Q.5. Describe structure and multiplication of bacteriophages. (2006, 09, 11)
Related Question -
Q. Describe mode of infection and reproduction cycle of Bactriophage. (2012)
Q. Write short notes on Bacteriophage. (2004, 07)
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 and carbohydrates and vaccinia virus contains fat, carbohydrate, proteins, enzymes and appreciable amount of water and copper.
Life Cycle of a Bacteriophage: -
The word bacteriophage means eater of bacteria but it is not true because they infect bacteria and interact. These bacteria multiply 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: -
Multiplication occure without destroying the host. Here host not only survive but transmits genetically the ability to produce phage particles.
In general when bacteriophage 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 coordination 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.6. Write short notes on viroids or prions.
Ans. Viroids: -
Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest discovered is a 220 nucleobase scRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV).In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size. The human pathogen hepatitis D is similar to viroids.
Viroids were discovered and given this name by Theodor Otto Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971.
Viroid RNA does not code for any protein. The replication mechanism involves RNA polymerase II, an enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes “rolling circle” synthesis of new RNA using the viroid’s RNA as template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates
The first viroid to be identified was the potato spindle tuber viroid (PSTVd). Some 33 species have been identified.
Viroids and RNA Silencing: -
There has long been confusion over how viroids are able to induce symptoms in plants without encoding any protein products within their sequences. Evidence now suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid like symptoms.
This evidence indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex. The viroid sRNAs actually contain sequences capable of complementary base pairing with the plant’s own messenger RNAs and induction of degradation or inhibition of translation is what causes the classic viroid symptoms.
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.