Viruses, Viroids, Prions and Lichens
Viruses, viroids, prions and lichens find no place in Whittaker's five kingdom classification. These notes on viruses, viroids, prions and lichens explain why viruses are called non-cellular, who discovered them, what they are made of and which diseases they cause. They then cover viroids as free RNA, prions as abnormally folded protein, and lichens as an alga-fungus partnership. Every scientist, year and pairing is listed for quick recall, with practice in the match-list and statement formats used in NEET.
- ★ Must learn Lichens and the acellular viruses, viroids and prions are not placed in Whittaker's five kingdoms.
- Viruses: non-cellular; inert crystalline structure outside the living cell; obligate parasites.
- ★ Must learn Ivanowsky (1892): tobacco mosaic agent passed bacteria-proof filters. Beijerinck (1898): named it virus, Contagium vivum fluidum. Stanley (1935): crystallised viruses, mostly protein.
- ★ Must learn A virus is a nucleoprotein; it has RNA or DNA, never both; the genetic material is infectious.
- Plant viruses: single-stranded RNA. Animal viruses: single- or double-stranded RNA, or double-stranded DNA. Bacteriophages: usually double-stranded DNA.
- Capsid: protein coat of capsomeres, arranged in helical or polyhedral forms.
- Human viral diseases: mumps, small pox, herpes, influenza, AIDS.
- ★ Must learn Viroids: T.O. Diener (1971); free, low molecular weight RNA without a protein coat; potato spindle tuber disease.
- ★ Must learn Prions: abnormally folded protein; BSE (mad cow disease) in cattle and CJD in humans.
- Lichens: phycobiont (alga, autotrophic) and mycobiont (fungus, heterotrophic); good pollution indicators.
1. Outside the Five Kingdoms
- ★ Exam imp Whittaker's five kingdom classification has no place for lichens or for some acellular organisms: viruses, viroids and prions.
- These four are therefore introduced separately.
2. Viruses
2.1 Nature of viruses
- The common cold and 'flu' are familiar illnesses caused by viruses.
- Viruses found no place in classification because they are not truly 'living', if living means having a cell structure.
- Viruses are non-cellular organisms.
- Outside the living cell, they have an inert crystalline structure.
- Once they infect a cell, they take over the host cell's machinery to replicate themselves, killing the host.
- Whether viruses are living or non-living is therefore an open question.
- Replicate inside a host cell
- Contain genetic material (RNA or DNA)
- The genetic material is infectious
- No cell structure
- Inert crystalline structure outside the cell
- Can be crystallised, as Stanley showed
2.2 Discovery of viruses
- The word virus means venom or poisonous fluid.
- Dmitri Ivanowsky (1892) recognised certain microbes as the cause of the mosaic disease of tobacco. They were found to be smaller than bacteria, because they passed through bacteria-proof filters.
- M.W. Beijerinck (1898) showed that the extract of infected tobacco plants could infect healthy plants. He named the new pathogen 'virus' and called the fluid Contagium vivum fluidum (infectious living fluid).
- W.M. Stanley (1935) showed that viruses could be crystallised and that the crystals consist largely of proteins.
- Viruses are inert outside their specific host cell.
- ★ Exam imp Viruses are obligate parasites.
Scientists in time order: I Bet Stanley Disagrees: Ivanowsky (1892), Beijerinck (1898), Stanley (1935), Diener (1971, viroids). Beijerinck gave the name 'virus'; Stanley made crystals.
★ Very important Contagium vivum fluidum (infectious living fluid) was the name given by M.W. Beijerinck (1898), who also named the pathogen 'virus'. Ivanowsky (1892) first recognised it.
2.3 Structure and genetic material
- Besides proteins, viruses contain genetic material, which may be either RNA or DNA.
- ★ Exam imp No virus contains both RNA and DNA; it has one or the other, never both.
- A virus is a nucleoprotein, and its genetic material is infectious.
- The protein coat, called the capsid, is made of small subunits called capsomeres. It protects the nucleic acid.
- Capsomeres are arranged in helical or polyhedral geometric forms.
| Virus infects | Genetic material (in general) |
|---|---|
| Plants | Single-stranded RNA |
| Animals | Single- or double-stranded RNA, or double-stranded DNA |
| Bacteria (bacteriophages) | Usually double-stranded DNA |
For genetic material, sort by host: Plant = single-stranded RNA, Bacteriophage = double-stranded DNA, Animal = any of single- or double-stranded RNA or double-stranded DNA. Bacteriophage labels run top to bottom: head, collar, sheath, tail fibres.
2.4 Viral diseases
- In humans, viruses cause mumps, small pox, herpes and influenza.
- AIDS in humans is also caused by a virus.
- In plants, the symptoms include mosaic formation, leaf rolling and curling, yellowing and vein clearing, and dwarfing and stunted growth.
Human viral diseases: My Sister Has Influenza Again (Mumps, Small pox, Herpes, Influenza, AIDS).
Plant symptoms: My Little Yellow Vine Dwarfs (Mosaic, Leaf rolling and curling, Yellowing, Vein clearing, Dwarfing and stunted growth).
Name the four labelled parts of a bacteriophage, from top to bottom.
Which two parts are labelled on the tobacco mosaic virus?
What are capsomeres?
Why did Ivanowsky conclude the agent was smaller than bacteria?
3. Viroids
- ★ Exam imp In 1971, T.O. Diener discovered a new infectious agent, smaller than viruses.
- It caused potato spindle tuber disease.
- It was found to be a free RNA: it lacked the protein coat found in viruses, hence the name viroid.
- The RNA of the viroid was of low molecular weight.
| Feature | Virus | Viroid |
|---|---|---|
| Size | Larger | Smaller than viruses |
| Protein coat (capsid) | Present | Absent |
| Genetic material | RNA or DNA | Free RNA of low molecular weight |
| Example | Tobacco mosaic virus | Potato spindle tuber disease agent |
★ Very important Viroid = free RNA without a protein coat, of low molecular weight. Discovered by T.O. Diener (1971) as the cause of potato spindle tuber disease.
4. Prions
- In modern medicine, certain infectious neurological diseases were found to spread through an agent made of abnormally folded protein.
- The agent was similar in size to viruses. These agents were called prions.
- ★ Exam imp The most notable prion disease of cattle is bovine spongiform encephalopathy (BSE), commonly called mad cow disease.
- Its analogous variant in humans is Creutzfeldt-Jacob disease (CJD).
| Prion disease | Host |
|---|---|
| Bovine spongiform encephalopathy (BSE), mad cow disease | Cattle |
| Creutzfeldt-Jacob disease (CJD) | Humans |
The infectious agents by what they are made of: Virus = nucleic acid + protein; Viroid = RNA only; Prion = protein only (abnormally folded). 'Bovine' points to cattle (BSE).
5. Lichens
- ★ Exam imp Lichens are symbiotic associations, that is, mutually useful associations, between algae and fungi.
- The algal component is called the phycobiont; the fungal component is the mycobiont.
- The phycobiont is autotrophic and the mycobiont is heterotrophic.
- Algae prepare food for the fungi.
- Fungi provide shelter and absorb mineral nutrients and water for their partner.
- The association is so close that a lichen in nature looks like one organism, not two different organisms.
- Lichens are very good pollution indicators: they do not grow in polluted areas.
| Partner | Name | Nutrition | Role |
|---|---|---|---|
| Alga | Phycobiont | Autotrophic | Prepares food for the fungus |
| Fungus | Mycobiont | Heterotrophic | Gives shelter; absorbs mineral nutrients and water |
Phyco = alga, Myco = fungus. The 'phyco' partner photosynthesises and supplies food; the 'myco' partner supplies shelter, water and mineral nutrients.
★ Very important Lichens are very good pollution indicators: they do not grow in polluted areas.
6. Exam Essentials: Traps, Exceptions, Numbers and Pairs
Points NEET turns into statements and match lists:
- Never both RNA and DNA in one virus.
- Stanley's crystals consisted largely of proteins.
- Viroid RNA is of low molecular weight; viroids are smaller than viruses.
- Prions are proteins; viroids are RNA; viruses are nucleoproteins.
- Phycobiont is autotrophic; mycobiont is heterotrophic.
Exceptions
- No virus contains both RNA and DNA.
- Viroids lack the protein coat that viruses have.
- Prions consist of abnormally folded protein, unlike viruses and viroids, which carry nucleic acid.
- Plant viruses generally have single-stranded RNA, but bacteriophages usually have double-stranded DNA.
- Viruses are inert outside their specific host cell.
- Lichens do not grow in polluted areas.
Numbers to Remember
- 1892: Dmitri Ivanowsky, mosaic disease of tobacco.
- 1898: M.W. Beijerinck, 'virus' and Contagium vivum fluidum.
- 1935: W.M. Stanley, crystallised viruses.
- 1971: T.O. Diener, viroids.
- 2 partners in a lichen; capsomeres arranged in 2 geometric forms (helical, polyhedral).
Pairs to Match
| Item | Matches with |
|---|---|
| Dmitri Ivanowsky (1892) | Tobacco mosaic agent passed bacteria-proof filters |
| M.W. Beijerinck (1898) | Named 'virus'; Contagium vivum fluidum |
| W.M. Stanley (1935) | Viruses crystallised; crystals largely protein |
| T.O. Diener (1971) | Viroids; potato spindle tuber disease |
| Plant viruses | Single-stranded RNA |
| Bacteriophages | Usually double-stranded DNA |
| Capsid | Protein coat made of capsomeres |
| Capsomeres | Helical or polyhedral arrangement |
| Viroid | Free RNA, no protein coat |
| Prion | Abnormally folded protein |
| BSE (mad cow disease) | Prion disease of cattle |
| CJD | Prion disease of humans |
| Phycobiont | Algal partner; autotrophic |
| Mycobiont | Fungal partner; heterotrophic |
| Lichens | Pollution indicators |
Examples to Remember
| Group | Examples |
|---|---|
| Viral diseases in humans | Mumps, small pox, herpes, influenza, AIDS |
| Viral symptoms in plants | Mosaic formation, leaf rolling and curling, yellowing, vein clearing, dwarfing and stunted growth |
| Viruses named | Tobacco mosaic virus (TMV); bacteriophage |
| Viroid disease | Potato spindle tuber disease |
| Prion diseases | BSE (mad cow disease) in cattle; CJD in humans |
What does the word virus mean?
Which disease is caused by a viroid?
Name the prion disease of humans.
Which lichen partner absorbs water and mineral nutrients?
7. Quick Revision
- Lichens, viruses, viroids and prions are not in Whittaker's five kingdoms.
- Viruses: non-cellular, inert crystals outside a cell; take over host machinery and kill the host.
- Virus means venom or poisonous fluid.
- Ivanowsky (1892): tobacco mosaic agent smaller than bacteria; passed bacteria-proof filters.
- Beijerinck (1898): named 'virus'; Contagium vivum fluidum.
- Stanley (1935): viruses crystallised; crystals largely protein.
- Viruses are obligate parasites and nucleoproteins with infectious genetic material.
- RNA or DNA, never both; plant viruses single-stranded RNA; phages usually double-stranded DNA.
- Capsid of capsomeres, helical or polyhedral.
- Diseases: mumps, small pox, herpes, influenza, AIDS; plant symptoms: mosaic, curling, yellowing, vein clearing, dwarfing.
- Viroids (Diener, 1971): free, low molecular weight RNA; potato spindle tuber disease.
- Prions: abnormally folded protein; BSE in cattle, CJD in humans.
- Lichens: phycobiont (autotroph) + mycobiont (heterotroph); pollution indicators.
8. Solved Examples
| List I | List II |
|---|---|
| A. Dmitri Ivanowsky | I. Discovery of viroids |
| B. M.W. Beijerinck | II. Crystallisation of viruses |
| C. W.M. Stanley | III. Contagium vivum fluidum |
| D. T.O. Diener | IV. Infectious agent passing bacteria-proof filters |
(1) A-IV, B-III, C-II, D-I
(2) A-III, B-IV, C-II, D-I
(3) A-IV, B-II, C-III, D-I
(4) A-I, B-III, C-II, D-IV
Answer: (1). Ivanowsky found the filterable agent (IV), Beijerinck named the infectious living fluid (III), Stanley crystallised viruses (II), and Diener discovered viroids (I).
A. No virus contains both RNA and DNA.
B. Viruses that infect plants generally have double-stranded DNA.
C. The capsid is made of small subunits called capsomeres.
D. Viruses are obligate parasites.
E. Viruses are metabolically active outside their host cells.
Choose the correct answer from the options given below:
(1) A, C and D only
(2) A, B and C only
(3) B, C and E only
(4) A, C, D and E only
Answer: (1). B is wrong: plant viruses generally have single-stranded RNA. E is wrong: viruses are inert outside their specific host cell.
A. Discovery of viroids
B. Crystallisation of viruses
C. Naming of the pathogen as 'virus'
D. Recognition of a filterable agent of tobacco mosaic disease
Choose the correct answer from the options given below:
(1) D, C, B, A
(2) C, D, B, A
(3) D, B, C, A
(4) D, C, A, B
Answer: (1). Ivanowsky 1892 (D), Beijerinck 1898 (C), Stanley 1935 (B), Diener 1971 (A).
(1) Bovine spongiform encephalopathy
(2) Potato spindle tuber disease
(3) Mumps
(4) Citrus canker
Answer: (1). BSE (mad cow disease) is a prion disease. Potato spindle tuber is caused by a viroid, mumps by a virus and citrus canker by a bacterium.
(1) They are symbiotic associations between algae and fungi.
(2) The phycobiont is autotrophic.
(3) The mycobiont prepares food for the algal partner.
(4) They are very good pollution indicators.
Answer: (3). The alga (phycobiont) prepares the food. The fungus (mycobiont) gives shelter and absorbs water and mineral nutrients.
9. Practice Questions
- How are viroids different from viruses?Answer: Viroids are smaller than viruses and are free RNA of low molecular weight without a protein coat. Viruses have a protein capsid around RNA or DNA.
- What do the terms phycobiont and mycobiont signify?Answer: In a lichen, the phycobiont is the algal partner (autotrophic) and the mycobiont is the fungal partner (heterotrophic).
- Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.Answer: A virus is a nucleoprotein: a capsid of capsomeres (helical or polyhedral) around infectious RNA or DNA, never both. Plant viruses have single-stranded RNA; animal viruses have single- or double-stranded RNA or double-stranded DNA; phages usually have double-stranded DNA. Diseases: mumps, small pox, herpes, influenza (also AIDS).
- Are viruses living or non-living? Give points for a class discussion.Answer: Living: they replicate inside host cells and carry infectious genetic material. Non-living: they have no cell structure, are inert outside a host and can be crystallised. So they lie on the border.
- Match List I (agent) with List II (nature).
Choose the correct answer from the options given below:
List I List II A. Virus I. Abnormally folded protein B. Viroid II. Free RNA without a protein coat C. Prion III. Nucleoprotein with RNA or DNA D. Lichen IV. Symbiotic association of an alga and a fungus
(1) A-III, B-II, C-I, D-IV
(2) A-II, B-III, C-I, D-IV
(3) A-III, B-I, C-II, D-IV
(4) A-IV, B-II, C-I, D-IIIAnswer: (1). Virus is a nucleoprotein, viroid is free RNA, prion is folded protein, and a lichen is an alga-fungus partnership. - Statement I: Viroids are smaller than viruses.
Statement II: The RNA of viroids has a high molecular weight.
(1) Both Statement I and Statement II are correct
(2) Both Statement I and Statement II are incorrect
(3) Statement I is correct but Statement II is incorrect
(4) Statement I is incorrect but Statement II is correctAnswer: (3). Viroid RNA is of low molecular weight. - Which of the following are NOT symptoms of viral infection in plants?
A. Mosaic formation
B. Formation of heterocysts
C. Vein clearing
D. Formation of fruiting bodies
E. Dwarfing and stunted growth
Choose the correct answer from the options given below:
(1) B and D only
(2) A and C only
(3) B, D and E only
(4) A, B and C onlyAnswer: (1). Heterocysts are nitrogen-fixing cells of some cyanobacteria, and fruiting bodies belong to fungi and slime moulds; A, C and E are viral symptoms.
Common Mistakes to Avoid
- Writing that a virus can carry both DNA and RNA. No virus contains both.
- Giving viroids a protein coat. Viroids are free RNA without a coat.
- Calling prions RNA agents. They are abnormally folded protein.
- Swapping Ivanowsky and Beijerinck. Beijerinck coined 'virus' and Contagium vivum fluidum; Ivanowsky found the filterable agent.
- Saying Stanley's virus crystals were mainly nucleic acid. They consisted largely of proteins.
- Giving plant viruses double-stranded DNA. They generally have single-stranded RNA; phages usually have double-stranded DNA.
- Swapping lichen partners. The phycobiont (alga) makes food; the mycobiont (fungus) gives shelter, water and minerals.
- Thinking lichens thrive near pollution. They do not grow in polluted areas.
Frequently Asked Questions
Why are viruses not placed in any of the five kingdoms?
Whittaker's five kingdoms are built on cellular organisms, and viruses are non-cellular. If living means having a cell structure, viruses are not truly living. Outside a host cell they are inert crystalline particles. So viruses, along with viroids, prions and lichens, are described separately from the five kingdoms.
Are viruses living or non-living?
Viruses show features of both. Inside a host cell they take over its machinery, replicate and carry infectious genetic material, which are features of life. Outside a cell they have no cell structure, are inert and can even be crystallised. Hence they are often described as lying between living and non-living.
Who discovered viruses?
Dmitri Ivanowsky (1892) first recognised the agent of tobacco mosaic disease as smaller than bacteria, since it passed bacteria-proof filters. M.W. Beijerinck (1898) named it virus and called the infective fluid Contagium vivum fluidum. W.M. Stanley (1935) crystallised viruses and showed the crystals were largely protein.
What is the genetic material of viruses?
A virus has either RNA or DNA, never both, and this genetic material is infectious. Plant viruses generally have single-stranded RNA. Animal viruses have single- or double-stranded RNA, or double-stranded DNA. Bacteriophages, the viruses of bacteria, usually have double-stranded DNA inside a protein capsid.
How are viroids different from viruses?
Viroids, discovered by T.O. Diener in 1971, are smaller than viruses. A viroid is free RNA of low molecular weight and lacks the protein coat that every virus has. A viroid causes potato spindle tuber disease, while viruses such as TMV have a capsid of capsomeres around their nucleic acid.
What are prions and which diseases do they cause?
Prions are infectious agents made of abnormally folded protein, similar in size to viruses. They cause infectious neurological diseases. The best known are bovine spongiform encephalopathy, or mad cow disease, in cattle, and its analogous variant in humans, Creutzfeldt-Jacob disease, or CJD.
What are the phycobiont and mycobiont of a lichen?
A lichen is a mutually useful association of an alga and a fungus. The algal partner, the phycobiont, is autotrophic and prepares food. The fungal partner, the mycobiont, is heterotrophic; it gives shelter and absorbs mineral nutrients and water. The two live so closely that a lichen looks like one organism.
Why are lichens called pollution indicators?
Lichens do not grow in polluted areas, so their presence suggests clean air and their absence suggests pollution. This makes them very good pollution indicators. NEET may pair lichens with this role in a match list, or test it as a true or false statement.
Previous year questions on Viruses, Viroids, Prions and Lichens
2 questions from past papers, each with a step-by-step solution.
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