Viruses, Viroids, Prions and Lichens Questions in English

(D) Bacteriophages are viruses that infect bacteria. Most bacteriophages contain $DNA$ as their genetic material,such as $T_2$,$T_4$,and $T_6$ phages.
However,the $MS_2$ phage is a well-known example of a bacteriophage that contains single-stranded $RNA$ as its genetic material.

(C) The $T_2$ and $T_4$ phages are virulent bacteriophages that possess a complex structure including a head,a contractile tail,a tail plate,and spikes (tail fibers) for attachment to the host cell.
Temperate phages (such as Lambda phage) are characterized by their ability to integrate their genome into the host genome (lysogeny) and often lack the complex contractile tail apparatus,tail plates,and spikes found in the $T$-even series of phages.

(C) $Bacteriophage$ is a virus that infects bacteria.
It typically possesses a complex structure consisting of an icosahedral head (capsid) that contains the genetic material and a helical tail that aids in the attachment and injection of $DNA$ into the host cell.
$TMV$ (Tobacco Mosaic Virus) is a helical virus,not icosahedral.
Therefore,the correct option is $C$.

(D) phage that infects a host cell without immediately destroying it is known as a temperate phage. In this process,the viral genome integrates into the host genome and replicates along with it,a cycle known as the lysogenic cycle. Among the given options,the $\lambda$ (lambda) phage is a classic example of a temperate phage,whereas $T_2$ phages are typically virulent,meaning they cause the lysis (destruction) of the host cell.

(D) The $T_4$ bacteriophage is a complex virus consisting of a head,a tail,and tail fibers. The head (capsid) is an elongated icosahedron. The total number of capsomeres in the $T_4$ phage structure is $2144$. This includes the capsomeres forming the head,the collar,the tail sheath,and the base plate.

(C) $Zymophage$ is a type of virus that specifically infects and acts as a parasite on $Yeast$ cells.
Since $Yeast$ is a fungus,$Zymophages$ are essentially mycoviruses that target yeast species.

(C) Bacteriophages are viruses that infect bacteria.
Those bacteriophages that specifically infect the colon bacterium,$Escherichia \ coli$ (commonly known as $E. \ coli$),are referred to as Coliphages.
Therefore,the correct option is $C$.

(B) Lysogeny is a form of viral reproduction involving the fusion of the nucleic acid of a bacteriophage with that of a host bacterium. In this process,the phage $DNA$ integrates into the host chromosome and becomes a prophage. The host cell continues to replicate normally,and the viral $DNA$ is copied along with the host $DNA$ during each cell division without causing immediate lysis of the host cell.

(B) Cyanophages are viruses that infect cyanobacteria. Like most bacteriophages,they contain $DNA$ as their genetic material. Therefore,the correct option is $B$.

(D) The $T_2$ bacteriophage is a complex virus that infects $E. coli$ bacteria.
Its head (capsid) is composed of protein subunits called capsomeres.
Scientific studies and structural analysis of the $T_2$ phage head indicate that it consists of exactly $2144$ capsomeres.
Therefore,the correct option is $D$.

(C) The ultrastructure of bacteriophage $T$ (specifically $T2$ bacteriophage) was studied by $M. Schlesinger$ in $1933$. He was the first to demonstrate that the bacteriophage consists of a protein head and a tail,providing early insights into the morphology of viruses.

(B) typical bacteriophage (like $T_2$ or $T_4$ phage) consists of a head and a tail. The head is polyhedral,while the tail is a hollow tube surrounded by a contractile sheath. The dimensions of the tail of a typical bacteriophage are approximately $1000\, \mathring{A}$ in length and $250\, \mathring{A}$ in diameter.

(A) Cyanophages are viruses that specifically infect and replicate within cyanobacteria (blue-green algae). The term is derived from 'cyano' (referring to cyanobacteria) and 'phage' (meaning to eat or destroy). Therefore,they are specialized bacteriophages that target cyanobacteria.

(B) The correct answer is $(b)$.
Satellite viruses are subviral agents that depend on the presence of another virus,known as a helper or activator virus,for their replication.
They lack the genes required for their own replication and therefore must infect a host cell that is already infected by a helper virus.
An example of a satellite virus is the tobacco mottle virus,which requires the tobacco mottle helper virus for its multiplication.

(B) Viroids are infectious agents that consist only of a short strand of circular,single-stranded $RNA$ without a protein coat.
Potato spindle tuber virus $(PSTV)$ is a well-known example of a viroid.
In contrast,viruses like $TMV$,Polyoma virus,and Mumps virus possess a protein coat called a capsid surrounding their genetic material.

(A) $W$.$M$. Stanley crystallized the Tobacco mosaic virus $(TMV)$ for the first time in $1935$. He demonstrated that viruses could be crystallized and that crystals consist largely of proteins. This discovery was a significant milestone in virology,proving that viruses are not just infectious agents but possess a definite chemical structure.

(A) The outermost proteinaceous covering of a virus is known as the capsid.
It is composed of small protein subunits called capsomeres.
The capsid serves to protect the viral genetic material ($DNA$ or $RNA$) from environmental damage.

(C) The lysogenic cycle is the reproductive cycle of a temperate,non-virulent phage (like $\lambda$ phage).
It has the potential to lyse the host cell,but it normally involves the integration of the viral genome with the host $DNA$.
The viral genome then replicates along with the host cell $DNA$ without immediately destroying the host.

(C) In the lysogenic cycle,the viral genome integrates into the host bacterial genome and becomes a prophage.
In this state,the virus does not replicate independently,and the host bacterium continues to survive and divide normally.
This phenomenon is known as lysogeny.

(C) Bacteriophages are viruses that infect bacteria. $M_{12}$ phage is a type of bacteriophage that is known to be virulent. $A$ virulent phage,also known as a lytic phage,follows the lytic cycle,which results in the destruction (lysis) of the host bacterial cell upon the release of new viral particles.

(B) Bacteriophages are viruses that infect bacteria. They consist of a head and a tail structure. Despite having a tail,they do not possess any mechanism for independent locomotion. They are obligate parasites that rely on Brownian motion or random collisions to encounter their host cells. Therefore,tailed bacteriophages are non-motile.

(A) 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 are obligate parasites that use the bacterial machinery to reproduce.

(A) Bacteriophages that exhibit a lysogenic cycle are known as temperate phages.
In the lysogenic cycle,the viral genome integrates into the host bacterial genome and replicates along with it without immediately destroying the host cell.
In contrast,virulent phages undergo the lytic cycle,which results in the immediate destruction (lysis) of the host cell.

(A) Viruses that infect bacteria are known as bacteriophages.
When these viruses enter the bacterial cell,they take over the host's machinery to replicate their own genetic material and synthesize viral proteins.
This process leads to the assembly of new viral particles.
Finally,the virus causes the rupture or lysis of the bacterial cell wall to release the progeny viruses.
Such viruses are referred to as $Lytic$ viruses because they complete the lytic cycle.

(A) The Tobacco Mosaic Virus $(TMV)$ is a plant virus that contains a single-stranded $RNA$ molecule as its genetic material.
It consists of a protein coat (capsid) surrounding the $RNA$ genome.

(A) virus that infects bacteria is known as a $Bacteriophage$.
$Bacteriophages$ are viruses that specifically target and replicate within bacterial cells.
$Cyanophages$ infect cyanobacteria,while $Telophase$ and $Prophase$ are stages of cell division (mitosis/meiosis),not viruses.

(B) Viruses are considered $acellular$ organisms because they lack a cellular structure. They consist of genetic material (either $DNA$ or $RNA$) enclosed within a protein coat called a capsid. They do not possess organelles,cytoplasm,or a cell membrane,which are characteristic of true cells.

(C) The defining characteristic of living organisms is the ability to reproduce or replicate their genetic material.
Viruses are considered to be at the border of living and non-living because they are inert outside a host cell but become active and replicate (duplicate themselves) once they enter a living host cell.
Therefore, the ability to duplicate themselves is the strongest evidence for their biological activity.

(D) Viruses are obligate parasites,meaning they require a living host cell to replicate and perform their biological activities.
They lack the metabolic machinery to produce energy or synthesize proteins independently.
Option $A$ is correct as viruses consist of a genetic material ($DNA$ or $RNA$) enclosed in a protein coat (capsid).
Option $B$ is correct as they are obligate intracellular parasites.
Option $C$ is correct because viruses do not possess the enzymes or organelles required for cellular respiration.
Option $D$ is incorrect because viruses cannot be grown in a simple culture medium like sugary liquid (broth); they require living cells (such as embryonated eggs or tissue cultures) to multiply.

(A) When a bacteriophage infects a bacterium,its genetic material can integrate into the host bacterial genome.
In this latent (non-virulent) state,the integrated viral $DNA$ is known as a prophage.
The enzyme $\lambda$ integrase facilitates the attachment of the phage $DNA$ to the specific site of the bacterial genome.
Therefore,the correct term for the integrated viral genome within the host bacterial genome is a prophage.

(C) The 'Mad Cow' disease,also known as Bovine Spongiform Encephalopathy $(BSE)$,is caused by infectious agents known as Prions.
Prions are abnormally folded proteins that can induce abnormal folding in normal proteins in the brain.
These were historically referred to as 'Scrapie proteins' because similar agents cause Scrapie disease in sheep.
Therefore,the correct option is $C$.

(C) $Cuscuta$ $reflexa$ (commonly known as "Dodder") is a parasitic plant that grows on angiosperms. It acts as a bridge for the transmission of various plant pathogens, including viruses and mycoplasmas, from one host to another.

(C) Blue-green algae,also known as $Cyanobacteria$,are photosynthetic prokaryotes.
$Nostoc$,$Anabaena$,and $Aulosiras$ are all well-known genera of $Cyanobacteria$.
$Lichen$ is not an alga; it is a symbiotic association between an alga (or $Cyanobacteria$) and a fungus.
Therefore,$Lichen$ is the correct answer.

(D) Lysogeny is a type of life cycle in bacteriophages where the viral $DNA$ integrates into the host bacterial genome.
In this state,the viral genome is known as a prophage.
The bacterial cell continues to replicate and divide normally without being destroyed or lysed by the phage.
This process allows the viral genetic material to be passed on to daughter cells during bacterial cell division.

(D) Historically,organisms that were immobile and spread out in their environment were often classified as plants. Moulds,mushrooms,and lichens all exhibit these characteristics,which led early naturalists to group them under the kingdom Plantae before the development of modern taxonomic systems like the five-kingdom classification.

(A) Lichens are symbiotic associations between fungi (mycobiont) and algae or cyanobacteria (phycobiont).
In lichens,the sexual reproduction is primarily carried out by the fungal partner (mycobiont),which produces fruiting bodies such as apothecia or perithecia.
The algal partner does not participate in the sexual reproductive process of the lichen.
Therefore,the correct option is $(A)$.

(A) The symbiotic association between fungi and algae is known as a $Lichen$. In this relationship,the algal component is called $phycobiont$ and the fungal component is called $mycobiont$. They live in a mutually beneficial association where the algae prepare food through photosynthesis,and the fungi provide protection,anchorage,and absorption of minerals and water for the algae. $Mycorrhiza$,on the other hand,is a symbiotic association between fungi and the roots of higher plants.

(C) The correct answer is $C$. Lichens are extremely slow-growing organisms. They do not grow at a rate of $2 \ cm$ per day; rather,they typically grow at a very slow rate,often around $1 \ mm$ per year. The other statements are correct: some species like $Cladonia$ (reindeer moss) are consumed by reindeers,they are well-known bio-indicators of air pollution (especially $SO_2$),and they represent a symbiotic association between an algal component (phycobiont) and a fungal component (mycobiont).

(A) Lichens are symbiotic associations between algae and fungi. The fungal partner (mycobiont) in the majority of lichens belongs to the class $Ascomycetes$. In some cases,it may belong to $Basidiomycetes$,but $Ascomycetes$ is the most common group involved in this symbiotic relationship.

(A) $Fruticose$ or filamentous lichens are branched and shrubby in appearance but possess a small base for attachment. Examples include $Cladonia$ and $Usnea$.

(C) Lichens represent a symbiotic association between a fungal partner (mycobiont) and an algal partner (phycobiont or photobiont).
In the majority of lichens,the fungal partner belongs to the class $Ascomycetes$.
The algal partner is usually a $Cyanobacteria$ (blue-green algae) or a green alga.
However,the most common association found in the majority of lichens involves blue-green algae and $Ascomycetes$.

(C) Lichens are known for their extremely slow growth rate,often growing at a rate of approximately $1 mm$ per year. Due to this slow metabolic activity and growth,they take a very long time to reach maturity. In many species,it takes about $100$ years for a lichen to reach its mature stage.

(A) $Crustose$ lichens grow as a thin,flat crust over rocks,soil,or the bark of trees,giving them an appearance of being painted on the surface. Examples include $Graphis$ and $Haematomma$.

(C) Lichens are highly sensitive to air pollution,particularly to higher $SO_2$ concentrations in the atmosphere.
This $SO_2$ causes plasmolysis of algal cells and converts the chlorophyll of algae into pheophytin by removing the magnesium $(Mg)$ ion.
This process results in chlorosis and the eventual death of the lichen.

(B) Lichens are ecologically significant because they act as pioneer species in primary succession on barren rocks.
They secrete organic acids that help in the weathering of rocks,which contributes to soil formation and allows other plants to colonize the area.

(C) Reindeer moss,scientifically known as $Cladonia\, rangiferina$,is a type of lichen found in the arctic and subarctic regions.
It serves as a vital food source for reindeer and caribou during the winter months.
Therefore,the correct option is $C$.

(C) $Usnea$ is a genus of fruticose lichens that grow on the bark of trees or rocks.
Lichens are symbiotic associations between a fungus (mycobiont) and an alga or cyanobacterium (phycobiont).
Therefore,$Usnea$ is classified as a lichen.

(D) $Helotism$ is a specific form of symbiosis observed in lichens,where one partner (the fungus) acts as a master and the other (the alga) acts as a slave.
In this relationship,the fungus provides protection,water,and mineral absorption,while the alga provides food through photosynthesis.
The fungus is considered to benefit more than the alga,hence the term $Helotism$ is used to describe this association between algae and fungi.