Viruses, Viroids, Prions and Lichens Questions in English

(D) Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont).
They are very slow-growing organisms,often taking many years to cover a small surface area.
They are known to be pollution indicators,especially for $SO_2$ (sulfur dioxide),as they do not grow in polluted areas.
They serve as a primary food source for reindeer in the Arctic tundra.
Therefore,the statement that they grow rapidly at a rate of $2 \ cm$ per year is false.

(B) Lichens are symbiotic associations between fungi (mycobiont) and algae or cyanobacteria (phycobiont).
In the majority of lichen associations,the fungal partner belongs to the class Ascomycetes,and the algal partner is either a green alga or a blue-green alga (cyanobacteria).
Therefore,most lichens consist of blue-green algae and Ascomycetes.

(B) Lichens are symbiotic associations between algae and fungi. They are highly sensitive to atmospheric pollutants,especially sulfur dioxide $(SO_2)$. Because they do not grow in polluted areas,they are widely used as biological indicators of air pollution.

(D) Lichens represent a symbiotic association between algae (phycobiont) and fungi (mycobiont).
In this relationship,the fungus provides protection,anchorage,and absorption of minerals and water for the algae.
In return,the algae provide food (carbohydrates) produced through photosynthesis to the fungus.
Since both organisms benefit from this association,it is classified as a symbiotic relationship.

(A) Lichens represent a symbiotic relationship between algae $(phycobiont)$ and fungi $(mycobiont)$.
In this association,the algal component is responsible for photosynthesis,providing food for the fungus.
Conversely,the fungal component provides protection,anchorage (support),and absorbs water and mineral nutrients from the environment for the algae.
Therefore,the fungus provides support,protection,and absorption for the algae.

(D) Bovine Spongiform Encephalopathy $(BSE)$,commonly known as 'mad cow disease',is a progressive neurological disorder in cattle.
It is caused by prions,which are misfolded proteins that can induce abnormal folding of normal cellular proteins in the brain.
In humans,the consumption of meat from $BSE$-infected cattle can lead to a variant form of Creutzfeldt-Jakob disease $(vCJD)$.
This is a fatal neurodegenerative disorder characterized by the rapid deterioration of brain tissue,leading to sponge-like holes in the brain.

(B) Lichens are symbiotic associations between fungi and algae.
They are highly sensitive to sulfur dioxide $(SO_2)$ pollution in the air.
In areas with high levels of $SO_2$,lichens fail to grow,making them excellent bioindicators for air quality monitoring.

(C) The correct answer is $C$.
$1$. Viroids consist of free $RNA$ and lack a protein coat,which makes statement $A$ correct.
$2$. Viruses are obligate parasites because they require a host cell to replicate,making statement $B$ correct.
$3$. In viruses,the genetic material ($DNA$ or $RNA$) is the infective constituent,not the protein coat (capsid). The protein coat only protects the genetic material. Therefore,statement $C$ is incorrect.
$4$. Prions are infectious agents composed entirely of abnormally folded proteins,making statement $D$ correct.

(C) $Viroid$ is a small infective segment of nucleic acid,usually $RNA$,which lacks a protein coat.
$Prions$ are infectious pathogenic agents composed mainly of proteins and are devoid of nucleic acid.
$Mycoplasmas$ are the smallest known living cells and they completely lack a cell wall.
$Rickettsiae$ are bacteria that possess a cell wall.

(C) Viruses are sub-microscopic infectious agents that replicate only inside the living cells of a host organism.
They are obligate parasites,meaning they require a host to complete their life cycle.
Because viruses are significantly smaller than bacteria,they can easily pass through bacterial-proof filters.
Therefore,the statement that viruses cannot pass through bacterial filters is incorrect.

(A) Viruses are considered to be at the borderline between living and non-living. They are not considered true organisms because they lack a cellular structure,independent metabolic machinery,and the ability to replicate outside a host cell.
They consist of genetic material ($DNA$ or $RNA$) enclosed in a protein coat (capsid),making them nucleoproteins.
Since they lack cell organelles and cannot perform life processes independently,the Reason correctly explains why they are not classified as organisms.

(B) The Assertion is correct because $TMV$ (Tobacco Mosaic Virus) is indeed the causative agent of mosaic disease in tobacco plants.
The Reason is also correct because the genetic material of $TMV$ is a single-stranded $RNA$ molecule.
However,the fact that $TMV$ has $RNA$ as its genetic material does not explain why it causes mosaic disease; these are two independent biological facts about the virus. Therefore,the Reason is not the correct explanation of the Assertion.

(A) Lichens are economically important in various industries, including the chemical industry.
Litmus is a well-known acid-base indicator used extensively in chemical laboratories, which is extracted from lichens like $Rocella montaignei$.
Orcein is another important dye and biological stain derived from lichens like $Rocella tinctoria$.
Since these chemical products are derived from lichens, the reason correctly explains why lichens are important for chemical industries.

(B) Mad cow disease,also known as Bovine Spongiform Encephalopathy $(BSE)$,is a neurodegenerative disease in cattle.
It is caused by prions.
Prions are infectious agents that consist entirely of abnormally folded proteins.
They lack nucleic acids ($DNA$ or $RNA$) and are smaller than viruses.
Therefore,the correct option is $B$.

(A) Lichens are symbiotic associations between algae and fungi.
$1$. The algal component is known as the phycobiont,which is autotrophic.
$2$. The fungal component is known as the mycobiont,which is heterotrophic.
$3$. Lichens are very sensitive to $SO_2$ pollution and do not grow in polluted areas,making them excellent pollution indicators.
Therefore,statement $A$ is correct.

(C) Viroids were discovered in $1971$ by $T.O.$ Diener.
They cause potato spindle tuber disease.
They are smaller in size than viruses.
They lack the protein coat found in viruses and contain free $RNA$ of low molecular weight.

(B) Phycobiont refers to the algal component of the lichens,and mycobiont refers to the fungal component.
Algae contain chlorophyll and prepare food for fungi,whereas the fungus provides shelter to algae and absorbs water and nutrients from the soil.
This type of mutually beneficial relationship is referred to as symbiotic.

(N/A) Viruses are sub-microscopic infectious agents that can infect all living organisms. $A$ virus consists of genetic material surrounded by a protein coat called a capsid.
The genetic material may be present in the form of $DNA$ or $RNA$.
Most plant viruses have single-stranded $RNA$ as their genetic material. Viruses infecting animals may have single or double-stranded $RNA$ or double-stranded $DNA$.
Bacteriophages,which are viruses that infect bacteria,usually possess double-stranded $DNA$. The protein coat (capsid) is made up of small subunits called capsomeres,which are arranged in helical or polyhedral geometric forms.
$AIDS$,smallpox,mumps,and influenza are four common examples of viral diseases.

(N/A) Viruses are microscopic entities that exhibit characteristics of both living and non-living organisms.
$1$. Evidence for living nature: They contain genetic material ($DNA$ or $RNA$) and can replicate,but only inside a host cell. They also exhibit mutations and can respond to environmental stimuli within the host.
$2$. Evidence for non-living nature: They are acellular,lack metabolic machinery (like ribosomes or enzymes for energy production),and cannot reproduce independently outside a host cell. They can also be crystallized,a property typical of non-living matter.
Conclusion: Because they bridge the gap between living and non-living,they are often considered 'obligate parasites' or 'borderline' entities in biological classification.

(N/A) $\rightarrow$ Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
$\rightarrow$ They are not truly 'living' and thus did not find a place in the classification system.
$\rightarrow$ Once they infect a host cell,they take over the host's machinery to replicate themselves,eventually killing the host.
$\rightarrow$ The term 'virus',meaning venom or poisonous fluid,was given by Pasteur.
$\rightarrow$ $D. I. Ivanowsky (1892)$ recognized certain microbes as the causal organisms of the mosaic disease of tobacco,which were smaller than bacteria as they passed through bacteria-proof filters.
$\rightarrow$ $M. W. Beijerinck (1898)$ demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called the fluid 'Contagium vivum fluidum' (infectious living fluid).
$\rightarrow$ $W. M. Stanley (1935)$ showed that viruses could be crystallized,and these crystals consist largely of proteins.
$\rightarrow$ Viruses are obligate parasites.
$\rightarrow$ Structure: They contain genetic material,which could be either $RNA$ or $DNA$,but never both.
$\rightarrow$ $A$ virus is a nucleoprotein,and the genetic material is infectious.
$\rightarrow$ Plant viruses usually have single-stranded $RNA$,while animal viruses have single or double-stranded $RNA$ or double-stranded $DNA$.
$\rightarrow$ Bacteriophages (viruses that infect bacteria) are usually double-stranded $DNA$ viruses.
$\rightarrow$ The protein coat,called the capsid,is made of small subunits called capsomeres,which protect the nucleic acid and are arranged in helical or polyhedral geometric forms.
$\rightarrow$ Viruses cause diseases like mumps,smallpox,herpes,influenza,and $AIDS$ in humans.

(N/A) Viroids were discovered by $T.O. Diener$ in $1971$ as a new infectious agent that is smaller than viruses.
$1$. Structure: They consist of a short strand of low molecular weight $RNA$ and lack the protein coat that is found in viruses,hence the name 'viroid'.
$2$. Genetic Material: The $RNA$ of the viroid is of low molecular weight.
$3$. Pathogenicity: They cause diseases in plants,such as the potato spindle tuber disease.
$4$. Replication: They replicate inside the host plant cells using the host's machinery.

(N/A) $\rightarrow$ Lichens are symbiotic associations,i.e.,mutually useful associations between algae and fungi.
$\rightarrow$ The algal component is known as the phycobiont and the fungal component as the mycobiont; they are autotrophic and heterotrophic,respectively.
$\rightarrow$ Algae prepare food for fungi,and fungi provide shelter and absorb mineral nutrients and water for their partner.
$\rightarrow$ Their association is so close that if one saw a lichen in nature,one would never imagine that they had two different organisms within them.
$\rightarrow$ Lichens are very good pollution indicators; they do not grow in polluted areas.

(C) Viruses are considered to be at the borderline between living (animate) and non-living (inanimate) entities.
They are considered inanimate (non-living) because they lack cellular machinery,metabolism,and the ability to reproduce independently outside a host cell.
They are considered animate (living) because they possess genetic material ($DNA$ or $RNA$) and can replicate,mutate,and evolve once they infect a suitable host cell.
Therefore,they exhibit characteristics of both living and non-living things depending on their environment.

(C) 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 their genetic material and produce new viral particles.
Each virus contains either $DNA$ or $RNA$ as its genetic material,but never both.
The protein coat,called a capsid,protects the nucleic acid.
They are smaller than bacteria and cannot be cultured on artificial media.

(N/A) Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
They are obligate parasites.
In addition to proteins,viruses also contain genetic material that could be either $RNA$ or $DNA$.
No virus contains both $RNA$ and $DNA$.
$A$ virus is a nucleoprotein,and the genetic material is infectious.
In general,viruses that infect plants have single-stranded $RNA$,and viruses that infect animals have either single or double-stranded $RNA$ or double-stranded $DNA$.
Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double-stranded $DNA$ viruses.
The protein coat called capsid made of small subunits called capsomeres,protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

(D) Lichens represent a symbiotic association between an alga (phycobiont) and a fungus (mycobiont).
In this relationship,the algal component performs photosynthesis to produce food,which is shared with the fungus.
The fungal component provides structural protection,absorbs water,and acquires mineral nutrients from the environment for the alga.
Since both organisms benefit from this mutualistic interaction,it is a classic example of symbiosis.

(N/A) Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont).
They are considered excellent indicators of air pollution,particularly sulfur dioxide $(SO_2)$ pollution,for the following reasons:
$1$. Lichens do not have roots and absorb nutrients and water directly from the atmosphere through their surface.
$2$. They lack a protective cuticle or stomatal regulation,making them highly sensitive to the accumulation of toxic gases and heavy metals present in the air.
$3$. In areas with high levels of $SO_2$ pollution,lichens fail to grow or die,as $SO_2$ interferes with their chlorophyll and metabolic processes.
$4$. Their absence in a specific area serves as a biological marker for high air pollution levels.

(N/A) Viruses are considered a connecting link between living and non-living organisms because they exhibit characteristics of both.
$1$. Non-living characteristics: Viruses are acellular and exist as inert crystalline structures when outside a host cell.
$2$. Living characteristics: Once they infect a host cell,they take over the host's machinery to replicate themselves,showing metabolic activity and reproduction.
Since they behave as non-living matter in a free state but exhibit life processes like replication within a host,they are defined as the connecting link between living and non-living entities.

(N/A) $(i)$ Capsid: It is the protein coat that surrounds the genetic material of a virus. It protects the nucleic acid from environmental damage.
$(ii)$ Capsomeres: These are the individual protein subunits that arrange themselves in a precise geometric pattern to form the capsid.

(B) .$J$. Ivanowsky $(1892)$ recognized certain microbes as the causative organisms of the mosaic disease of tobacco.
He demonstrated that these microbes were smaller than bacteria because they passed through bacteria-proof filters.
His work laid the foundation for the discovery of viruses.

(B) In $1898$, $M. W. Beijerinck$ demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants.
He called the fluid as $Contagium \text{ } vivum \text{ } fluidum$ (infectious living fluid).
This was a significant milestone in the study of viruses, as it suggested that the causative agent was not a bacterium but something smaller and capable of multiplication within the host.

(B) In $1935$,$W$. $M$. Stanley demonstrated that viruses could be crystallized.
He further showed that these crystals consist largely of proteins.
This discovery was a landmark in virology as it suggested that viruses are non-cellular entities that can be treated like chemical substances.

(B) Louis Pasteur is credited with coining the term $virus$.
In Latin,the word $virus$ means 'venom' or 'poisonous fluid'.
He used this term to describe the infectious agents that caused diseases like rabies.

(B) In $1971$,$T$. $O$. Diener discovered a new infectious agent that was smaller than viruses.
This agent was named 'viroids'.
Viroids consist of a short strand of low molecular weight $RNA$ and lack a protein coat.
They are known to cause the potato spindle tuber disease.

(B) $(i)$ The name virus,which means venom or poisonous fluid,was given by $Louis Pasteur$.
$(ii)$ $T. O. Diener$ discovered viroids in $1971$ as an infectious agent that was smaller than viruses and lacked the protein coat.

(A) Lichens are symbiotic associations between algae and fungi.
The algal component is known as the $Phycobiont$ (or $phyco$ meaning algae).
The fungal component is known as the $Mycobiont$ (or $myco$ meaning fungi).
Therefore,the correct term for the algal component is $Phycobiont$.

(B) $\rightarrow$ The correct answer is $B$.
$\rightarrow$ Lichen represents an obligate mutualistic symbiosis between algae (phycobiont) and fungi (mycobiont). Both organisms live together for their mutual benefit.
$\rightarrow$ The fungal component provides protection,anchorage,and absorbs water and minerals from the environment,which it shares with the algal partner. In return,the algal component performs photosynthesis to provide organic food (carbohydrates) to the fungus.
$\rightarrow$ Because of this high degree of physiological interdependence,if the two partners are separated,they cannot survive independently in the harsh environments (like bare rocks) where lichens typically grow. Thus,both components are unable to sustain themselves without each other,justifying the association as an obligate symbiosis.

(N/A) The three viruses that contain $RNA$ as their genetic material are:
$(i)$ $TMV$ (Tobacco Mosaic Virus)
$(ii)$ $HIV$ (Human Immunodeficiency Virus)
$(iii)$ $Influenza$ virus

(N/A) Tobacco mosaic virus $(TMV)$ is a well-known example of a rod-shaped virus. It consists of a single-stranded $RNA$ genome enclosed within a protein coat called a capsid,which is composed of many identical protein subunits known as capsomeres arranged in a helical structure.

(N/A) Viruses that infect and parasitize bacteria are known as bacteriophages.
These viruses do not consume bacteria as food; instead,they attach to the bacterial surface,inject their genetic material,and utilize the host's cellular machinery to replicate themselves,eventually leading to the lysis of the bacterial cell.
[Diagram: $A$ bacteriophage showing parts like Head,Collar,Tail,Plate,Pins,and Prongs.]

(C) Viroids were discovered by $T.O. Diener$ in $1971$. They are infectious agents that are smaller than viruses. Structurally,they consist of a low molecular weight free $RNA$ molecule and lack the protein coat that is found in viruses. Therefore,they are essentially 'naked' $RNA$ particles.

(D) Viroids are infectious agents that are smaller than viruses.
They consist of a short strand of circular,single-stranded $RNA$ without a protein coat (capsid).
Because they lack a protein coat,they are distinct from viruses.
Therefore,the correct statement is that they have free $RNA$ without a protein coat.

(D) Bacteriophages,often referred to as $phage$ viruses,are viruses that specifically infect and replicate within bacteria. $TMV$ (Tobacco Mosaic Virus) infects plants,while Retroviruses and Polioviruses typically infect animal cells. Therefore,the correct option is $D$.

(D) In many viruses,$RNA$ serves as the genetic material instead of $DNA$.
$1$. Retroviruses (e.g.,$HIV$) contain $RNA$ as their genetic material.
$2$. Poliovirus is a single-stranded $RNA$ virus.
$3$. Tobacco Mosaic Virus $(TMV)$ also contains $RNA$ as its genetic material.
Therefore,all the listed viruses use $RNA$ as their genetic material.

(B) Lichens are known as bio-indicators of air pollution,specifically sulfur dioxide $(SO_2)$ pollution. They are highly sensitive to pollutants and cannot grow in areas where the air is contaminated. Therefore,they are absent in polluted regions.

(A) Mosaic diseases in plants,such as Tobacco Mosaic Virus $(TMV)$,are caused by viruses. These pathogens infect plant cells,leading to characteristic mottled or mosaic patterns on leaves due to the destruction of chlorophyll.

(D) Prions are abnormal,misfolded proteins that can induce abnormal folding in normal cellular proteins in the brain. They are known to cause several neurodegenerative diseases such as Bovine Spongiform Encephalopathy $(BSE)$ in cattle and Creutzfeldt-Jakob disease $(CJD)$ in humans. Unlike viruses or viroids,they do not contain nucleic acids ($DNA$ or $RNA$).

(A) The provided image shows the structure of a $Bacteriophage$.
$Bacteriophages$ are viruses that infect bacteria.
They consist of a head (labeled $X$), a collar (labeled $Y$), a tail (labeled $Z$), and tail fibers (labeled $c$).
These structures are characteristic of a $Bacteriophage$ virus, which is distinct from the rod-shaped $Tobacco Mosaic Virus$ or other microorganisms.

(C) The given figure represents a bacteriophage,which is a virus that infects bacteria.
In the diagram:
$X$ points to the head (capsid) of the bacteriophage.
$Y$ points to the collar region,which connects the head to the tail.
Therefore,$X$ represents the head and $Y$ represents the collar.