Q. 1. Give a brief account of viruses?
Related Question: -
Q. What are viruses? (2012)
Q. Write short notes on Nature of virus. (2012, 13)
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 nucleoproties 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.2. Describe the structure of virus. (2005)
Related Question: -
Q. Discuss the external symptoms of virus. (2005)
Q. Describe to internal symptoms of viruses. (2006, 09)
Ans. F. L. Bawden and N.W. Parie of England described that viruses are composed of proteins and nucleic acid. They isolated the virus from tomato plant in crystalline form and made it clear that the powder of virus is nothing but made up of protein and nucleic acid. It is revealed by observation under electron microscope that viruses are smaller than bacteria in size. Fully developed virus particles are called “virions” and it consists of three parts-
(1) Capsid: - In the central part of virion, nucleic acid, DNA or RNA is found which surrounded by capsid made up of protein. Sometimes an outer delicate envelope is found surrounding the capsid which is made of lipid or lipoprotein. Nucleic acid of virion is responsible for producing infectivity to the host and capsid, which is made of proteins, provides specificity to host.
Virions surrounded with envelope are also sensitive to lipid solents like other chloroform etc. The viruses that are not surrounded with envelopes are known as naked viruses. Capsid is made up of a number of sub-units which are known as capsomeres. Generally three types of symmetry are observed in viruses:
In polygonal symmerty, three types of arrangement is found .
(i) Tetrahedral - It consists of four layers.
(ii) Octahedral - It consists of eight layers.
(iii) Icosahedral - It is made up of about twenty layers.
Viruses are often icosahedral and their study can be made only through electron microscope and X-ray diffraction. Capsid consists of several sub-units capsomers. Capsomers are made up of several monomers. Each monomere has one or more polypeptide chains. According to the presence of five or six units, capsomeres are divided into two types pentamere & hexamere. The number of capsomers in Polio virus is 32 and in Ribo virus is 92 and in Herpes virus is 192.
(2) Envelope or Sheath: - In some isosahedral and Herpes virus, Influenza virus and Mumps virus capsid is covered with a 100 - 15 A0 thick coat made of protein, carbohydrate and lipid. Its formation has been already been done by plasma membrane of host cell.
(3) Nucleic Acid: - There may be present only one nucleic acid RNA or DNA in virus. But both types of nucleic acids DNA and RNA are never found tegether in viruses. In plant virus, RNA is usually single stranded or double stranded but in animal virus DNA is found in the form of a single stranded. In bacteriophage, DNA is present in the form of single or double strand.
Q.3. Give a short account of the general characters and classification of Bacteria.
Ans. Bacteria was first seen by Leeuwenhoek in 1676 (16 32 - 1723) by this simple microscope. Earlier bacteriologists regarded the bacteria as small animals, because of motility of exhibited by many bacteria, motility by rapid cell division (a character of many protozoa). The bacteria differed from animals in the smaller size, absence of nucleus and a mouth or gullet.
General characters of Bacteria:-
(1) They are unicellular achlorophyllous minute orgnisms leading to autophytic, saprophytic and parasitic lives. These may be motile or non -motile and if motile, it is due to presence of flagella.
(2) The nuclear material found in bacteria, is termed incipient nucleus.
(3) The characteristic method of propogation in bacteria is the binary fission.
(4) Sexual reproduction it absent but it has been recently reported in Escherichia coli.
(5) The only and main function of bacteria is to multiply.
(6) These are, on one hand employed for various industrial purposes but on the other cause many diseases in men, animals and plants.
Classification of bacteria:-
At present the most widely accepted and standard system of classification is as published in Bergey’s Manual (1953). According to this system, classification class schizomycetes is divisible into 10 orders.
Kingdom - Plant,
Division - Protophyta.
Class - Schizomycetes.
Order I. Pseudomonadales: -
Bacteria of this order have cells straight, curved or spiral, rigid, rod shaped, usually polar flagellate sometimes non-motile gram negative, endospores not formed cells with photosynthetic, pigments, some may be soil inhabiting, some may be parasitic while others are pathogenic e.g., Nitrosomonas, Nitrobactor, Thiobacillus, Pseudomonas, etc.
Order II. Eubacterials :-
Includes there bacteria. Occurring in water soil and air and in bodies of animals and plants. Cells are rigid, simple, Spherical or rods, motile or non -motile, Gram positive and Gram negative e.g. Azotobactor, Rhizobium, Micrococus, Bacillus etc.
Order III. Actinomycetes: -
Enclosed hypha like cells, known as mould like bacteria e.g., Mycobacterium, Actinomyces, Streptomyces etc. Mostly found in soil and fresh water, few are pathogenic.
Order IV. Chlamydobacteriales:-
Filamentous bacteria often shethed, motile forms have polar flagell only.
Order V. Hyphomicrobioles :-
Includes stalked bacteria, pear, shaped reproduction by budding.
Order IV. Caryophanales:-
Filaments bacteria, motile form have flagella arising all over the cell.
Order VII. Beggiatoales:-
Occur usually in multicelled filaments some of which show gliding type of mobility over surface, often have sulphur granules.
Order VIII. Myxobacteriales:-
Includes slime bacteria, flexible roads exhibiting movement on solid surface.
Order IX. Spirochetales.
Order X. Mycoplosmatales:-
Include high pleomorphic organism filterable.