Viruses – Structure, Classification and Replication
A virus sits awkwardly on the boundary of what biologists are willing to call living. It carries genetic information and it evolves, but it has no cytoplasm, no ribosomes and no way of generating its own energy, so outside a host it is closer to a chemical particle than to an organism. Everything a virus does that looks like life happens inside a cell it has taken over. That single fact explains why viruses are studied separately from bacteria, fungi and protists, and why the strategies used to control them differ so sharply from antibiotics. This note covers what viruses are made of, how they are classified, how they replicate in bacterial and animal hosts, the damage they cause, and the two stranger infectious agents that are not viruses at all.
What separates a virus from a cell
Cells, whether bacterial or eukaryotic, share a common minimum: a plasma membrane, cytoplasm, ribosomes, and the metabolic machinery to build molecules and generate ATP. A virus has none of this. It is described as acellular, meaning it lacks cells entirely rather than simply being small. What a virus does carry is a genome and a protein shell to protect and deliver it.
Because it cannot make proteins or copy its own genome unaided, a virus is an obligate intracellular parasite. It must reach a host cell, get its genome inside, and redirect that cell’s ribosomes, enzymes and nucleotides to build new virus particles. A complete, assembled virus particle outside a cell is called a virion, and in that state it is metabolically inert. It can persist, but it cannot grow, respond or reproduce.
Structure: capsid, genome and envelope
The genome sits at the core. Unlike cellular life, which always stores its genetic information as double-stranded DNA, viral genomes may be DNA or RNA, and either single-stranded or double-stranded. This variability is one of the defining features of viruses and it has direct consequences for how each virus replicates once inside a cell.
Around the genome is the capsid, a protein coat built from repeating subunits called capsomeres. Using many copies of a few protein types is economical: a small genome does not have the coding capacity to specify a large, complicated shell. The capsid gives the virion its characteristic shape, most commonly helical, icosahedral, or a complex form such as the head-and-tail structure seen in many bacteriophages.
Some viruses add an envelope, a lipid bilayer taken from the membrane of the host cell as the virion leaves. Embedded in it are viral glycoproteins that recognise receptors on the next host cell. Enveloped viruses are generally more fragile outside a host because that lipid layer is easily disrupted by drying, heat and detergents, which is one reason simple hand washing is effective against several of them.
How viruses are classified
Morphology was the first basis for grouping viruses, and shape plus the presence or absence of an envelope still appears in every description. On its own it is too coarse to be useful, because unrelated viruses can look alike under an electron microscope.
The more informative scheme groups viruses by genome type and by the route each takes to produce messenger RNA, since making mRNA is the one step every virus must accomplish. This gives seven groups: double-stranded DNA, single-stranded DNA, double-stranded RNA, single-stranded RNA of positive polarity, single-stranded RNA of negative polarity, single-stranded RNA that is reverse transcribed into DNA, and double-stranded DNA that replicates through an RNA intermediate.
Polarity is worth pausing on. A positive-polarity RNA genome reads like mRNA and can be translated by host ribosomes as soon as it enters the cell. A negative-polarity genome is complementary to mRNA, so the virus must carry its own polymerase into the cell to transcribe a usable copy first. Two viruses that look identical can therefore behave completely differently in the first minutes of infection.
Bacteriophages: the lytic and lysogenic cycles
A bacteriophage is a virus that infects bacteria, and phage replication is the clearest textbook illustration of the two strategies open to a virus.
In the lytic cycle the phage attaches to the bacterial surface, injects its genome, and immediately shuts down host synthesis. The cell’s machinery is redirected to making phage genomes and capsid proteins, these assemble into new virions, and the cell finally bursts. Lysis kills the host and releases the progeny in one step, so the infection is fast and obviously destructive.
In the lysogenic cycle the phage genome instead integrates into the bacterial chromosome and is copied passively every time the bacterium divides. The host survives and may carry the integrated genome for many generations without visible symptoms. Stress signals such as DNA damage can trigger excision, at which point the virus switches to the lytic pathway. Lysogeny matters medically because integrated phage genes sometimes encode toxins that turn a harmless bacterium into a pathogenic one.
Replication in animal cells
Animal viruses follow the same broad sequence but differ in the detail of each step. Attachment depends on a specific fit between a viral surface protein and a receptor on the target cell. That specificity determines host range and tissue tropism, and it is why most viruses infect a narrow set of species and often only certain cell types within them.
Entry happens either by endocytosis, where the cell engulfs the whole virion, or by fusion, where an enveloped virus merges its envelope directly with the plasma membrane. Once inside, the capsid is removed in a step called uncoating, exposing the genome to the cell’s machinery.
Replication and protein synthesis then follow the route dictated by the genome type. New genomes and capsid proteins are assembled into virions, which leave either by lysis or by budding. In budding, virions push out through the plasma membrane individually and take a piece of it with them as their envelope, so the host cell is not necessarily destroyed and can keep releasing virus for a prolonged period.
Not every infection proceeds immediately. Some viruses establish latency, persisting in the body in a quiet state and producing intermittent symptoms when they reactivate. This is why some viral infections recur throughout life from a single original exposure.
The damage viruses cause in animals and plants
Viral disease is described by its course as much as by its cause. An acute disease rises and resolves within a short period. A chronic infection persists for a long time. An asymptomatic infection produces no signs at all and is detected only by laboratory testing, which is what makes some viruses so difficult to contain.
At the cellular level the effects fall into recognisable patterns. Cytopathic viruses damage the cells they infect, sometimes causing cell necrosis outright. Others disturb the control of division, producing hyperplasia, abnormally high growth, or hypoplasia, abnormally low growth. A small number are oncogenic and can contribute to cancer by interfering with the regulation of the cell cycle.
Plant viruses cause serious agricultural losses and often produce distinctive symptoms such as mosaic patterning on leaves, stunted growth, or galls. Because plant cells are protected by a cell wall, these viruses usually depend on mechanical wounding or on an insect vector to get inside, and control tends to focus on the vector and on planting resistant varieties rather than on treating the infected plant.
Vaccines, antiviral drugs and viruses as tools
Prevention is more effective than treatment for most viral disease, and vaccination is the principal tool. Attenuated vaccines use a weakened live virus produced by repeatedly passaging it until it loses virulence; they provoke a strong, durable response but carry a small risk of back mutation, where the weakened strain reverts towards a disease-causing form. Killed or inactivated vaccines remove that risk but often produce a weaker response and need boosters.
Antiviral drugs are harder to design than antibiotics because viruses use the host’s own machinery, so a drug that blocks viral replication can easily damage the patient’s cells too. Useful targets are the enzymes the virus supplies itself. Reverse transcriptase inhibitors such as AZT work on this principle, blocking an enzyme that has no counterpart in an uninfected human cell. Protease inhibitors act later, preventing the processing of viral proteins into their functional forms.
The same properties that make viruses dangerous also make them useful. Their ability to deliver nucleic acid into a specific cell type is exploited in gene therapy, where a modified virus carries a working copy of a gene into a patient’s cells. Phages are also used as narrowly targeted antibacterial agents, an approach of renewed interest as antibiotic resistance spreads.
Prions and viroids: infectious but not viral
Two other acellular agents are studied alongside viruses because they transmit disease, but neither fits the definition of a virus.
A prion is an infectious protein with no nucleic acid at all. It is a misfolded version of a protein the host already makes, and it propagates by inducing normally folded copies to adopt the same abnormal shape. The accumulating misfolded protein damages nervous tissue, producing the transmissible spongiform encephalopathies, a group that includes scrapie in sheep, bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. Because there is no genome to target, prions are not affected by any of the strategies used against viruses.
A viroid is the mirror image of that problem: a short circular RNA molecule with no protein coat. Viroids infect plants and cause disease not by coding for proteins, since they code for none, but by interfering with the host’s own gene regulation. Between them, prions and viroids show that the minimum requirement for an infectious agent is lower than a complete virus particle.
Quick revision summary
- A virus is acellular and metabolically inert outside a host, so it can only replicate as an obligate intracellular parasite.
- The particle is a genome plus a protein capsid built from repeating capsomeres, sometimes with a host-derived lipid envelope.
- Viral genomes may be DNA or RNA, single- or double-stranded; classification into seven groups follows the route each takes to making mRNA.
- Bacteriophages either lyse the host immediately in the lytic cycle or integrate into its chromosome and replicate passively in the lysogenic cycle.
- Animal viruses attach through a specific receptor, enter by endocytosis or fusion, uncoat, replicate, and exit by lysis or budding.
- Vaccines are the main defence; antivirals target virus-supplied enzymes such as reverse transcriptase, while prions and viroids fall outside the virus definition entirely.
