Viruses, Viroids and Prions

Whittaker's five-kingdom scheme sorts every organism into Monera, Protista, Fungi, Plantae or Animalia, and every one of those five boxes assumes the thing being sorted has a cell. Viruses, viroids and prions do not, so NCERT tucks them in as a deliberate addendum to biological classification rather than a sixth kingdom, and treats the three as genuinely separate categories rather than three names for one idea. That distinction is exactly where UPSC likes to set its traps. This note is about that trio specifically: what a virus is actually built from, how a bacteriophage's two possible life cycles differ, and what makes a viroid and a prion each miss a different piece of what a virus has. It is deliberately narrow: the broader story of cells, genes and human evolution is covered separately, and so is the microbial world that sewage treatment, biofertilisers and genetic engineering draw on.

A particle that argues about being alive

The word virus is Latin for poison or venom, a name that predates any understanding of what these particles actually are. Dmitri Ivanowsky (1892), studying a mosaic-patterned disease destroying tobacco crops, found that the diseased sap stayed infectious even after being pushed through a porcelain filter fine enough to trap every known bacterium, a strong hint that whatever was causing the disease was smaller than any bacterial cell. Martinus Beijerinck repeated the filtration in 1898, confirmed the filtered fluid alone could infect a healthy plant, and gave the agent its modern identity: he called it a contagium vivum fluidum, reasoning that it behaved like something alive even though it passed through a filter as if it were dissolved. The picture sharpened further in 1935, when Wendell Stanley isolated the tobacco mosaic agent as an actual crystal, something no living cell can be reduced to and survive, and showed the crystal was overwhelmingly protein. That single result is why a virus is usually described as sitting on the fence between chemistry and biology: crystallised and sitting on a shelf, it behaves like an inert compound; the moment it reaches a susceptible cell, it behaves like an organism single-mindedly making copies of itself.

Structurally, every virus is built from the same two ingredients: a nucleic acid core, either DNA or RNA but never both within one natural virus, wrapped in a protein shell called the capsid. The capsid itself is made of repeating protein subunits, capsomeres, which self-assemble into one of two basic shapes: a helical rod, as in tobacco mosaic virus, or a polyhedral shell with multiple flat faces, called icosahedral symmetry, as in the viruses causing polio and the common cold. Some viruses pick up a further layer on their way out of an infected cell: a lipid envelope torn from the host's own membrane, carrying virus-made glycoprotein spikes on its surface. Others exit without ever acquiring one and are described as naked. What none of these particles have, envelope or not, is a cytoplasm, an independent set of ribosomes, or any means of generating energy or building proteins on their own. Every step of making a new virus particle borrows the host cell's machinery entirely, which is the sense in which a virus is called an obligate intracellular parasite: not simply dependent on a host, but incapable of any metabolic activity whatsoever outside one.

DNA genome or RNA genome, never both

A useful rule of thumb from NCERT: plant viruses lean toward carrying a single strand of RNA (tobacco mosaic virus again), animal viruses carry either RNA (single or double stranded) or DNA, and viruses that infect bacteria are usually double-stranded DNA. Treat that as a starting pattern rather than a fixed law, because the named exceptions are exactly what UPSC likes to test. Smallpox's variola virus and the herpesviruses carry double-stranded DNA. Influenza and rabies carry single-stranded RNA. HIV belongs to a family called retroviruses, which carry single-stranded RNA but cannot use it directly the way most RNA viruses do: HIV first has to convert its own RNA into a DNA copy, using an enzyme it packages inside itself called reverse transcriptase, before that DNA copy is spliced into a chromosome inside the infected human cell. Hepatitis B runs the same trick in the opposite direction: it is classified as a DNA virus, yet part of its replication cycle runs through an RNA intermediate that later gets copied back into DNA. Between them, HIV (RNA to DNA) and hepatitis B (DNA to RNA to DNA) are the two standard named examples of a virus crossing between the two kinds of nucleic acid during its own life cycle, a detail worth holding onto precisely because it is the kind of fact a statement-based question loves to get backwards.

Inside a bacteriophage, and the lytic route

A bacteriophage, a virus that infects bacteria, has a distinctive architecture worth knowing by its parts: a polyhedral head packaging the genome, a short connecting collar, a tail built as a contractile sheath, and tail fibres at the very end that recognise and lock onto a specific receptor molecule on the bacterial cell wall. That lock-and-key fit is why a given phage strain usually infects only one bacterial species, or even one strain of it.

A phage that only ever destroys the cell it infects is called virulent, and it does so through what is known as the lytic cycle, five stages in sequence. Attachment: the tail fibres bind the matching surface receptor. Penetration: the tail sheath contracts and drives the genome through the bacterial cell wall, while the emptied capsid is left stuck to the outside, unlike most animal viruses, which typically enter a host cell whole. Biosynthesis: the bacterium's own enzymes and ribosomes, now redirected by the injected viral genome, start manufacturing fresh copies of viral genetic material and viral proteins instead of the bacterium's own. Maturation: those newly made parts self-assemble into complete virus particles inside the cell. Lysis: a phage-coded enzyme dissolves the bacterial cell wall from within, and dozens to hundreds of new virions burst out, killing the host cell in the process.

The other option: lysogeny and phage lambda

Not every phage runs straight through the lytic sequence. A temperate phage, of which bacteriophage lambda is the standard teaching example, has a second option available immediately after its genome is injected: rather than commandeering the cell at once, the phage DNA splices itself into the bacterial chromosome and is carried along passively, copied alongside the host's own genes every time the bacterium divides. Genetic material integrated this way is called a prophage, and a bacterium carrying one is a lysogen. A lysogen behaves like an ordinary, healthy bacterium for any number of generations, showing no outward sign of infection at all.

Two separate things can happen from there. Environmental stress, such as ultraviolet exposure or a chemical trigger, can cause induction: the dormant prophage excises itself back out of the chromosome and switches into the lytic cycle it had effectively been postponing, killing the cell it had been quietly riding inside. Or, without ever leaving the lysogenic state at all, the prophage can permanently change what the bacterium is capable of, an effect called lysogenic conversion. The two best-known real cases are genuinely important beyond biology class: the toxin gene behind diphtheria, in Corynebacterium diphtheriae, and the toxin gene behind cholera, in Vibrio cholerae, are not native bacterial genes at all. Both arrived carried on a prophage, and each bacterium produces its disease-causing toxin only because that phage's genetic material is sitting inside it.

Viroids: infectious RNA with no coat at all

By the early 1970s, plant pathologists had spent decades trying to isolate the virus behind potato spindle tuber disease, a wasting illness that leaves potato tubers thin and elongated. Theodor Diener, working with the United States Department of Agriculture, published the answer in 1971, and it was not a virus at all: the infectious agent was a bare strand of RNA, circular and only a few hundred nucleotides long, with no capsid, no protein of any kind wrapped around it, and nowhere near sizeable enough to code for one either. Diener named this new category of pathogen a viroid specifically because it lacked the one feature that, by definition, makes a virus a virus: a protein coat. Because a viroid carries no genes for enzymes of its own, it survives only by hijacking enzymes the host plant cell already uses for its own RNA processing, and current understanding is that it causes disease by disrupting that processing rather than by producing any toxic protein. The International Committee on Taxonomy of Viruses eventually gave viroids their own taxonomic order, formally separate from viruses rather than a stripped- down version of one.

Prions: infectious protein with no nucleic acid at all

If a viroid is a virus missing its protein, a prion is the mirror-image gap: a virus missing its nucleic acid. Stanley Prusiner, in 1982, proposed that the infectious agent behind a cluster of fatal neurodegenerative diseases carried no genetic material whatsoever, an idea that broke the assumption, unchallenged until then, that every infectious agent has to carry DNA or RNA in order to copy itself. Every mammal's nerve cells already produce the protein at the centre of this story, PrP, folded into a harmless, normal shape. The infectious form is the identical protein, same amino acid sequence, folded into a different, abnormal shape, and it propagates purely by contact: it forces every normal PrP molecule it touches to flip into its own misfolded conformation, a chain reaction of protein reshaping rather than the template-copying that DNA or RNA replication relies on. This is precisely why prions resist the sterilisation methods that reliably destroy viruses and bacteria, since there is no genetic material to damage in the first place. The named diseases are worth knowing individually: bovine spongiform encephalopathy (BSE, popularly "mad cow disease") in cattle, its human counterpart variant Creutzfeldt-Jakob disease, kuru (first documented in Papua New Guinea), and scrapie in sheep. Prusiner's 1997 Nobel Prize in Physiology or Medicine credited the discovery as, in the Nobel committee's own words, "a new biological principle of infection", language chosen precisely because a prion breaks the one rule every virus and every viroid still obeys.

Why the five kingdoms have no room for any of the three

Line the three up and the pattern that keeps them all outside Whittaker's scheme becomes obvious. A virus carries nucleic acid wrapped in protein, but no cell. A viroid carries nucleic acid with no protein coat around it at all. A prion carries protein with no nucleic acid whatsoever. Each is missing something the five-kingdom system takes for granted a living organism has, which is exactly why NCERT treats this trio as an addendum bolted onto classification rather than folding it into Monera, the kingdom that already houses the simplest cellular life. It is worth one line of continuity with the wider syllabus here: some viruses do useful work rather than harm, baculoviruses used as narrow-spectrum biocontrol agents against crop pests being the standard example, and Alexander Fleming's unrelated discovery of penicillin came from a completely different microbe, a mould, not a virus at all, a distinction worth keeping straight since both surface in the same broader chapter cluster.

The exam angle

The single most reliable UPSC trap in this chapter is the three-way matching question: virus, viroid and prion, matched against "has nucleic acid and protein", "has nucleic acid only, no protein" and "has protein only, no nucleic acid". Get the composition of each wrong and every downstream statement in that question collapses, so it is worth being able to state, without hesitating, that a viroid is smaller and simpler than a virus (RNA with no coat) while a prion is not simpler at all, it is a different class of agent entirely (protein with no genetic material, which is why it cannot mutate the way a virus can). A second recurring pattern is discoverer-and-year matching across Ivanowsky (1892, filtration), Beijerinck (1898, naming and the living-fluid idea), Stanley (1935, crystallisation), Diener (1971, viroids) and Prusiner (1982 discovery, 1997 Nobel Prize), often scrambled so that one name is paired with another's year or contribution. A third is the DNA-virus-versus-RNA-virus statement, where the specific trap is HIV: candidates default to assuming a retrovirus's DNA stage means it "is" a DNA virus, when the genome it actually packages and transmits is RNA; hepatitis B is the useful counter-example UPSC can pair it against, since hepatitis B is genuinely DNA-classified despite running an RNA step internally. Finally, lytic versus lysogenic cycle statements are common: a lytic infection always kills the host cell it enters, while a lysogenic one can persist silently for generations and, through lysogenic conversion, permanently change what the bacterium does (the diphtheria and cholera toxin genes are the concrete, testable examples), without ever killing a single cell.

Quick revision points

• Viruses, viroids and prions sit outside Whittaker's five kingdoms because none of the three has a cell; NCERT covers them as an addendum, not a sixth kingdom.
• Discovery sequence: Ivanowsky (1892, filtration through a bacteria-proof filter), Beijerinck (1898, named the agent, proposed it was a living fluid), Stanley (1935, crystallised tobacco mosaic virus, showed it was mostly protein).
• A virus is nucleic acid (DNA or RNA, never both) wrapped in a protein capsid made of capsomeres, sometimes with a further lipid envelope; it is an obligate intracellular parasite with no independent metabolism at all.
• Capsid symmetry: helical (tobacco mosaic virus) or icosahedral (poliovirus and similar).
• DNA viruses: smallpox (variola), herpesviruses, hepatitis B (despite an internal RNA step). RNA viruses: tobacco mosaic virus, influenza, rabies. HIV is a retrovirus: RNA genome, converted to DNA inside the host by reverse transcriptase.
• Bacteriophage structure: polyhedral head, collar, contractile tail sheath, tail fibres that bind a specific bacterial receptor.
• Lytic cycle (virulent phages): attachment, penetration, biosynthesis, maturation, lysis, always kills the host cell.
• Lysogenic cycle (temperate phages, e.g. bacteriophage lambda): the genome integrates as a prophage, the bacterium is a lysogen; induction later triggers the lytic cycle, while lysogenic conversion can change the bacterium's traits without ever killing it (diphtheria and cholera toxin genes both arrived this way).
• Viroids (Diener, 1971, potato spindle tuber disease): naked, circular, single-stranded RNA, no protein coat, smaller than any virus genome.
• Prions (Prusiner, 1982 discovery, 1997 Nobel Prize): infectious misfolded protein, no nucleic acid at all; BSE, variant CJD, kuru and scrapie are the named diseases.

Work through the linked questions below to see how UPSC turns the virus-viroid-prion composition and the lytic-lysogenic distinction into statement-matching traps.