Viruses and viroids Practice Questions
20 free Viruses and viroids practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following describes the nature of viruses outside a living host cell?
- A Metabolically active and reproductive
- B Saprophytic decomposers
- C Unicellular prokaryotes
- D Inert crystalline structure
Correct answer: Inert crystalline structure
Viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell. Once they infect a host cell, they take over the machinery of the host to replicate themselves.
The name 'virus', which means venom or poisonous fluid, was given by:
- A D.J. Ivanowsky
- B M.W. Beijerinck
- C Louis Pasteur
- D W.M. Stanley
Correct answer: Louis Pasteur
Louis Pasteur coined the term 'virus' to signify the poisonous nature of the fluids associated with infectious diseases. Later researchers like Ivanowsky and Beijerinck provided experimental evidence for their existence as distinct pathogens.
D.J. Ivanowsky (1892) recognized certain microbes as the causal organism of the mosaic disease of tobacco based on which observation?
- A They could be crystallized
- B They were much larger than typical bacteria
- C They passed through bacteria-proof filters
- D They contained both DNA and RNA
Correct answer: They passed through bacteria-proof filters
Ivanowsky found that the pathogens causing tobacco mosaic disease were smaller than bacteria because they passed through filters that typically trapped bacteria. This was a key step in identifying viruses as a unique class of pathogens.
The phrase 'Contagium vivum fluidum' (infectious living fluid) was proposed by:
- A W.M. Stanley
- B Robert Hooke
- C Theodor O. Diener
- D M.W. Beijerinck
Correct answer: M.W. Beijerinck
M.W. Beijerinck demonstrated that the extract of infected tobacco plants could cause infection in healthy plants. He called this infectious agent 'Contagium vivum fluidum' to describe its living, fluid nature.
Who first demonstrated that viruses could be crystallized and that the crystals consist largely of proteins?
- A D.J. Ivanowsky
- B M.W. Beijerinck
- C L. Pasteur
- D W.M. Stanley
Correct answer: W.M. Stanley
In 1935, W.M. Stanley showed that viruses could be crystallized, which was a revolutionary discovery. These crystals are primarily composed of proteins and remain inert until they enter a host cell.
In addition to proteins, all viruses contain genetic material which could be:
- A Either RNA or DNA but not both
- B Both RNA and DNA simultaneously
- C Only double-stranded DNA
- D Only single-stranded RNA
Correct answer: Either RNA or DNA but not both
A virus is a nucleoprotein where the genetic material is infectious. No virus contains both RNA and DNA; it must be one or the other.
Generally, viruses that infect plants have:
- A Single-stranded DNA
- B Double-stranded DNA
- C Single-stranded RNA
- D Double-stranded RNA
Correct answer: Single-stranded RNA
Most plant-infecting viruses, such as the Tobacco Mosaic Virus (TMV), possess single-stranded RNA as their genetic material. This is a common trend observed in plant virology.
Bacteriophages, the viruses that infect bacteria, usually possess:
- A Single-stranded RNA
- B Single-stranded DNA
- C No genetic material
- D Double-stranded DNA
Correct answer: Double-stranded DNA
Bacteriophages are typically complex viruses that contain double-stranded DNA. They use this genetic material to hijack the bacterial machinery for viral replication.
The protein coat of a virus that protects the nucleic acid is called the:
- A Capsid
- B Capsomere
- C Virion
- D Envelope
Correct answer: Capsid
The capsid is the protective protein shell of a virus. It encloses the viral genome and is composed of smaller subunits called capsomeres.
The individual small subunits that make up the protein coat of a virus are known as:
- A Peptones
- B Amino acids
- C Viroids
- D Capsomeres
Correct answer: Capsomeres
Capsomeres are the basic protein subunits that assemble to form the capsid. These capsomeres are arranged in polyhedral or helical geometric forms.
Which of the following diseases in humans is NOT caused by a virus?
- A Mumps
- B Smallpox
- C Influenza
- D Typhoid
Correct answer: Typhoid
Typhoid is a bacterial disease caused by Salmonella typhi. Mumps, smallpox, and influenza are all well-known viral infections in humans.
In plants, symptoms like mosaic formation, leaf rolling, curling, yellowing, and vein clearing are indicative of:
- A Viral infection
- B Bacterial infection
- C Fungal infection
- D Abiotic stress
Correct answer: Viral infection
Viruses cause various systemic symptoms in plants by disrupting cellular processes. Common signs include mosaic patterns, dwarfing, and stunted growth.
Viroids were discovered by T.O. Diener in 1971. How do they differ from viruses?
- A They do not have a protein coat
- B They have a protein coat but no RNA
- C They are larger than viruses
- D They contain double-stranded DNA
Correct answer: They do not have a protein coat
Viroids are infectious agents that are smaller than viruses and consist only of free RNA. They lack the protein coat (capsid) that is found in all viruses.
Which disease is specifically caused by viroids?
- A Potato spindle tuber disease
- B Mosaic Disease of Tobacco plants
- C Creutzfeldt-Jakob disease
- D Leaf curl of Papaya
Correct answer: Potato spindle tuber disease
Potato spindle tuber disease was the first disease found to be caused by viroids. Viroids are known to infect plants and cause significant agricultural damage.
The RNA of the viroid is characterized by having:
- A High molecular weight
- B Low molecular weight
- C Associated proteins
- D Complex branching
Correct answer: Low molecular weight
The genetic material of a viroid is a circular, single-stranded RNA molecule of low molecular weight. This simplicity distinguishes it from the more complex genomes of viruses.
What is the primary reason viruses are not included in the five-kingdom classification system?
- A They are prokaryotic
- B They are non-living/non-cellular
- C They are too small to be categorized
- D They do not have DNA
Correct answer: They are non-living/non-cellular
Whittaker's classification is based on cellular organization, but viruses are non-cellular. Because they do not exhibit living characteristics outside a host, they are not considered 'truly living'.
Prions are infectious agents that consist of:
- A Abnormally folded proteins
- B Free RNA without protein
- C DNA wrapped in polysaccharides
- D A mixture of RNA and DNA
Correct answer: Abnormally folded proteins
Prions are similar in size to viruses but consist entirely of abnormally folded proteins. They cause several neurological diseases by inducing normal proteins to misfold.
The disease 'Bovine Spongiform Encephalopathy' (BSE) in cattle is caused by:
- A Viruses
- B Viroids
- C Prions
- D Fungi
Correct answer: Prions
BSE, commonly known as Mad Cow Disease, is caused by prions. The human analog of this disease is Creutzfeldt-Jakob disease (CJD).
Lichens are symbiotic associations between:
- A Virus and Bacteria
- B Algae and Fungi
- C Bacteria and Fungi
- D Algae and Virus
Correct answer: Algae and Fungi
Lichens represent a mutually beneficial relationship between an algal component (phycobiont) and a fungal component (mycobiont). The algae prepare food while the fungi provide shelter and absorb minerals.
Lichens are very good indicators of:
- A Water pollution
- B Air pollution
- C Soil erosion
- D Viral presence
Correct answer: Air pollution
Lichens are highly sensitive to air pollutants, especially sulfur dioxide. They do not grow in polluted areas, making them effective bioindicators for air quality.