Viruses, Viroids, Prions and Lichens Questions in English

(A) $1$. Viroids and viruses are significantly smaller than bacteria. Because of their extremely small size,they can easily pass through the pores of bacteria-proof filters.
$2$. Bacteria-proof filters are designed with pore sizes that are small enough to trap bacteria but large enough to allow smaller particles like viruses and viroids to pass through.
$3$. Therefore,Assertion $A$ is true because they pass through these filters.
$4$. Reason $R$ is also true because the primary function of these filters is to retain bacteria.
$5$. Since the ability of viruses and viroids to pass through the filter is directly due to their size being smaller than the bacteria that the filter is designed to trap,$R$ is the correct explanation of $A$.

$(A)$ The correct matches are as follows:
$(A)$ Tobacco Mosaic Virus $(TMV)$ is a plant virus that infects tobacco plants. Thus, $(A-s)$.
$(B)$ Potato Leaf Roll is a viral disease affecting potato plants. Thus, $(B-t)$.
$(C)$ Mumps is a viral disease caused by the mumps virus, which is an animal virus infecting humans. Thus, $(C-u)$.
$(D)$ Bacteriophage is a virus that infects bacteria. Thus, $(D-q)$.
$(E)$ Polio virus is an animal virus that infects humans. Thus, $(E-p)$.
Combining these, we get $(A-s), (B-t), (C-u), (D-q), (E-p)$. Therefore, option $(A)$ is correct.

(B) The ascending order of size for the given biological entities is as follows:
$1$. Viroids are the smallest infectious agents, consisting only of short strands of circular $RNA$.
$2$. Viruses are larger than viroids, consisting of genetic material enclosed in a protein coat.
$3$. Bacteria are prokaryotic cells, which are significantly larger than viruses.
$4$. Yeast is a unicellular fungus (eukaryotic), which is larger than bacteria.
$5$. Ascomycetes (like mushrooms or multicellular fungi) are the largest among these, being multicellular eukaryotes.
Therefore, the correct sequence is: $Viroid < Virus < Bacteria < Yeast < Ascomycetes$.

(B) $1$. Assertion $(S)$: Lichens are composite organisms consisting of a symbiotic association between an alga (phycobiont) and a fungus (mycobiont). This statement is correct.
$2$. Reason $(R)$: The scientific study of lichens is indeed known as lichenology. This statement is also correct.
$3$. Relationship: While both statements are scientifically accurate,the definition of a lichen $(S)$ does not depend on the fact that its study is called lichenology $(R)$. Therefore,$R$ is not the explanation for $S$.

(B) 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).
Unlike viruses,they do not contain $DNA$ or proteins.
Therefore,the correct answer is $RNA$.

(C) Lichens represent a symbiotic association between algae (phycobiont) and fungi (mycobiont).
In this association,the algal component is responsible for photosynthesis and food production.
The fungal component provides protection,anchors the lichen to the substrate,and is responsible for the absorption of water and mineral nutrients from the environment for the algal partner.

(D) Lichens are symbiotic associations between algae and fungi.
They reproduce through various methods:
$1$. Asexual reproduction: This occurs through the formation of specialized structures like soredia,isidia,and fragmentation.
$2$. Sexual reproduction: This is primarily performed by the fungal partner (mycobiont),which produces fruiting bodies such as apothecia or perithecia.
$3$. Vegetative reproduction: This occurs through simple fragmentation of the thallus.
Since lichens exhibit both asexual and sexual modes of reproduction,the most appropriate answer is that they can undergo both.

(C) Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont).
$Strigula$ is a genus of foliicolous lichens (lichens that grow on leaves).
$Usnea$ is a well-known genus of fruticose lichens,often called 'old man's beard'.
$Spirogyra$ is a genus of filamentous green algae and is not a lichen.
Therefore,both $Strigula$ and $Usnea$ are examples of lichens.

(A) Lichens are symbiotic associations between algae and fungi.
Statement $X$ is true because lichens consist of two main components: the algal component (phycobiont) and the fungal component (mycobiont).
Statement $Y$ is false because the phycobiont (algae) is autotrophic (prepares its own food) and the mycobiont (fungi) is heterotrophic (provides protection and absorbs minerals/water for the algae).
Therefore,Statement $X$ is true and Statement $Y$ is false.

(C) The figure shows the structure of the Tobacco Mosaic Virus $(TMV)$.
$TMV$ is a plant virus that has a helical symmetry.
It consists of a single-stranded $RNA$ molecule surrounded by a protein coat called a capsid,which is made up of subunits called capsomeres.
This specific rod-like helical structure is characteristic of $TMV$ as shown in $NCERT$ biology textbooks.

(B) The figure shows the Tobacco Mosaic Virus $(TMV)$.
$TMV$ is a plant virus that infects a wide range of plants,but it is most famously known for causing the mosaic disease in tobacco plants.
It was the first virus to be discovered and crystallized.

(A) The figure represents a $Bacteriophage$.
$Bacteriophages$ are viruses that infect bacteria.
They typically consist of a polyhedral head containing genetic material and a tail structure used for attachment to the host cell surface.

(C) The figure shows a bacteriophage,which is a virus that infects bacteria. Bacteriophages are viruses that specifically target and replicate within bacterial cells.

(B) The organism shown in the figure is a type of lichen,specifically 'Cladonia',which is known for its cup-shaped structures called podetia.
Lichens are symbiotic associations between fungi and algae.
While 'Parmelia' (option $D$) is also a type of lichen,the specific cup-like structure shown in the image is characteristic of 'Cladonia'.
However,in the context of general classification,the organism is a Lichen.

(C) The image shows a lichen,specifically a species of Cladonia. The cup-shaped structures visible on the thallus are the reproductive bodies known as apothecia. Apothecia are the fruiting bodies (ascocarps) of many fungi,particularly those in the class Ascomycetes,which form the fungal component of these lichens. They are responsible for producing and dispersing ascospores.

(B) The organism shown in the figure is a lichen. Lichens are symbiotic associations between algae and fungi. The algal component is known as the phycobiont,and the fungal component is known as the mycobiont. Therefore,the phycobiont is algae.

(C) The term $Virus$ (meaning venom or poisonous fluid) was given by Louis Pasteur.
$D.J. Ivanowsky$ discovered the causal organism of tobacco mosaic disease, but the name $Virus$ was coined by $Louis Pasteur$.

(B) In $1892$, $D.J. \text{Ivanowsky}$ recognized certain microbes as causal organisms of the mosaic disease of tobacco.
These were found to be smaller than bacteria because they passed through bacteria-proof filters.
Therefore, the correct answer is Tobacco.

(C) In $1892$,$D.J. Ivanowsky$ recognized certain microbes as causal organisms of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters. He named these infectious agents as viruses,which means 'venom' or 'poisonous fluid'.

(D) Viruses are nucleoproteins,meaning they consist of a genetic material core surrounded by a protein coat called a capsid.
In viruses,the genetic material is either $RNA$ or $DNA$,but never both.
Therefore,a virus can contain either $RNA$ or $DNA$ as its nucleic acid.

(C) The capsid is the protein coat of a virus that encloses its genetic material. It is composed of smaller protein subunits called capsomeres. These capsomeres are arranged in a precise geometric pattern to protect the viral genome.

(A) Viruses are considered as a connecting link between living and non-living organisms because they exhibit characteristics of both.
They lack independent metabolic machinery and cannot replicate outside a host cell,which is a non-living trait.
However,once they enter a host cell,they hijack the host's machinery to replicate their genetic material,which is a living trait.
Therefore,both the assertion and the reason are true,and the reason correctly explains why viruses are considered a link between living and non-living entities.

(A) Viruses are obligate parasites that are significantly smaller than bacteria. Because of their extremely small size (typically ranging from $20-300 \ nm$),they are capable of passing through filters that are designed to trap bacteria (bacteria-proof filters). Therefore,both the assertion and the reason are true,and the reason correctly explains why viruses can pass through these filters.

(B) Viruses are considered to be at the borderline between living and non-living organisms.
They are non-living outside a host cell because they lack cellular machinery,metabolic pathways,and the ability to reproduce independently.
However,once they enter a living host cell,they hijack the host's machinery to replicate their genetic material and synthesize viral proteins,exhibiting characteristics of living organisms.

(C) bacteriophage is a type of virus that infects and replicates within bacteria. The term is derived from 'bacteria' and the Greek word 'phagein' meaning 'to eat'. They consist of a genetic material ($DNA$ or $RNA$) enclosed within a protein coat (capsid). Therefore,the correct option is $C$.

(A) virus is a nucleoprotein entity.
It consists of a genetic material,which can be either $DNA$ or $RNA$,enclosed within a protein coat called a capsid.
Viruses never contain both $DNA$ and $RNA$ simultaneously.
Therefore,the correct composition is proteins and $DNA$ or $RNA$.

(D) The term $Virus$ is derived from the Latin word meaning '$poison$' or '$venom$'.
Historically, $Louis \ Pasteur$ coined the term $Virus$ to describe the infectious agents that cause diseases.
$Viruses$ are often described as '$living \ chemicals$' because they consist of genetic material ($DNA$ or $RNA$) enclosed in a protein coat, lacking independent metabolic machinery, and only replicating inside a living host cell.
Therefore, all the given options describe different aspects or historical contexts of viruses.

(A) Dmitri Ivanovsky,a Russian botanist,discovered the Tobacco Mosaic Virus $(TMV)$ in $1892$. He demonstrated that the causative agent of tobacco mosaic disease could pass through bacterial-proof filters,which was the first evidence of the existence of viruses.

(C) In $1892$,$D$.$J$. Ivanowsky recognized certain microbes as causal organisms of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters. This led to the discovery of the Tobacco Mosaic Virus $(TMV)$.

(B) The first virus to be discovered was the $TMV$ $(Tobacco \ Mosaic \ Virus)$.
In $1892$,Dmitri Ivanovsky discovered that certain microbes caused tobacco mosaic disease,which could pass through bacteria-proof filters.
Later,in $1898$,Martinus Beijerinck demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called the fluid '$Contagium \ vivum \ fluidum$' (infectious living fluid).
This discovery marked the beginning of virology.

(A) Viruses are significantly smaller than bacteria. Because of their extremely small size (typically ranging from $20 \ nm$ to $300 \ nm$),they can easily pass through standard bacteria-proof filters (such as $Chamberland$ filters) that are designed to retain bacteria. Therefore,viruses are often referred to as filterable agents.

(C) virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Structurally,a virus consists of genetic material (either $DNA$ or $RNA$) enclosed within a protein coat called a capsid. Together,the nucleic acid and the protein coat form a complex known as a nucleoprotein. Therefore,the main structural component of a virus is nucleoprotein.

(A) Viruses are considered obligate parasites.
When they are outside a living host cell,they are inert and exist in a crystalline or free-living state.
They do not show any metabolic activity outside the host cell.
Therefore,the habitat of a virus in its free-living state is outside a living cell.

(B) Viruses are considered to be at the border of living and non-living organisms.
When they are outside a host cell (free-living state),they do not show any metabolic activities and are considered inert or non-living.
They only exhibit characteristics of living organisms when they enter a host cell and hijack its machinery to replicate.
Therefore,in a free-living state,they are inactive and non-living.

(D) Viruses are considered to be at the border of living and non-living organisms.
When a virus is outside a host cell,it is inert or inactive.
However,once it enters a host cell,it takes over the host's machinery to replicate its genetic material and synthesize viral proteins.
Therefore,inside a host cell,a virus exhibits characteristics of a living organism and is considered active.

(C) Louis Pasteur was the first scientist to use the term 'virus' (which means venom or poisonous fluid) to describe the causative agents of infectious diseases,such as rabies. Dmitri Ivanowsky is known for discovering the tobacco mosaic virus,but the term itself was coined by Pasteur.

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

(A) The protein coat of a virus is known as the $Capsid$.
This $Capsid$ is composed of small structural subunits called $Capsomeres$.
These $Capsomeres$ are arranged in helical or polyhedral geometric forms to protect the viral nucleic acid.

(B) Viruses are classified based on the type of host they infect.
$1$. Plant viruses usually contain single-stranded $RNA$.
$2$. Bacterial viruses (bacteriophages) usually contain double-stranded $DNA$.
$3$. Animal viruses can contain either $DNA$ or $RNA$ as their genetic material.
Among the given options,the Polio virus is a well-known animal virus that infects humans.

(D) Viruses that infect plants are known as plant viruses.
Tobacco mosaic virus $(TMV)$ is a well-known plant virus that causes mosaic disease in tobacco plants.
Potato tuber virus is also a type of plant virus that infects potato crops.
Bacteriophages are viruses that infect bacteria,not plants.
Therefore,both $(A)$ and $(B)$ are examples of plant viruses.

(C) virus consists of a genetic material ($DNA$ or $RNA$) enclosed within a protein coat called a capsid.
The capsid is composed of small protein subunits known as capsomeres.
Since these subunits are proteins,they are essentially chains of amino acids,which are referred to as polypeptides.
Therefore,a capsomere is made of polypeptides.

(C) Viruses are considered to be at the border of living and non-living entities.
They are non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
However,once they infect a host cell,they take over the machinery of the host cell to replicate themselves.
Due to this dual nature—being inert like non-living matter outside a host and showing characteristics of life (like replication) inside a host—they are known as the connecting link between living and non-living organisms.

(B) The genetic material of a virus is enclosed within a protective protein coat called the $Capsid$.
This $Capsid$ is made up of small protein subunits known as $Capsomeres$.
The $Capsid$ protects the viral nucleic acid ($DNA$ or $RNA$) from environmental damage.

(D) Viruses are considered a connecting link between living and non-living things because they exhibit characteristics of both.
$1$. They behave as inert,non-living matter (like crystals) when they are outside a host cell.
$2$. They become active and exhibit characteristics of living organisms (like replication) only when they enter a host cell.
$3$. Since they exist in both states (free state as non-living and host cell as living),they are classified as a bridge between the two.

(D) Viruses are considered to be at the border between living and non-living organisms. They are obligate parasites that require a host cell to replicate their genetic material and synthesize proteins. Outside the host cell,viruses are inert particles that do not exhibit any metabolic activities,growth,or reproduction. Therefore,they behave as non-living entities outside the host.

(B) Viruses are considered to be at the border between living and non-living organisms. They behave as non-living entities outside a host cell because they can be crystallized and stored like chemical substances. They lack metabolic machinery and cellular structure,which are characteristic of living organisms. They only show signs of life (reproduction) when they enter a host cell.

(D) Viruses are considered to be at the boundary between living and non-living organisms. They lack the cellular machinery required for independent metabolism and reproduction. They only show characteristics of life (such as replication) when they infect a host cell. Outside a host,they behave like inert,non-living particles. Therefore,they are distinct from true living cells.

(A) Viruses are considered obligate parasites because they lack the machinery required for independent metabolism and replication.
They are inert outside a host cell and only become active and replicate when they enter a living host cell.
While they are ultramicroscopic and contain nucleic acids,the most defining biological characteristic that distinguishes them from other life forms is their absolute dependence on a host cell for metabolic activity and reproduction.

(C) bacteriophage is a type of virus that specifically infects and replicates within bacteria.
The term 'bacteriophage' literally means 'bacteria eater' (from Greek 'phagein' meaning 'to eat').
These viruses attach to the surface of a bacterium and inject their genetic material into the host cell to hijack its machinery for viral replication.
Therefore,option $C$ is the correct description.