Viruses, Viroids, Prions and Lichens Questions in English

(D) The given figure represents a bacteriophage,which is a virus that infects bacteria.
The structure consists of a polyhedral head containing genetic material and a tail assembly.
The part labeled as $A$ points to the thread-like structures at the base of the tail,which are known as tail fibers.
These tail fibers are responsible for recognizing and attaching to specific receptor sites on the surface of the host bacterium.

(C) Viroids are infectious agents that consist only of a short strand of circular,single-stranded $RNA$ without a protein coat (capsid).
In contrast,viruses consist of genetic material ($DNA$ or $RNA$) enclosed within a protein coat called a capsid.
Therefore,the defining characteristic of viroids is that they are $RNA$ molecules without a protein coat.

(B) The correct answer is $B$. Viroids are infectious agents that consist only of a short strand of circular single-stranded $RNA$ without a protein coat. $A$ key characteristic of viroid $RNA$ is that it has a low molecular weight compared to the $RNA$ found in viruses. Therefore,the statement that their $RNA$ is of high molecular weight is incorrect.

(C) : Viroids are infectious $RNA$ particles which were discovered by $T$.$O$. Diener $(1971)$.
These are devoid of protein coat and cause diseases in plants only,e.g.,potato spindle tuber,chrysanthemum stunt etc.
Therefore,the statement that viroids were discovered by $D$.$J$. Ivanowsky is incorrect,as $D$.$J$. Ivanowsky discovered the causal organism of tobacco mosaic disease.

(B) : Tobacco mosaic virus $(TMV)$ is a $RNA$ virus that causes tobacco mosaic disease.
It contains a single-stranded coiled $RNA$ molecule as its genetic material.
The protein coat,known as the capsid,consists of approximately $2130$ capsomeres.
These capsomeres are arranged in a helical pattern to form a hollow cylinder with a diameter of about $4 \ nm$.

(A) The correct answer is $A$.
Viruses are non-cellular,nucleoprotein entities that utilize the synthetic machinery of a living host cell for their replication.
They do not possess a cellular structure,such as a nucleus or organelles.
Their genetic material consists of either $DNA$ or $RNA$,but never both.
This genetic material is enclosed within a protective protein coat called a capsid.

(B) : In viruses,three architectural forms are found: helical (elongated body,e.g.,$TMV$),cuboidal (short broad body with rhombic,rounded,polyhedral shape,e.g.,poliovirus),and binal (with both cuboidal and helical parts,e.g.,$T_2$ phage). Therefore,the statement that all viruses have helical symmetry is incorrect.

(A) The correct answer is $A$. The nucleic acid of a virus is surrounded by a protective protein coat called the capsid. The capsid is composed of small protein subunits known as capsomeres. In some viruses,this capsid is further covered by an outer layer called an envelope,which typically consists of a combination of lipids,proteins,and carbohydrates.

(C) Based on the structure of a bacteriophage:
$A$ represents the Head,which contains the viral genetic material.
$B$ represents the Sheath (or tail tube),which is the contractile part of the tail.
$C$ represents the Collar,which connects the head to the tail.
$D$ represents the Tail fibres,which are used for attachment to the host cell surface.
Therefore,the correct identification is $A$-Head,$B$-Sheath,$C$-Collar,$D$-Tail fibres,which corresponds to option $C$.

(D) The correct answer is $(d)$.
Viroids are infectious $RNA$ particles that were discovered by $T$.$O$. Diener in $1971$.
These particles are devoid of a protein coat (capsid) and consist only of a short strand of low molecular weight $RNA$.
Viroids cause various diseases in plants,such as potato spindle tuber disease and chrysanthemum stunt disease.

(D) : Plant viruses often contain parasites of their own,referred to as satellites.
Satellite $RNAs$ are highly dependent on their helper virus for both replication and encapsidation.
Their size varies from $194$ to $1500$ nucleotides (approx.).
The larger satellites contain open reading frames and express proteins,whereas smaller satellites do not produce functional proteins.

(B) : Prions were named by Stanley Prusiner (who received the Nobel Prize in $1997$).
Prions are infectious agents composed entirely of proteins,lacking any nucleic acid.
Prions are the causative agents of diseases such as scrapie in sheep and bovine spongiform encephalopathy (mad cow disease) in cattle.

(C) : Mad cow disease is the common term for Bovine Spongiform Encephalopathy $(BSE)$,a progressive neurological disorder of cattle.
It is caused by prions.
Symptoms include an excitable or nervous temperament to external stimuli such as touch to the skin.
$A$ prion (short for proteinaceous infectious particle) is a unique type of infectious agent,as it is made only of protein.
Prions are abnormally structured forms of a host protein,which are able to convert normal molecules of the protein into the abnormal structure.

(C) : Lichens are highly sensitive to $SO_2$ pollution. They are unable to survive in areas with high concentrations of sulfur dioxide in the atmosphere. Because of this sensitivity,they serve as excellent biological indicators for monitoring $SO_2$ pollution levels in the environment.

(A) The genetic material in most organisms is $DNA$,but in some viruses,$RNA$ acts as the genetic material.
Tobacco mosaic virus $(TMV)$ is a well-known example of a virus that contains $RNA$ as its genetic material.
Cyanobacteria,Cladophora (an alga),and Sieve cells (in plants) all contain $DNA$ as their primary genetic material.
Therefore,the correct option is $A$.

(D) Lichens are symbiotic associations between fungi and algae. They are highly sensitive to atmospheric pollutants,especially sulfur dioxide $(SO_2)$. Therefore,they do not grow in polluted areas and act as natural pollution indicators.

(D) Prions are abnormal,misfolded proteins that act as infectious agents.
They do not contain any nucleic acids ($DNA$ or $RNA$).
They are responsible for several neurodegenerative diseases in animals and humans,such as bovine spongiform encephalopathy $(BSE)$ in cattle and Creutzfeldt-Jakob disease $(CJD)$ in humans.
Therefore,the correct option is $D$.

(B) Prions are abnormally folded proteins that cause infectious diseases. They differ from viruses or bacteria because they do not contain nucleic acids ($DNA$ or $RNA$). Therefore,prions are known as 'proteinaceous infectious particles'.

(C) Mutualism is a type of biological interaction between two species where both organisms benefit from the relationship.
Lichen represents a classic example of mutualism between a fungus and an alga (or cyanobacteria).
The fungus provides protection,water,and minerals to the alga,while the alga provides food (carbohydrates) through photosynthesis.
Therefore,the correct option is $C$.

(D) Lichens are pioneer species in primary succession on bare rocks. They secrete organic acids (such as oxalic acid) that help in the weathering and dissolution of rock surfaces,which contributes to soil formation.

(B) Animal viruses can contain either $DNA$ or $RNA$ as their genetic material. In the case of $RNA$ viruses,the genome can be single-stranded $(ssRNA)$ or double-stranded $(dsRNA)$. Therefore,the number of $RNA$ strands present in an animal virus can be one (in $ssRNA$ viruses) or two (in $dsRNA$ viruses). Thus,the correct answer is one or two.

(D) $W$.$M$. $Stanley$ demonstrated in $1935$ that viruses could be crystallized and that crystals consist largely of proteins. This discovery was a significant milestone in virology,showing that viruses could be treated like chemical substances while retaining their infectious nature.

(B) Lichens are symbiotic associations between fungi and algae.
In this association,the fungal component is known as the $Mycobiont$,which is usually an Ascomycete or Basidiomycete.
The algal component is known as the $Phycobiont$,which is usually a green alga or a cyanobacterium.
Therefore,the correct option is $B$.

(D) In $1892$,$D.J. Ivanowsky$ recognized certain microbes as the causative organisms of the mosaic disease of tobacco. He found that these microbes were smaller than bacteria because they could pass through bacteria-proof filters.

(C) Viroids are infectious agents that are smaller than viruses. They consist solely of a short strand of circular,single-stranded $RNA$ without a protein coat (capsid). In contrast,viruses consist of genetic material ($DNA$ or $RNA$) enclosed within a protein coat. Therefore,viroids are the smallest known infectious agents.

(A) Viroids were discovered by $T.O. Diener$ in $1971$ as an infectious agent that is smaller than viruses.
They consist of a low molecular weight $RNA$ molecule.
Unlike viruses,viroids lack the protein coat (capsid) that is typically found in viruses.
Therefore,they are essentially free $RNA$ molecules that cause diseases in plants.

(D) Lichens are symbiotic associations between algae and fungi. They are highly sensitive to sulfur dioxide $(SO_2)$ pollution. Because they lack roots and absorb nutrients directly from the atmosphere,they accumulate pollutants. In areas with high levels of $SO_2$,lichens fail to grow,making them excellent bioindicators for air quality monitoring.

(B) The Tobacco mosaic virus $(TMV)$ is a well-studied plant virus.
It consists of a single-stranded $RNA$ molecule that is helically coiled.
This $RNA$ strand is protected by a protein coat called a capsid,which is composed of smaller subunits known as capsomeres.
The helical arrangement of capsomeres around the $RNA$ gives the virus its characteristic rod-like shape.

(D) $1$. $W.M. Stanley$ $(1935)$ demonstrated that viruses could be crystallized and crystals consist largely of proteins. This statement is correct.
$2$. The term 'contagium vivum fluidum' (infectious living fluid) was coined by $M.W. Beijerinck$ $(1898)$ for the extract of infected tobacco plants. This statement is correct.
$3$. Mosaic disease in tobacco is caused by $Tobacco Mosaic Virus$ $(TMV)$,and $AIDS$ in humans is caused by $Human Immunodeficiency Virus$ $(HIV)$. Both are viral diseases. This statement is correct.
$4$. Viroids were discovered by $T.O. Diener$ in $1971$,not $D.J. Ivanowski$. $D.J. Ivanowski$ $(1892)$ recognized certain microbes as causal organisms of the mosaic disease of tobacco. Thus,option $D$ is the wrong statement.

(C) Lichens are symbiotic associations between algae and fungi. They are known as pollution indicators,especially for sulfur dioxide $(SO_2)$. They are very sensitive to air pollution and do not grow in areas where the air is contaminated with pollutants.

(A) Potato spindle tuber disease is caused by $Viroids$.
$Viroids$ are infectious agents that consist of a short strand of circular,single-stranded $RNA$ without a protein coat.
They were discovered by $T.O. Diener$ in $1971$.
Unlike viruses,they lack a capsid and are much smaller in size.

(D) Viruses are considered obligate parasites because they lack their own metabolic machinery and require a living host cell to replicate.
Option $A$ is incorrect because the protein coat of a virus is called a capsid,not the nucleic acid.
Option $B$ is incorrect because viruses do not have their own metabolic system; they rely entirely on the host cell's machinery.
Option $C$ is incorrect because a virus contains either $RNA$ or $DNA$ as its genetic material,never both.

(B) The Tobacco Mosaic Virus $(TMV)$ is a well-studied plant virus.
Its genetic material consists of a single molecule of single-stranded $RNA$ $(ssRNA)$.
This $RNA$ is encapsulated within a protein coat called a capsid,which is made up of repeating protein subunits known as capsomeres.
Therefore,the correct description of $TMV$ genes is that they are single-stranded $RNA$.

(B) Tobacco Mosaic Virus $(TMV)$ is a well-known plant virus that infects a wide range of plants,especially tobacco.
It consists of a single strand of $RNA$ encapsulated within a protein coat called a capsid.
Therefore,the genetic material of $TMV$ is single-stranded $RNA$ $(ssRNA)$.

(B) The $Influenza$ virus belongs to the family $Orthomyxoviridae$.
It is an enveloped virus that contains a segmented,single-stranded $RNA$ genome.
Unlike some other viruses,it does not contain $DNA$ as its genetic material.
Therefore,the correct option is $RNA$.

(B) Viruses are obligate parasites that lack their own metabolic machinery. They consist of genetic material,which is either $DNA$ or $RNA$,enclosed in a protein coat. Because they lack the cellular machinery targeted by antibiotics (such as cell wall synthesis or bacterial protein synthesis),they are not affected by antibiotics. Therefore,the statement that viruses contain either $DNA$ or $RNA$ is correct.

(A) Cauliflower mosaic virus $(CaMV)$ is a member of the family $Caulimoviridae$.
Unlike most plant viruses which contain $RNA$ as their genetic material,$CaMV$ contains double-stranded $DNA$ $(dsDNA)$ as its genetic material.
Therefore,the correct option is $A$.

(D) The Tobacco Mosaic Virus $(TMV)$ is a well-studied plant virus.
It has a rod-shaped or tubular structure.
The dimensions of the $TMV$ particle are approximately $300 \, nm$ in length and $18 \, nm$ in diameter.
Therefore,the correct size is $300 \times 18 \, nm$.

(D) Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
They are obligate parasites,meaning they require a living host cell to replicate as they lack their own metabolic machinery.
The protein coat of a virus is called a capsid,not the nucleic acid.
Viruses contain either $RNA$ or $DNA$,never both.

(A) Viruses are considered to be at the border of living and non-living organisms. They are inert outside a host cell because they lack the metabolic machinery required for self-replication. Similarly,isolated chromosomes are just genetic material that cannot replicate or perform metabolic functions on their own. Therefore,both viruses and isolated chromosomes require a living cellular environment to replicate their genetic material.

(B) Viruses that infect bacteria are known as bacteriophages. When these viruses enter a bacterial cell,they take over the host's machinery to replicate. In the lytic cycle,the virus multiplies rapidly and eventually causes the host cell to burst or undergo lysis,releasing the new viral particles. Therefore,these are referred to as lytic viruses.

(A) In $1971$,$T$.$O$. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. This agent was found to be a free infectious $RNA$ molecule that lacked the protein coat typically found in viruses. This infectious agent was named a 'viroid'.

(A) virus consists of a genetic material ($DNA$ or $RNA$) enclosed within a protective protein shell. This protective protein coat is known as the $Capsid$. The $Capsid$ is made up of smaller protein subunits called $Capsomeres$.

(C) Viruses are obligate parasites that require a living host cell to replicate. Statement $A$ is true.
Viruses exhibit various symmetries,such as icosahedral,helical,or complex,not just helical symmetry. Therefore,statement $B$ is false.
Viruses lack the metabolic machinery (ribosomes,enzymes) to synthesize their own nucleic acids and proteins independently; they hijack the host cell's machinery for this purpose. Therefore,statement $C$ is also false.
Viruses do not possess the targets (like cell walls or specific metabolic pathways) that antibiotics act upon,so they are unaffected by antibiotics. Statement $D$ is true.
In the context of standard biology multiple-choice questions,statement $C$ is the most fundamentally incorrect regarding their biological autonomy.

(B) The Tobacco mosaic virus $(TMV)$ is a well-studied plant virus.
It consists of a single-stranded $RNA$ molecule that is coiled in a helical structure.
This $RNA$ is protected by a protein coat called a capsid,which is composed of smaller structural subunits known as capsomeres.
Therefore,$TMV$ is characterized by its helical $RNA$ and the presence of capsomeres.

(A) The correct answer is $A$.
$1$. Viroids were discovered by $T. O. Diener$ in $1971$, not by $D. J. Ivanowsky$.
$2$. $W. M. Stanley$ demonstrated that viruses could be crystallized and crystals consist largely of proteins.
$3$. $M. W. Beijerinck$ demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as $Contagium \text{ } vivum \text{ } fluidum$ (infectious living fluid).
$4$. Tobacco mosaic disease and $AIDS$ are indeed viral diseases.

(C) Viroids were discovered by $T.O. Diener$ in $1971$ as a new infectious agent that is smaller than viruses.
They consist of a free,low molecular weight $RNA$ and lack the protein coat that is found in viruses.
Therefore,the statement that their $RNA$ has a high molecular weight is false,as they contain low molecular weight $RNA$.

(D) Viroids were discovered by $T.O. Diener$ in $1971$ as a new infectious agent that is smaller than viruses.
They consist of a low molecular weight $RNA$ molecule.
Unlike viruses,viroids lack the protein coat (capsid) that is typically found in viruses.
Therefore,viroids are characterized as free $RNA$ molecules without a protein coat.

(C) Lichens are composite organisms consisting of a symbiotic association between algae (phycobiont) and fungi (mycobiont).
They are well-known as biological indicators of air pollution,specifically sulfur dioxide $(SO_2)$.
Lichens are highly sensitive to $SO_2$ and cannot grow in areas where the concentration of $SO_2$ is high.
Therefore,their absence in a particular area serves as an indicator of $SO_2$ pollution.