Viruses explained: structure, genome and replication strategy

What makes a virus acellular?

A virus is not a miniature cell. It has genetic material enclosed by a protein capsid and, in some viruses, a lipid envelope acquired from a host membrane. It lacks cytoplasm, ribosomes and the independent metabolic machinery needed to synthesize proteins or generate ATP.

Because replication depends on host enzymes, ribosomes, nucleotides and energy, viruses are obligate intracellular parasites. Outside a suitable host cell, a complete infectious particle—the virion—can be transported and remain infective, but it does not grow or reproduce independently.

The architecture of a virion

The viral genome may be DNA or RNA, single- or double-stranded, linear, circular or segmented. The capsid protects that genome and is built from protein subunits called capsomeres. Together, genome and capsid form the nucleocapsid.

Enveloped viruses carry a host-derived lipid membrane containing viral glycoprotein spikes. Those spikes bind specific receptors and help determine host and tissue specificity. Because lipid envelopes are disrupted by detergents and drying, many enveloped viruses are less stable outside the body than non-enveloped viruses.

The central challenge: making mRNA

Host ribosomes translate mRNA, so every virus must provide or produce readable positive-sense mRNA. A positive-sense RNA genome can function directly as mRNA. A negative-sense RNA genome must first be copied into a complementary positive strand by an RNA-dependent RNA polymerase.

DNA viruses usually transcribe mRNA from DNA. Retroviruses reverse-transcribe RNA into DNA, integrate that DNA into the host genome, and then use host transcription to produce viral RNA. Hepatitis B is a DNA virus with a reverse-transcription stage, which is why genome type and replication route must be considered together.

• Positive-sense RNA: genome can be translated directly.
• Negative-sense RNA: viral polymerase must first make positive mRNA.
• Double-stranded DNA: mRNA is transcribed from the DNA template.
• Retroviral RNA: reverse transcription creates an integrated DNA intermediate.

A general replication sequence

Despite enormous diversity, viral replication can be organised into attachment, entry, uncoating, biosynthesis, assembly and release. Attachment requires a match between viral proteins and host receptors. Entry may involve membrane fusion, endocytosis or genome injection, depending on the virus.

After uncoating, genome replication and viral protein synthesis use a mixture of host and viral enzymes. Newly produced components self-assemble. Viruses leave through cell lysis, budding or exocytosis; budding allows an enveloped virus to acquire membrane as it exits.

How to reason through virus questions

First identify what material is present. Protein only suggests a prion; naked infectious RNA in plants suggests a viroid; nucleic acid inside a capsid describes a virus. Then ask whether the question concerns the free virion or a genome stage inside the host.

For replication questions, work backward from mRNA. Ask how this genome can produce a positive-sense message for ribosomes. That single question resolves many apparently difficult classification problems.