Viral Structure and Classification

DNA viruses versus RNA viruses

A virus's genome is either DNA or RNA, never both. This is the single most fundamental split in virology and the primary organising axis for classifying every virus.

DNA viruses generally replicate their genome using host DNA polymerase, since DNA-templated DNA synthesis is chemically identical to normal host cell replication. This replication most commonly occurs within the host cell nucleus. Poxviruses are the major exception, replicating entirely within the cytoplasm and carrying their own DNA-dependent RNA polymerase to make this possible.

RNA viruses generally must supply their own RNA-dependent RNA polymerase, because host cells possess no enzyme capable of copying RNA into RNA. Most RNA viruses replicate within the cytoplasm. Influenza is a notable exception among negative-sense RNA viruses, replicating its genome inside the nucleus.

Single stranded versus double stranded genomes

This distinction applies to both DNA and RNA viruses. Double-stranded genomes are directly transcribable by conventional means. Single-stranded genomes require additional steps before transcription can proceed: consideration of strand sense for RNA viruses, or synthesis of a second complementary strand for single-stranded DNA viruses such as parvoviruses.

Positive sense RNA

A positive-sense genome can function directly as mRNA. It is immediately translated by host ribosomes upon release into the cytoplasm, with no prior transcription step required. This property makes a purified positive-sense RNA genome, on its own, directly infectious if introduced into a susceptible cell, since translation can begin the moment it enters, even in the complete absence of any virion protein.

Negative sense RNA

A negative-sense genome is complementary to mRNA and cannot be translated directly. The virus must first transcribe its genome into a positive-sense, mRNA-equivalent copy using an RNA-dependent RNA polymerase. This enzyme must therefore be present and functional immediately upon entry, which is why it is packaged directly within the virion itself. The host cell provides no equivalent enzyme, and unlike a positive-sense genome, an isolated negative-sense genome alone is not infectious.

Segmented genomes

Some viruses carry their genome divided across multiple separate physical segments rather than as a single continuous molecule. Influenza is the classic example, carrying eight segments. Several other medically important groups, including some bunyaviruses, also carry segmented genomes.

Segmentation enables reassortment. When two different strains of a segmented virus co-infect the same host cell, progeny virions can be assembled from a mixture of segments drawn from both parent strains, producing a novel genetic combination in a single step. This is a fundamentally different, far more abrupt mechanism of genetic change than the gradual accumulation of point mutations.

Linear versus circular genomes

Most viral genomes are linear. Some DNA viruses, including papillomaviruses and polyomaviruses, carry circular genomes instead. This structural feature is relevant to their specific replication strategy, and for some circular DNA viruses, to their capacity to persist within the host cell nucleus as an episome, a circular DNA element that replicates independently of, but alongside, the host chromosome.

Capsomeres are the repeating protein subunits from which the capsid is built. They self-assemble into the final capsid structure without requiring additional host or viral scaffolding in most cases, an efficient strategy that lets a comparatively small viral genome encode only one or a few distinct capsid protein types while still producing a large, structurally complex final shell.

Icosahedral symmetry produces a near-spherical structure built from twenty triangular faces. It is the most space-efficient way to enclose a genome using the minimum quantity of protein, and it is the most common capsid architecture among viruses generally, seen in adenoviruses, herpesviruses, picornaviruses, and many others.

Helical symmetry arranges capsomeres in a spiral around the nucleic acid, producing a rod-shaped or filamentous nucleocapsid. It is common among negative-sense RNA viruses, including influenza's internal ribonucleoprotein complex, rabies virus, and Ebola virus.

Complex symmetry describes a structure that fits neither simple geometric category, often incorporating additional external structural elements. Poxviruses are the classic medically important example, with a complex, brick-shaped or ovoid structure built from multiple internal layers rather than a single discrete geometric capsid.

The lipid envelope is a lipid bilayer surrounding the nucleocapsid in enveloped viruses, acquired by budding through a host cell membrane.

Viral glycoproteins are virus-encoded proteins embedded within the envelope, projecting outward as surface spikes. They mediate receptor attachment and membrane fusion during entry, and they function as the principal target of neutralising antibodies, making them the dominant target of vaccine design against essentially every effective enveloped-virus vaccine.

The envelope's host-derived membrane component is taken directly from a host cell membrane, whether plasma membrane, nuclear membrane, or a membrane of the endoplasmic reticulum or Golgi apparatus, depending on the specific virus and its budding site. The envelope itself therefore carries no virus-encoded lipid, only virus-encoded protein embedded within otherwise host-derived lipid.

This has a direct structural consequence: environmental fragility. Because the envelope is a delicate lipid bilayer rather than a robust protein shell, enveloped viruses are comparatively fragile outside a host, vulnerable to drying, heat, and disruption by lipid solvents such as detergents, alcohol, and soap. This is a sharp contrast to the comparatively resilient protein-only capsid of nonenveloped viruses.

The clinical and infection-control consequence of this table is significant and directly testable. Alcohol-based hand sanitiser is reliably effective against enveloped viruses but substantially less effective against nonenveloped viruses. Outbreaks caused by nonenveloped viruses, norovirus being the classic example, specifically require soap-and-water hand hygiene rather than reliance on alcohol gel alone.

Acid resistance among nonenveloped enteric viruses is the direct structural basis for their capacity to survive passage through the stomach and establish infection via the faecal-oral route, a property most enveloped viruses lack.