Q.20 Describe the structure and replication of bacteriophage. (2007, 09)
Related Questions -
Q. Write short notes on Bacteriophage. (2011, 13)
Q. Give a detailed account of Bacteriophages. (2012)
Ans. Structure: -
It is a kind of virus parasitizes on bacteria. The phages are charachteristically smaller in size showing relative morphological simplicity and rapid reproduction. The well studied example is T4 bacteriophage of E. coli.which has overall dimensions of about 65 *95mu.Its protein body consists of polyhederal head , neck , collar, sheath , end plate, and six tail fibres. An exceptionally long and completely arranged double stranded spherical molecule of DNA of nearly 5200mu is found contorted in the head. The sheath is specialized in function and acts like shocker spring during infection which permits the penetration of core inside the bacterium cell.
These two examples explain the general structure of viruses. Both these contain nucleic acid and protein. Some viruses besides nucleoprotein contain a few more substances eg. influenza virus contains lipid s and carbohydrates and vaccinia virus contains fat, carbohydrate, proteins, enzymes and appreciable amount of water and copper.
Life Cycle of a Bacteriophage: -
The word bacteriphage means eater of bacteria but it is not true because they infect nacteria and interact. These mmultiply in the cell of bacterium. The period of production of new phage particles inside the bacterium is called vegetative phase during which replication of genetic material, synthesis of phage protein and morphological elaboration of phage particles occur. After vegetative phase, phage particles mature to an infective form.
There are few categories of infective viruses.
(i) Virulent: -
Multiplication of phage within bacterial cell is followed by dissolution or lysis of bacterial cell.
(ii) Temperate: -
Multiplicatipon occure without destroying the host. Here host not only survive but transmits genetically the ability to produce phage particles.
In general when bacteriphage infects, its DNA core enters but the protein coat remains outside the bacterial cell where the entire body is transferred in the host and multiply in a specific part of the host cell. DNA of bacteriophage when enters in bacterium it becomes incorporated in the bacterial chromosome. This integrated phage is called prophage.
A prophage acts like a gene which exercises its potential for phenotypic expression. It replicates in coordinatiommn with the replication of bacterial chromosome. It does not direct for the synthesis of other phage material and organization of mature phage particles. After the multiplication of large number of phage genetic material and absorbing the protein surrounding each, new bacteriophages are formed.
Q.21 Describe the different modes of transmission in viruses. (2006)
Related Question -
Q. Write an account on transmission of viruses. (2015,05)
Q. Describe the nature and modes of transmission of plant viruses. (2009)
Ans. All viruses are infectious and transmissible. There can be two broad lines of transmission.
(1) Artificial Methods of Transmission: -
(a) Sap Inoculation: -
Sap is extracted from the virusinfected parts of the plant and is purified.It is now applied to the healthy surfaces of the leaf or it can be inoculated as such.
(b) Grafting: -
For the transmission of virus by grafting technique, bud from the diseased plant is taken out and grafted in a healthy plant. At times stem grafting may be made in which case either scionor stalk should be from infected plant. Common example of transmission by grafting is peach yellow virus.
(2) Natural Methods of Transmission: -
(a) By Soil: -
These are also known as soil borne viral diseses and their transmission may be both –natural by means of fungi and nematodes and mechanical by the persisting viruses in plant debris.
(b) By Seed: -
Viruses may persist in the seeds of some plants which during formation of new plant develop disease. The common example is bean mosaic virus.
(c) By Pollen Grains: -
Pollen grains of an infected plant, when for fertilization reach the healthy plant, transmits the disease. After fertilization viruses make house in young developing seeds. Germination of such seeds into new plants develop viral diseases.
(d) By Natural Grafting: -
Parsitic angiosperms like cuscuta make natural grafts place to place on various parts of different plants and when grow on more than one plant including diseased and healthy ones, they transmit the disease.
(e) By Vegetative Propagation of Plants: -
Certain plants reproduce by formation of vegetative propogules which transmit viral disease from parent to young plant.
(f) By Roots: -
Injuries produced in the roots during cultivation also become infected by viruses from soil.
(g) By Contact: -
Certain contagious viral diseases spread by mere contact in between healthy and diseased plants eg, TMV.
(h) By Weeds: -
Occassionally weeds serve as off seaso alternative host which transmit viral diseases during the growth of original host.
(i) By Vectors: -
Vectors constitute one of the most effective and notorious agencies of viral transmission. Two basic categories of such transmission have been recognized-
(i) Simple Mechanical Transmission: -
Rabies and beetles of dogs are known to transmit TYMV(Turnip yellow mosaic virus). When beetles, grasshoppers or ear wigs feed an infected plant, their mouth parts become contaminated with viruses which are later transmitted to healthy plants. Common vector of TYMV is turnip flea beetle.Such insects have no salivary glands.
Another category is that of aphids which are helpful in mechanical transmission of virus. Aphids commonly feed phloem sap of diseased plants and sip the viruses along with. These aphids can transmit viruses only immediately after feeding.
(ii) Virus and Vectors are Intimately Connected: -
Aphids: -
Afew viruses have intimate relationship with aphids, eg. peoenation mosaic virus and barley yellow dwarf virus. Such viruses are found in phloem sap and when they are extracted by aphid they reach in aphid‘s gut. Viruses do not multiply in the gut. These are transmitted after moulding stage of insects. Another category of viruses multiplies in aphid‘s gut. These are called propogative viruses. Common example is potato leaf roll virus in the aphid .
Leaf Hoppers: -
Propogative type of transmission is also seen among grass hoppers. Besides a few examples of stylet borne and circulative transmission is found in case of dwarf disease of rice by Nephteltix apicalis.
Mosquitoes: -
These are common transmitters of animal viruses and methods of transmission are similar to that of aphids and grass hoppers.
Nematodes: -
These are one of the agencies of spreading soil borne disease is well known for the transmission of diseases such as grape fan leaf and tobacco ring spot disease of virus.
Fungi: -
Olphidium brasscae transmits tobacco necrosis virus and lettuce big vein by infecting a wide range of roots. Though it caused a heavy transmission but only few fungi are known for viral transmission.
Q.22. Write definition of viruses and also explain nature of virus. (2014,16)
Ans. Definition:-
Viruses are the smallest living entity, which are parasitic, ultramicroscopic and disease producing organism. These were investigated by Iwanoski in 1892.
Brief History:-
The earliest known reference to a plant virus is in 1756 describing a condition known as Tulip Break. The word ‘virus ‘comes from the Latin meaning ‘poison’ has been used for centuries in this literal sense, but with the development science of microbiology, it was loosely applied to various microorganisms e.g., ‘chicken cholera virus’ etc. In 1886 Mayer described Tobacco mosaic disease but could not found its causal Agent.
Iwanowski (1892) was first to observe and showed bacteria free environment included mosaic disease in tobacco plant.
Frobischer (1957) devided viruses into four groups: -
(i) Plant viruses (ii) Mammalian viruses (iii) Insect viruses, (iv) Bacterial viruses.
They differ from other micro-organism because (i) of their ultramicroscopic size which enabled them to pass through bacterial filters and (ii) as they could not be grown in artificial, cell free media. (iii) These can be crystallized and precipitated like chemicals. The crystals become active like in living cell. In short - viruses may be recognized to be a special class obligatory interacellular parasites.
Characters of viruses:-
1. Viruses are ultramicroscopic infectious particles ranging form 2 nm to 300 nm in diameter.
2. Viruses are highly pathogenic.
3. Viruses are Perfect obligate parasites.
4. They grow only in living cells.
5. They are made of nucleoprotiens in which nucleic acid is RNA or DNA.
6. They Cause infection in host by nucleic acids.
7. Viruses are easily transmitted so their infection is very fast.
8. They can be crystallized.
Viruses have characters of both living or non living by which some scientists keep them in living and some in non living objects.
Living Characters of viruses:-
1. Viruses are particulate and strictly obligate parasites.
2. They respond to environment such as heat and ultraviolet rays.
3. They can multiply or active in living cells.
4. These shows properties of mutation.
5. They choose specific host.
Non living characters of viruses:-
1. Viruses do not respire.
2.There is absence of protoplasm.
3. They have no complete cellular structure.
4. They can take crystallised forms which retain capacity of infection.
5. Virus possesses high specific gravity.
Q.23. Write about the structure and multiplication of plant viruses. (2005)
Related Questions -
Q. Write short note on structure of viruses. (2012)
Ans. Viruses are very small and can only be observed with an electron microscope. The structure of a virus is given by its coat of proteins, which surround the viral genome. Assembly of viral particles takes place spontaneously.
Over 50% of known plant viruses are rod shaped (flexuous or rigid). The length of the particle is normally dependent on the genome but it is usually between 300–500 nm with a diameter of 15–20 nm. Protein subunits can be placed around the circumference of a circle to form a disc. In the presence of the viral genome, the discs are stacked, then a tube is created with room for the nucleic acid genome in the middle.
The second most common structure amongst plant viruses are isometric particles. They are 40–50 nm in diameter. In cases when there is only a single coat protein, the basic structure consists of 60 T subunits, where T is an integer. Some viruses may have 2 coat proteins are the associate to form a icosahedral shaped particle.
There are three genera of Geminiviridae that possess geminate particles which are like two isometric particles stuck together.
A very small number of plant viruses have, in addition to their coat proteins, a lipid envelope. This is derived from the plant cell membrane as the virus particle buds off from the cell.
As mentioned above, 90% of plant viruses have genomes that consist of single stranded RNA, meaning that they are in the same sense orientation as messenger RNA. Viruses use the plant ribosomes to produce the 4-10 proteins encoded by their genome. However, since all of the proteins are encoded on a single strand (that is, they are polycistronic) this will mean that the ribosome will either only produce one protein, as it will terminate translation at the first stop codon, or that a polyprotein will be produced. Plant viruses have had to evolve special techniques to allow the production of viral proteins by plant cells.
5' Cap: -
In order for translation to occur, eukaryotic mRNAs require a 5' Cap structure. This means that viruses must also have one. This normally consists of 7MeGpppN where N is normally adenine or guanine. The viruses encode a protein, normally a replicase, with a methyltransferase activity to allow this.
Some viruses are cap-snatchers. During this process, a 7mG-capped host mRNA is recruited by the viral transcriptase complex and subsequently cleaved by a virally encoded endonuclease. The resulting capped leader RNA is used to prime transcription on the viral genome
However some plant viruses do not use cap, yet translate efficiently due to cap-independent translation enhancers present in 5' and 3' untranslated regions of viral mRNA.
Read through: -
Some viruses (e.g. tobacco mosaic virus (TMV) have RNA sequences that contain a “leaky” stop codon. In TMV 95% of the time the host ribosome will terminate the synthesis of the polypeptide at this codon but the rest of the time it continues past it. This means that 5% of the proteins produced are larger and different to the others normally produced. In TMV this extra sequence of polypeptide is an RNA polymerase which replicates its genome.
Production of Sub-Genomic RNAs: -
Some viruses use the production of sub-genomic RNAs to ensure the translation of all proteins within their genomes. In this process the first protein encoded on the genome, and this the first to be translated, is a replicase. This protein will act of the rest of the genome producing negative strand sub-genomic RNAs then act upon these to form positive strand sub-genomic RNAs that are essentially mRNAs ready for translation.
Segmented Genomes: -
Some viral families, such as the Bromoviridae instead opt to have multi-partite genomes, genomes split between multiple viral particles. For infection to occur, the plant must be infected with all particles across the genome. For instance Brome mosaic virus has a genome split between 3 viral particles, and all 3 particles with the different RNAs are required for infection to take place.
Q.24. Write short note on Prions. (2014, 15)
Ans. Slow viruses (Prions): -
Prions are the proteinceous particles though to cause a number of diseases including the slow virus diseases. Prions were named by Stanley B. Prusiner. prions can survive heat, radiation and chemical treatments that normally inactivate viruses.They appear to be composed of only proteins. The viruses causes in their hosts a range of infections viz., acute, inaparent, chronic, persistent, latent, slow progressive and tumorigenic infections. The so-called slow viruses are involved in slow progressive diseases. The agents which are presumed responsible for slow progressive diseases have been misanmed slow viruses, or also as prions. Many of these affect the centra nervous system and one of the best known is the scrapie agent of sheep and goat which causes the animal to scrape or scratch it self against obstacles.
Q.25. Write an essay on economic importance of viruses. (2013)
Ans. Economic importance of viruses
Virotherapy is an experimental form of cancer treatment using biotechnology to convert viruses into cancer-fighting agents by reprogramming viruses to attack cancerous cells, while healthy cells remained relatively undamaged. Usually the viruses used are herpes simplex virus or Adenoviruses.
It uses viruses as treatment against various diseases, most commonly as a vector used to specifically target cells and DNA in particular. It is not a new idea - as early as the 1950s doctors were noticing that cancer patients who suffered a non-related viral infection, or who had been vaccinated recently, showed signs of improvement[citation needed]: this has been largely attributed to the production of interferon and tumour necrosis factors in response to viral infection, but oncolytic viruses are being designed that selectively target and lyse only cancerous cells.
In the 1940s and 1950s, studies were conducted in animal models to evaluate the use of viruses in the treatment of tumors. In 1956 some of the earliest human clinical trials with oncolytic viruses for the treatment of advanced-stage cervical cancer were started[citation needed]. However, for several years research in this field was delayed due to the inadequate technology available. Research has now started to move forward more quickly in finding ways to use viruses therapeutically.