# Viral Structure and Replication > Viral structure and replication for FMGE Microbiology: capsid symmetry, envelopes, Baltimore classes, eclipse period and stepwise replication NBE asks. - Canonical URL: https://prepelephant.com/topics/fmge/microbiology/viral-structure-replication-fmge - Exam / course: FMGE · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Viral Structure and Replication", PrepElephant, https://prepelephant.com/topics/fmge/microbiology/viral-structure-replication-fmge ## 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.