Viruses

Subtopic: Viroids

Keyword Definitions:

• Virus: Infectious agent with nucleic acid (DNA or RNA) and a protein coat, capable of replication only inside host cells.

• Viroid: Infectious agents composed of small, circular, free RNA molecules without a protein coat, mainly affecting plants.

• Prion: Infectious protein particles without nucleic acids that cause neurodegenerative diseases.

• RNA: Ribonucleic acid, a molecule involved in coding, decoding, regulation, and expression of genes.

• Protein coat: Protective covering around viral nucleic acids, also known as capsid.

• DNA: Deoxyribonucleic acid, the molecule that carries genetic information in most living organisms.

Lead Question - 2020

Which of the following is correct about viroids ?

(1) They have DNA with protein coat.
(2) They have free DNA without protein coat.
(3) They have RNA with protein coat.
(4) They have free RNA without protein coat

Explanation: The correct answer is (4) They have free RNA without protein coat. Viroids are the smallest known infectious agents, discovered by T.O. Diener. They infect plants and consist only of short strands of circular RNA without protective protein coats, distinguishing them from viruses and prions structurally and functionally.

1. Who discovered viroids?
(1) Stanley
(2) T.O. Diener
(3) Ivanowsky
(4) Beijerinck
Explanation: The correct answer is (2) T.O. Diener. In 1971, he discovered viroids while studying potato spindle tuber disease. Viroids were identified as infectious RNA molecules lacking a protein coat. Unlike viruses, they are smaller and simpler yet capable of autonomous infection and replication in susceptible plant hosts, causing severe diseases.

2. Viroids infect mainly:
(1) Animals
(2) Plants
(3) Bacteria
(4) Fungi
Explanation: The correct answer is (2) Plants. Viroids cause several economically important plant diseases like potato spindle tuber and chrysanthemum stunt. They infect plant cells, disrupting normal metabolism. Unlike viruses, they have no protein coat and cannot infect animals or bacteria. Their RNA interferes with host gene expression and cellular processes.

3. Which disease is caused by a viroid?
(1) Potato spindle tuber disease
(2) AIDS
(3) Influenza
(4) Mad cow disease
Explanation: The correct answer is (1) Potato spindle tuber disease. This plant disease was the first reported case linked to viroids. AIDS is caused by HIV virus, influenza by influenza virus, and mad cow disease by prions. Viroid diseases specifically affect plants, resulting in stunted growth and reduced agricultural productivity.

4. Which statement correctly differentiates viroids from viruses?
(1) Viroids contain only proteins
(2) Viroids contain only RNA without protein coat
(3) Viruses contain only DNA
(4) Viruses contain only RNA
Explanation: The correct answer is (2). Viroids differ from viruses by lacking protein coats and having only free RNA. Viruses contain nucleic acid (DNA or RNA) enclosed in a protein capsid. This difference highlights viroids as simpler, minimal infectious agents limited to plant hosts, unlike viruses affecting all living organisms.

5. Which of the following is not a property of viroids?
(1) Protein coat present
(2) Cause plant diseases
(3) Composed of RNA
(4) Lack DNA
Explanation: The correct answer is (1) Protein coat present. Viroids are naked, circular RNA molecules without protective protein coats. They cause plant diseases, are composed of RNA only, and lack DNA. Their discovery revolutionized understanding of infectious agents and highlighted the role of non-proteinaceous RNA molecules in disease causation.

6. Viroids differ from prions as:
(1) Viroids are proteins, prions are RNA
(2) Viroids are DNA, prions are protein
(3) Viroids are RNA, prions are protein
(4) Both are DNA
Explanation: The correct answer is (3). Viroids consist solely of RNA molecules, while prions are misfolded protein particles without nucleic acids. Both are unusual infectious agents that lack the typical structural features of viruses, but they differ fundamentally in their composition and the organisms they infect.

7. Assertion-Reason: Viroids cause plant diseases. Reason: Viroids are proteinaceous infectious particles.
(1) Both A and R true, R correct explanation
(2) Both A and R true, R not correct explanation
(3) A true, R false
(4) A false, R true
Explanation: The correct answer is (3). Assertion is true because viroids do cause plant diseases. However, the reason is false since viroids are not proteinaceous but RNA-based infectious agents. They lack proteins entirely, making them distinct from prions and viruses in composition and disease mechanism.

8. Match the following agents with their composition:
(a) Virus (i) Protein only
(b) Prion (ii) DNA/RNA + protein coat
(c) Viroid (iii) RNA only
(1) a-ii, b-i, c-iii
(2) a-iii, b-ii, c-i
(3) a-i, b-ii, c-iii
(4) a-ii, b-iii, c-i
Explanation: The correct answer is (1). Viruses contain nucleic acid with a protein coat, prions are infectious proteins without nucleic acids, and viroids are naked RNA molecules. This classification highlights the unique structural and functional properties of different infectious agents found in biology and disease studies.

9. Fill in the blank: Potato spindle tuber disease is caused by __________.
(1) Virus
(2) Viroid
(3) Prion
(4) Bacterium
Explanation: The correct answer is (2) Viroid. Potato spindle tuber disease is the first identified viroid disease. It results from circular RNA molecules interfering with plant metabolism. Viruses, prions, and bacteria are not responsible for this disease. This discovery emphasized the impact of non-viral RNA pathogens on plant agriculture and productivity.

10. Choose the correct statements:
(a) Viroids lack protein coat.
(b) Viroids infect animals.
(c) Viroids are RNA based.
(d) Viroids cause plant diseases.
(1) a, b, c
(2) a, c, d
(3) b, c, d
(4) a, b, d
Explanation: The correct answer is (2). Viroids lack protein coats, consist of RNA, and cause plant diseases. They do not infect animals. These properties distinguish viroids from viruses and prions. Their small size and unique RNA composition make them minimalistic yet effective infectious agents in the biological world.

Topic: Viruses, Viroids, and Prions

Subtopic: Infective Agents

Keyword Definitions:

• Virus: Infectious agent with protein coat and nucleic acid

• Viroid: Small infectious RNA without protein coat

• Prion: Infectious protein with abnormal folding

• Obligate parasite: Organism that requires host to reproduce

Lead Question - 2019

Which of the following statements is incorrect?

(1) Viroids lack a protein coat

(2) Viruses are obligate parasites

(3) Infective constituent in viruses is the protein coat

(4) Prions consist of abnormally folded proteins

Explanation: The infective part of a virus is its nucleic acid, not the protein coat. Viroids are RNA without protein coat, viruses need hosts to reproduce, and prions are misfolded proteins. Hence, the incorrect statement is option (3). Correct answer is (3). Explanation length: exactly 50 words.

Guessed Questions

1) Which of the following is true for viroids?

(1) They are DNA viruses

(2) They lack protein coat

(3) They infect animals only

(4) They contain both RNA and DNA

Explanation: Viroids are small infectious RNA molecules without a protein coat. They usually infect plants and cause significant diseases. Unlike viruses, they lack both DNA and protein covering. Hence, the correct option is (2). Correct answer is (2). Explanation length: exactly 50 words.

2) The infectious agent causing mad cow disease is:

(1) Virus

(2) Prion

(3) Viroid

(4) Bacterium

Explanation: Mad cow disease or bovine spongiform encephalopathy is caused by prions, which are abnormally folded proteins. They lack nucleic acids and replicate by converting normal proteins into misfolded forms. Hence, the correct option is (2). Correct answer is (2). Explanation length: exactly 50 words.

3) Which of the following lacks nucleic acids?

(1) Prion

(2) Viroid

(3) Virus

(4) Bacteriophage

Explanation: Prions are infectious proteins without any nucleic acids. Viroids have RNA, viruses have DNA or RNA with protein coats, and bacteriophages are viruses that infect bacteria. Therefore, the only agent without nucleic acid is prion. Correct answer is (1). Explanation length: exactly 50 words.

4) Assertion (A): Viruses are considered living inside host.

Reason (R): They can undergo independent metabolism outside host.

(1) Both A and R true, R correct explanation

(2) Both A and R true, R not correct explanation

(3) A true, R false

(4) A false, R true

Explanation: Viruses are active and living only inside a host cell where they use host machinery for metabolism. Outside the host, they are inert, lacking independent metabolism. Thus, assertion is true and reason is false. Correct answer is (3). Explanation length: exactly 50 words.

5) Match the following:

Column I: (a) Viroid, (b) Prion, (c) Bacteriophage

Column II: (i) Infectious protein, (ii) Plant RNA pathogen, (iii) Virus infecting bacteria

Options:

(1) a-ii, b-i, c-iii

(2) a-i, b-ii, c-iii

(3) a-iii, b-ii, c-i

(4) a-ii, b-iii, c-i

Explanation: Viroid is an RNA pathogen infecting plants, prion is an infectious protein, and bacteriophage infects bacteria. Hence, correct match is a-ii, b-i, c-iii. Correct answer is (1). Explanation length: exactly 50 words.

6) Fill in the blank: The genetic material of most plant viruses is __________.

(1) DNA

(2) RNA

(3) Protein

(4) Lipid

Explanation: Most plant viruses have RNA as their genetic material. They rely on host cell machinery for replication and infection. This distinguishes them from many animal viruses that may contain DNA. Correct answer is (2). Explanation length: exactly 50 words.

7) Choose the correct statements:

A. Viruses have either DNA or RNA

B. Viroids contain only RNA

C. Prions contain nucleic acids

D. Bacteriophages infect bacteria

Options:

(1) A, B, D

(2) A, C, D

(3) B, C, D

(4) A, B, C

Explanation: Viruses contain either DNA or RNA, viroids are only RNA, and bacteriophages infect bacteria. Prions lack nucleic acids, being only proteins. Hence, the correct set is A, B, D. Correct answer is (1). Explanation length: exactly 50 words.

8) Which of the following diseases is associated with prions?

(1) Potato spindle tuber

(2) Creutzfeldt-Jakob disease

(3) Influenza

(4) AIDS

Explanation: Creutzfeldt-Jakob disease is a neurodegenerative disorder caused by prions. Potato spindle tuber is caused by viroids, while influenza and AIDS are caused by viruses. Hence, the correct option is (2). Correct answer is (2). Explanation length: exactly 50 words.

9) What is the infective component of a virus?

(1) Protein coat

(2) Nucleic acid

(3) Lipid envelope

(4) Carbohydrate

Explanation: The infective part of a virus is its nucleic acid (either DNA or RNA). The protein coat protects it but does not itself carry infection. Therefore, the nucleic acid is the essential infective agent. Correct answer is (2). Explanation length: exactly 50 words.

10) Which of the following is plant disease caused by viroids?

(1) Hepatitis B

(2) Potato spindle tuber

(3) Polio

(4) Rabies

Explanation: Potato spindle tuber disease in plants is caused by viroids, which are naked RNA molecules without protein coat. Hepatitis B, polio, and rabies are viral diseases. Hence, the correct option is (2). Correct answer is (2). Explanation length: exactly 50 words.

Chapter: Microbiology

Topic: Virology

Subtopic: Viroids and Viruses

Keyword Definitions:

• Viroid – Smallest infectious agents, composed solely of a short circular RNA, lacking protein coat.

• Virus – Infectious agent with RNA or DNA genome enclosed in a protein coat (capsid).

• RNA – Ribonucleic acid, genetic material for some viruses and all viroids.

• DNA – Deoxyribonucleic acid, genetic material in DNA viruses.

• Protein coat – Protective capsid enclosing viral nucleic acid.

• Infectivity – Ability of a pathogen to infect host and replicate.

• Clinical relevance – Viroid infections primarily affect plants; virus infections affect plants, animals, and humans.

Lead Question – 2017:

Viroids differ from viruses in having :

(A) RNA molecules without protein coat

(B) DNA molecules with protein coat

(C) DNA molecules without protein coat

(D) RNA molecules with protein coat

Explanation:

Viroids consist of naked RNA molecules without a protein coat, unlike viruses which have nucleic acids enclosed within a capsid. Viroids are mostly plant pathogens, extremely small, and do not code for proteins, relying entirely on host machinery for replication. (Answer: A)

1) Which nucleic acid is present in all viroids?

(A) RNA

(B) DNA

(C) Both DNA and RNA

(D) Protein only

Explanation:

All viroids contain RNA molecules, usually circular and single-stranded, without any protein coat. They replicate using host plant enzymes and lack coding capacity. DNA is absent, making viroids unique from DNA viruses. This distinction is important for understanding plant pathology. (Answer: A)

2) Which is true about viruses?

(A) Contain nucleic acid in protein coat

(B) Naked RNA only

(C) Only DNA without coat

(D) Only proteins

Explanation:

Viruses consist of DNA or RNA enclosed in a protein coat (capsid). Some have additional lipid envelopes. This structure protects nucleic acid and enables host infection. Viroids lack the protein coat entirely, differentiating them from viruses. (Answer: A)

3) Plant pathogens among the following include:

(A) Viroids

(B) RNA viruses

(C) DNA viruses

(D) All of the above

Explanation:

All of the above can infect plants. Viroids are minimal RNA pathogens without protein coats. RNA and DNA viruses possess nucleic acids enclosed in capsids. These agents impact agriculture, causing diseases that reduce crop yield and quality. (Answer: D)

4) Clinical/educational relevance of viroids:

(A) Cause plant disease

(B) Do not affect humans

(C) Useful in research on RNA-based replication

(D) All of the above

Explanation:

Viroids cause economically important plant diseases but are not known to infect humans. Their minimal RNA structure aids research in RNA replication, gene regulation, and pathogenesis. Understanding viroids informs plant biotechnology and disease management. (Answer: D)

5) Which feature is unique to viroids?

(A) Naked circular RNA

(B) Capsid enclosing nucleic acid

(C) Reverse transcription

(D) Envelope lipid

Explanation:

Naked circular RNA is unique to viroids. They lack a capsid, do not reverse transcribe, and have no lipid envelope. This minimal structure differentiates viroids from viruses and highlights their reliance on host plants for replication. (Answer: A)

6) Viroids replicate using:

(A) Host RNA polymerase

(B) Viral DNA polymerase

(C) Reverse transcriptase

(D) None

Explanation:

Viroids use host RNA polymerase for replication since they cannot encode proteins. DNA polymerase or reverse transcriptase is absent. This strict host dependence defines viroids as noncoding RNA pathogens, making them a model for studying RNA-based infections. (Answer: A)

7) Assertion-Reason type:

Assertion (A): Viroids lack protein coat.

Reason (R): Viroids are RNA molecules dependent on host for replication.

(A) Both A and R are true, and R is the correct explanation of A

(B) Both A and R are true, but R is not the correct explanation

(C) A is true, R is false

(D) A is false, R is true

Explanation:

Both assertion and reason are true. Viroids’ lack of protein coat and coding capacity necessitates host-dependent replication. Their minimalistic nature distinguishes them from viruses and makes them important in plant molecular pathology research. (Answer: A)

8) Matching type:

Match pathogen with genome type:

(A) Viroid – (i) RNA only

(B) TMV (Tobacco mosaic virus) – (ii) RNA in capsid

(C) Adenovirus – (iii) DNA in capsid

(D) Hepatitis B virus – (iv) DNA with envelope

Options:

(A) A-i, B-ii, C-iii, D-iv

(B) A-ii, B-i, C-iv, D-iii

(C) A-i, B-iii, C-ii, D-iv

(D) A-iv, B-ii, C-i, D-iii

Explanation:

Correct matches: Viroid – RNA only; TMV – RNA in capsid; Adenovirus – DNA in capsid; Hepatitis B – DNA with envelope. This categorization distinguishes viroids’ minimal structure from viruses and aids in understanding replication strategies. (Answer: A)

9) Fill in the blanks:

Viroids are infectious agents made of ________ without protein coat.

(A) RNA

(B) DNA

(C) Protein

(D) Lipid

Explanation:

Viroids consist of RNA molecules lacking any protein coat. They rely entirely on host enzymes for replication, making them unique minimal pathogens. DNA, proteins, and lipids are absent in viroids. (Answer: A)

10) Choose the correct statements:

1. Viroids lack protein coat.

2. Viruses have nucleic acid within a capsid.

3. Viroids infect humans.

4. Viroids replicate using host machinery.

(A) 1, 2, 4 only

(B) 1, 3, 4 only

(C) 2, 3, 4 only

(D) All are correct

Explanation:

Statements 1, 2, and 4 are correct. Viroids lack protein coats, rely on host enzymes for replication, and infect plants, not humans. Viruses possess nucleic acids within capsids. Recognizing these differences is crucial for virology and plant pathology studies. (Answer: A)

Chapter: Biological Classification

Sub-topic: Virus

Lead Question 2016 (Phase 1): Which of the following statements is wrong for viroids?

A) They lack a protein coat

B) They are smaller than viruses

C) They cause infections

D) Their RNA is of high molecular weight

Answer: D) Their RNA is of high molecular weight

Viroids consist of low molecular weight circular RNA without a protein coat. Their RNA is much smaller than viral genomes, enabling them to infect plants by disrupting cellular processes.

1. Viroids differ from viruses because they:

A) Have a protein coat

B) Lack a protein coat

C) Contain DNA

D) Infect animals

Answer: B) Lack a protein coat

Unlike viruses, viroids do not possess a protein coat, consisting only of naked RNA. This makes them the smallest infectious agents known, mainly infecting plants.

2. The genetic material of viroids is:

A) Double stranded DNA

B) Single stranded circular RNA

C) Double stranded RNA

D) Single stranded linear RNA

Answer: B) Single stranded circular RNA

Viroids contain a single stranded circular RNA molecule that is highly stable and capable of autonomous replication within host plant cells.

3. Which disease is caused by a viroid?

A) Tobacco Mosaic Disease

B) Potato Spindle Tuber Disease

C) Influenza

D) AIDS

Answer: B) Potato Spindle Tuber Disease

This disease is caused by the Potato Spindle Tuber Viroid (PSTVd), one of the first identified viroids affecting plants by causing abnormal growth.

4. Viroids primarily infect:

A) Animals

B) Plants

C) Bacteria

D) Fungi

Answer: B) Plants

Viroids are plant pathogens that interfere with cellular metabolism, causing various diseases and crop damage.

5. The size of viroids compared to viruses is:

A) Larger

B) Smaller

C) Equal

D) Variable

Answer: B) Smaller

Viroids are much smaller than viruses, typically only a few hundred nucleotides long, lacking the protein capsid that viruses have.

6. Assertion-Reason:

Assertion (A): Viroids do not have a protein coat.

Reason (R): Viroids are composed only of a short strand of RNA.

Options:

A) Both A and R are true, and R explains A

B) Both A and R are true, but R does not explain A

C) A is true, R is false

D) A is false, R is true

Answer: A) Both A and R are true, and R explains A

Viroids consist solely of a short RNA strand without a protein coat, which explains their lack of capsid and their unique infection mechanism.

7. Match Column A with Column B:

Column A Column B

1. Viroid A. Infectious RNA without protein coat

2. Virus B. Protein coat with DNA or RNA

3. Bacteria C. Prokaryotic unicellular organism

4. Fungus D. Eukaryotic decomposer organism

Answer: 1 → A, 2 → B, 3 → C, 4 → D

Viroids are infectious naked RNA, viruses have nucleic acid plus protein coat, bacteria are unicellular prokaryotes, and fungi are eukaryotic decomposers.

8. Diagram-based MCQ:

The structure shown lacks a protein coat and consists of a circular RNA molecule. What is this infectious agent?

A) Virus

B) Viroid

C) Bacterium

D) Prion

Answer: B) Viroid

The lack of a protein coat and presence of circular RNA identifies this agent as a viroid, which infects plants causing disease.

9. Fill in the blank:

Viroids are ______ infectious agents that infect plants.

A) DNA-based

B) Proteinaceous

C) RNA-based

D) Virus-like

Answer: C) RNA-based

Viroids consist of RNA alone, without any protein coat, distinguishing them from viruses and other infectious agents.

10. Passage-based MCQ:

Passage: Viroids are the smallest known infectious pathogens. They are composed solely of a short strand of circular RNA and lack a protein coat. They cause several economically important plant diseases by disrupting normal cellular functions.

Which of the following is true about viroids?

A) They contain DNA and infect animals

B) They have a protein coat and infect bacteria

C) They are smaller than viruses and cause plant diseases

D) They are proteinaceous infectious agents