Viruses, Viroids, Prions and Lichens Questions in English

(A) $W$. $M$. Stanley was the first to isolate a plant virus (Tobacco Mosaic Virus) in crystalline form in $1935$.
He demonstrated that these crystals were largely composed of proteins and remained infectious even after crystallization.

(C) Viruses are nucleoproteins,meaning they consist of a genetic material (nucleic acid) enclosed within a protein coat called a capsid.
$(a)$ The protein coat (capsid) protects the genetic material.
$(b)$ The nucleic acid can be either $DNA$ or $RNA$,but never both in the same virus.
Therefore,a virus is composed of both protein and nucleic acid.

(D) The $Cauliflower\, mosaic\, virus$ $(CaMV)$ belongs to the family $Caulimoviridae$.
It is a well-known plant virus that contains a double-stranded $DNA$ $(dsDNA)$ genome.
Unlike most plant viruses which typically have $RNA$ genomes,$CaMV$ is unique because it replicates its $dsDNA$ genome through an $RNA$ intermediate using reverse transcription.

(D) $virion$ is defined as the complete, infective form of a virus outside a host cell.
It consists of an $RNA$ or $DNA$ core with a protein coat (capsid) and sometimes an external envelope.
Therefore, the term $virion$ refers to a single, individual virus particle.

(D) Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell. They lack cellular machinery,including a cell wall,cell membrane,and cytoplasm. Therefore,all the listed components are absent in a virus.

(D) Viruses are nucleoprotein entities where the genetic material is infectious.
In viruses,the genetic material is either $RNA$ or $DNA$,but never both.
Therefore,a virus contains only one type of nucleic acid as its genetic material.

(A) The Tobacco Mosaic Virus $(TMV)$ is a well-studied plant virus. Its protein coat (capsid) is composed of many identical protein subunits called capsomeres. Each individual capsomere of $TMV$ consists of a single polypeptide chain containing exactly $158$ amino acids.

(C) The term $Ribovira$ refers to a group of viruses that possess $RNA$ as their genetic material.
These viruses are characterized by the presence of ribonucleic acid $(RNA)$ instead of deoxyribonucleic acid $(DNA)$ as the primary hereditary molecule.

(A) The Foot and mouth disease virus $(FMDV)$ is known to be one of the smallest viruses,with a diameter of approximately $20-30 \ nm$.
Tobacco mosaic virus $(TMV)$ is a rod-shaped virus with a length of about $300 \ nm$.
Coliphage lambda and Wound tumour viruses are significantly larger in size compared to the Foot and mouth disease virus.
Therefore,the correct option is $A$.

(C) The $Pox$ virus is considered one of the largest known viruses.
It is a large,complex,brick-shaped virus that contains a double-stranded $DNA$ genome.
Its size is approximately $200-300 \ nm$ in length,making it significantly larger than many other common viruses.

(C) $1$. Most plant viruses contain single-stranded $RNA$ as their genetic material.
$2$. Most animal viruses contain either $DNA$ or $RNA$ (often double-stranded $DNA$ or single-stranded $RNA$).
$3$. $TMV$ (Tobacco Mosaic Virus) is a well-known plant virus that contains a single-stranded $RNA$ molecule,not double-stranded $RNA$. Therefore,statement $C$ is false.
$4$. $T_4$ bacteriophage is a virus that infects bacteria and contains a double-stranded $DNA$ molecule.

(A) The Dahlia mosaic virus belongs to the family Caulimoviridae. Viruses in this family are characterized by having a double-stranded $DNA$ $(dsDNA)$ genome. Therefore,the genetic material of the Dahlia mosaic virus is $DNA$.

(C) The wound tumour virus $(WTV)$ is a plant virus that belongs to the family $Reoviridae$.
Members of the family $Reoviridae$ are characterized by having a genome composed of double-stranded $RNA$ $(dsRNA)$.
Therefore,the correct answer is double-stranded $RNA$.

(C) Most plant viruses are $RNA$ viruses,meaning they contain $RNA$ as their genetic material.
However,the $Cauliflower$ $mosaic$ $virus$ $(CaMV)$ is a well-known exception that contains double-stranded $DNA$ $(dsDNA)$ as its genetic material.
Therefore,the correct option is $C$.

(D) Viruses that infect algae are known as $Phycophages$.
$Cyanophages$ are viruses that infect cyanobacteria.
$Mycophages$ are viruses that infect fungi.
$Binal$ $viruses$ refer to viruses with a complex symmetry,such as $T4$ bacteriophage.

(B) Viruses are considered to be at the boundary between living and non-living organisms.
Respiration is a fundamental metabolic process characteristic of all living organisms,as it provides energy for cellular activities.
Since viruses lack their own metabolic machinery and do not perform respiration,this absence of respiration is considered a non-living character of a virus.

(B) The term "filterable agent" was historically used to describe viruses.
In the late $19^{th}$ century, researchers like Dmitri Ivanovsky and Martinus Beijerinck demonstrated that the causative agent of Tobacco Mosaic Disease could pass through bacterial-proof filters, unlike bacteria, which are retained by these filters.
Therefore, viruses are characterized as filterable agents because of their extremely small size compared to bacteria and fungi.

(D) Viruses consist of a genetic material ($DNA$ or $RNA$) enclosed within a protein coat called a capsid.
This capsid is composed of smaller structural subunits known as capsomeres.
Since capsomeres are proteinaceous in nature,they are essentially polymers of amino acids.
Therefore,the protein of capsomeres is made up of amino acids.

(A) The phanerogram of viruses,which is a system of classification based on the properties of the virion,was proposed by $Lwoff$ and $Tournier$ in $1966$. This system classifies viruses based on criteria such as the type of nucleic acid ($DNA$ or $RNA$),the symmetry of the capsid,and the presence or absence of an envelope.

(D) Cephaloridine is a cephalosporin antibiotic that functions by inhibiting bacterial cell wall synthesis.
Viruses,Mycoplasma,and Viroids lack a peptidoglycan cell wall.
Viruses and Viroids are acellular entities,and Mycoplasma lacks a cell wall entirely.
Therefore,Cephaloridine is ineffective against all of these.

(B) Most plant viruses contain single-stranded $RNA$ as their genetic material. While some viruses may contain $DNA$,the vast majority of viruses that infect plants are $RNA$ viruses.

(D) The term 'Virus' means 'venom' or 'poisonous fluid'. This name was given by Louis Pasteur.
$1.$ Dmitri Ivanowski $(1892)$ recognized certain microbes as causal organisms of the mosaic disease of tobacco.
$2.$ $M$.$W$. Beijerinck $(1898)$ demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as 'Contagium vivum fluidum' (infectious living fluid).
$3.$ $W$.$M$. Stanley $(1935)$ showed that viruses could be crystallized and crystals consist largely of proteins.

(A) Dmitri Ivanowsky $(1892)$ recognized certain microbes as causal organisms of the mosaic disease of tobacco. These were found to be smaller than bacteria because they could pass through bacteria-proof filters. However, it was $M.W.$ Beijerinck $(1898)$ who demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as $Contagium$ $vivum$ $fluidum$ (infectious living fluid). Therefore, the filterable property was specifically highlighted by the experiments of Ivanowsky and confirmed by Beijerinck.

(D) The term $Contagium \text{ } vivum \text{ } fluidum$ (meaning infectious living fluid) was coined by the Dutch microbiologist $M.W. \text{ } Beijerinck$ in $1898$.
He demonstrated that the extract of infected tobacco plants could cause infection in healthy tobacco plants, suggesting that the causative agent was a fluid that was alive.

(B) $TMV$ stands for Tobacco Mosaic Virus.
It is a rod-shaped virus.
The dimensions of the $TMV$ particle are approximately $17.5 \, \mathring A$ in diameter and $300 \, \mathring A$ in length.
Therefore,the correct size is $17.5 \times 300\, \mathring A$.

(B) The barley stripe mosaic virus $(BSMV)$ is a rod-shaped virus belonging to the genus $Hordeivirus$.
Scientific studies and electron microscopy measurements have determined that the dimensions of the $BSMV$ virion are approximately $130 \, nm$ in length and $25 \, nm$ in diameter.
Therefore,the correct size is $130 \times 25 \, nm$.

(B) The capsid of an Adenovirus is an icosahedral structure.
It is composed of exactly $252$ capsomeres.
These capsomeres consist of $240$ hexons and $12$ pentons.

(B) $TMV$ stands for Tobacco Mosaic Virus.
It is a plant virus.
The genetic material of $TMV$ is single-stranded $RNA$.

(C) Viruses are nucleoproteins,meaning they consist of genetic material protected by a protein coat.
According to biological principles,a virus contains either $DNA$ or $RNA$ as its genetic material,but never both.
Therefore,the statement that a virus contains both $DNA$ and $RNA$ is incorrect.

(A) Viruses are considered to be at the borderline between living and non-living entities.
They are characterized as obligate parasites because they require a living host cell to replicate and perform metabolic activities.
Outside of a host cell,they are inert and behave like non-living chemicals.
Therefore,being an obligate parasite is a key living characteristic of a virus.

(B) The protein coat,known as the capsid,of the tobacco mosaic virus $(TMV)$ is composed of $2130$ identical protein subunits called capsomeres. These capsomeres are arranged in a helical symmetry around the viral $RNA$ genome.

(C) Reoviruses belong to the family $Reoviridae$.
These viruses are characterized by having a genome composed of double-stranded $RNA$ $(ds \, RNA)$.
Unlike most other $RNA$ viruses which contain single-stranded $RNA$,reoviruses possess a segmented $ds \, RNA$ genome.

(A) Viruses are obligate intracellular parasites. This means they lack the metabolic machinery to replicate on their own and require the host cell's machinery to synthesize their components. Therefore, they can only multiply $In \text{ } vivo$ (inside a living host cell) and cannot multiply $In \text{ } vitro$ (in a cell-free culture medium).

(A) Viruses are classified based on the type of tissue they primarily infect.
$1$. Neurotropic viruses are those that have an affinity for the nervous system.
$2$. The poliovirus causes poliomyelitis,which affects the central nervous system,and the rabies virus (causing hydrophobia) also targets the nervous system.
$3$. Therefore,both are classified as neurotropic viruses.

(C) Whether it is a virus or any other biological system,hereditary information is always carried by nucleic acids. The sequence of nucleotide bases in a molecule of nucleic acid constitutes the genetic or hereditary information.

(C) Viruses are obligate parasites that require a living host cell to replicate. They lack the machinery for independent metabolism and reproduction. Viruses are highly specific and can only infect and multiply within specific types of living cells,as they require specific receptors on the host cell surface to attach and enter.

(B) Viruses are obligate parasites that lack the machinery required for independent replication.
They do not possess their own metabolic system and therefore must hijack the cellular machinery of a living host cell to multiply.
Option $A$ is incorrect because viruses can contain either $DNA$ or $RNA$ as their genetic material,but never both.
Option $C$ is incorrect because viruses can infect plants,animals,and bacteria (bacteriophages).
Option $D$ is incorrect because,as mentioned,they can contain either $DNA$ or $RNA$.

(C) Viruses are considered to be at the border line between living and non-living organisms.
They are considered non-living because they lack cellular machinery,cannot reproduce on their own,and can be crystallized.
They are considered living because they possess genetic material ($DNA$ or $RNA$) and can replicate inside a host cell.

(B) Potato spindle tuber disease is caused by a viroid known as Potato Spindle Tuber Viroid $(PSTVd)$.
Viroids are infectious agents that consist only of a short strand of circular,single-stranded $RNA$ without a protein coat.
They are smaller than viruses and were discovered by $T.O. Diener$ in $1971$.

(C) $X$-bodies,also known as inclusion bodies or crystalline inclusions,are characteristic structures formed within the host cells during certain viral infections. These bodies are often composed of viral proteins or aggregates of viral particles and serve as sites for viral replication or assembly. Therefore,$X$-bodies are specifically associated with viral infections.

(D) The Banana bunchy top disease is a viral disease caused by the Banana bunchy top virus $(BBTV)$.
This virus is transmitted by the banana aphid,$Pentalonia$ $nigronervosa$.
The disease is characterized by the stunted growth of the plant,with leaves appearing bunched at the top,giving it a 'bunchy top' appearance.

(B) Viral infections often lead to the death of host cells or tissues,a process known as necrosis.
Necrosis is characterized by the premature death of cells in living tissue due to external factors such as infection,toxins,or trauma.
In the context of plant pathology,viral infections frequently cause localized or systemic necrosis,which is a common symptom of viral disease.

(B) The rugose mosaic disease of potato is primarily caused by the Potato virus $Y$ $(PVY)$.
While Potato virus $X$ $(PVX)$ can also cause mosaic symptoms,the specific 'rugose' (wrinkled) mosaic condition is characteristically associated with $PVY$ infection,often in synergistic combination with $PVX$ in field conditions.
However,in standard biological contexts,$PVY$ is the primary causative agent for the rugose mosaic symptom.

(B) On the basis of the host infected,viruses are primarily classified into three main types: animal viruses,plant viruses,and bacterial viruses (also known as bacteriophages).
However,many other types of viruses have been discovered that infect a wide range of hosts,such as cyanophages (infecting cyanobacteria),mycophages (infecting fungi),zymophages (infecting yeast),and phycophages (infecting algae).

(A) The influenza virus is an enveloped virus that contains a single-stranded $RNA$ genome. Viruses are obligate parasites and contain either $DNA$ or $RNA$ as their genetic material,but never both. Therefore,the correct option is $A$.

(B) The potato leaf-roll disease is a viral disease caused by the Potato leaf-roll virus $(PLRV)$.
It is transmitted primarily by aphids,such as $Myzus$ $persicae$.
The virus infects the phloem tissue of the plant,leading to the characteristic rolling of leaves and stunted growth.

(C) bacteriophage is a virus that infects bacteria.
It consists of a genetic material (usually $DNA$) enclosed within a protein coat called a capsid.
Therefore,it is composed of nucleoprotein,which is a combination of nucleic acid and protein.

(B) The size of the given organisms is as follows:
$1$. $E. coli$ (a bacterium) is approximately $1-2 \mu m$ in length.
$2$. $Neurospora$ (a fungus) is a multicellular organism,much larger than bacteria and viruses.
$3$. $Bacteriophage$ (a virus) typically ranges from $20-200 \ nm$.
$4$. $TMV$ (Tobacco Mosaic Virus) is a rod-shaped virus with a size of approximately $300 \ nm \times 18 \ nm$.
Among these,$TMV$ and $Bacteriophage$ are viruses,but $TMV$ is generally considered smaller in terms of overall volume and complexity compared to many complex bacteriophages. However,in standard biological comparisons,$TMV$ is often cited as one of the smallest infectious agents listed here.

(D) The correct answer is $D$. Bacteriophages are viruses that infect bacteria. They possess a distinct tadpole-like structure consisting of a polyhedral (hexagonal) head and a cylindrical tail.

(C) virulent bacteriophage is one that follows the lytic cycle,leading to the destruction (lysis) of the host cell.
$T_4$ phage is a well-known example of a virulent bacteriophage that infects $E. coli$ and causes the lysis of the bacterial cell.
In contrast,temperate phages like $λ$ phage can follow the lysogenic cycle,where they integrate into the host genome without immediately killing the cell.