Describe the structure and properties of TMV (Tobacco Mosaic Virus). How does it differ from bacteriophage ?
Q. Describe the structure and properties of TMV (Tobacco Mosaic Virus). How does it differ from bacteriophage ?
Ans. Structure: TMV is an elongated rod shaped virus. It measures 300 ml x 15 ml. in diameter. It exhibits helical symmetry. It consists of about 95% protein coat and about 5% RNA. The protein coat is called capsid. Capsid consists of 2130 sub units (capsomeres). These capsomeres are closely packed and arranged in a helical manner around RNA forming a hollow cylinder.
Franklin et al., (1957) have described the structure of TMV particle. The particles appear as bundle of rods or needles under electron microscope. Each rod is approximately 3000 Å (300 nm) in length and 170 Å (17 nm) in diameter and has a molecular weight of 39 x 10° daltons. These particles have a protein coat (capsid) and a nucleic acid core. The protein coat is made up of approximately 2130 identical protein subunits called capsomeres. Each capsomere consists of a single polypeptide chain of 158 amino acids and its molecular weight is 18,000 daltons. The capsomeres are helically arranged around a central hole of 40 Å (4 nm). Inside the protein coat, there is a single stranded RNA (ss RNA) molecule, which is also spirally coiled to form helix, with a pitch of 23 Å and at a radius of about 40 Å. The RNA does occupy the hole, but is deeply embedded in the protein subunits, thus having its own helix. The viral RNA consists of 6,500 nucleotides and has a molecular weight of 2.4 millions, daltons.
Fig. Tobacco mosaic virus (TMV)-(a) the rod, (b) side arrangement of protein subunits (PS) and nucleic acid (NA), (c) cross section view of the virus rod.
HC – hollow core
The total length of each rod has about 130 helicals (turns) and in each helix there are approximately 16 1/3 capsomeres. In each helix of RNA there are 49 nucleotides and the helix has a pitch of 23 Å.
Properties: On the basis of the modern studies the under mentioned views have been put to explain the properties of TMV.
(i) It is found as extracellular (outside the cell) and intercellular (inside the cell) stages. The extracellular stage is called ‘Viron’. The viron is externally covered by capsids which are made up of protein. The capsids are covered with RNA, or DNA and thus the whole structure is nucleocapsids, or nucleoprotein. The intercellular virus is found inside the cell and is made of nucleic acid.
(ii) It is the smallest organism and ultramicroscopic.
(iii) It does not survive in dead tissues.
(iv) It bears the property of growth and reproduction.
(v) The viral cells are not surrounded by any membrane.
(vi) Inside the viral cell the cell organelles like mitochondria , Goigi bodies, ribosome, lysosomes etc. are not present.
(vii) The viral cell may be crystallised.
(viii) It is transmissible from unhealthy plant to healthy one.
(ix) It cannot be filtered through filter paper.
(x) It can multiply only within the living host cells.
(xi) It possesses the capacity of variations and adaptations.
(xii) It is highly resistant to high and low temperatures, sunlight, acids, alkalies and salts.
(xiii) Direct sunlight has no effect on viral cell.
Difference between bacteriophage and TMV : Bacteriophage:
(i) Shape: Tadpole like with head and tail.
(ii) Symmetry : Head cuboidal and tail helical.
(iii) Nucleic acid: Nucleoid DNA or RNA.
(iv) Adsorption : Adsorption over a host cell is highly specific and complex.
(v) Phagocytosis: The virus is not taken inside the host cells by phagocytosis.
(vi) Intercellular virion : Only DNA portion enters the host cell and is termed intercellular virion.
(vii) Separation : Protein coat and nucleic acids are separated outside the host cell.
(viii) Lysis: After eliciple phase lysis of host cell takes place.
(ix) Prophage : Prophase condition is present.
TMV
(i) Shape: rod shaped.
(ii) Symmetry: helical symmetry.
(iii) Nucleic acid nucleioid RNA.
(iv) Adsorption : adsorption over host cell is simple.
(v) Phagocytosis: the virus is taken inside the host cell by phagocytosis.
(vi) Intracellular virion: the entire virus enters into the host cell.
(vii) Separation : protein coat and RNA are separated inside the host cell.
(viii) Lysis: The host cell after eclipse phase may or may not lyse.
(ix) Prophage: No prophage condition present.
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