaviruses; helical symmetry belongs to enveloped RNA viruses like influenza, rabies and measles, while poxviruses are complex.
What happens during the eclipse period?
After uncoating, no infectious virus is detectable inside the cell while genomes and proteins are being synthesised; infectious progeny reappear only at assembly.
Why do naked viruses survive the gastrointestinal tract?
Their protein capsid resists acid, bile and detergents that would strip a lipid envelope, allowing faeco-oral transmission — the logic behind enterovirus and rotavirus epidemiology.
What is antigenic shift in influenza?
Reassortment of segmented genomes when two strains co-infect one host, creating a novel haemagglutinin or neuraminidase against which populations have little immunity — the pandemic mechanism.
Which enzyme must negative-sense RNA viruses carry?
An RNA-dependent RNA polymerase packed within the virion, because host cells cannot read or copy negative-sense RNA without it.