Page 1 : Viruses, Viruses (vira means poison: in Latin) are "noncellular, submicroscopic, obligatoy intracellular parasites, composed of proteinaceous covering around the nucleic acid (either DNA or RNA) and capable of selfreplication within the living of the host cells using host machinery for the synthesis of the specialized, particles- the virion which contain the viral genome and transfer it to the other cell., Viruses are a unique class of infectious agents truly distinctive by their simple organization and, mechanism of replication. They are among the most numerous 'living identity' on our planet and infect all, types of cellular organisms, Viruses exist in two states, the extracellular infectious particle or virion and the intracellular state, consisting of viral nucleic acid inside the host cell. In extracellular state, a virus is minute particles, containing nucleic acid surrounded by protein and occasionally possessed an envelope (membrane obtain, from the host cell). In typical form, outside the host cell, the virus particle (Virion) is metabolically inert. In, intracellular state, virus nucleic acid replicate, new copies of the viral genome are produced and the, components that make up the virus coat (protein coat) are also synthesized using host cell machinery., Since viruses contain nucleic acids and proteins. They are also called as nucleoprotein or, nucleocapsid. The protein coat of the virus is called capsid which is in contact with and the nucleic acid, (RNA or DNA). Capsid is formed by aggregation of protein sub-units called Capsomeres., Highly pathogenic virus is called virulent. A vector indicates an organism, (Such as insect) that, carries and/or transmits disease causing viruses. Aster yellow virus was the first plant virus shown to, multiply in insect vectors (i.e. propagatave virus). Satellite tobacco necrosis virus (STNV) is the smallest, known plant virus. Pox virus is the largest animal Virus., First successful vaccination against small pox was carried out by E. Jenner. First successful isolation, of virus the bacteriophage-WLL from the bacterium E. coli was done by M. Schlesinger (1933)., , DISCOVERY, It was Adolf Mayer, working in Netherlands, who in 1882 first describe an important disease of, tobacco which he called tobacco mosaic disease, he observe mosaic like pattern on leaves of diseased plants, and succeeded in reproducing the disease injecting juice extracted from infected tobacco leaves onto healthy, one, he could not identify the real agent that caused the disease. However Mayer's contribution will always, be remembered as he was the first person who first stem forward in the development of a new discipline, later recognized as „Virology‟., A few years later D. Ivanowski (1892), first experimentally demonstrated that the tobacco mosaic, disease has been cause by agents which successfully passed the Chamberland-filter (bacterial-proof filter), that retain even the smallest bacteria. It was an important clue, but contrary to his result and despite his, inability to isolate any bacterium Ivanowski still maintained that either the „pathogenic bacterium' somehow, passed through the filter or a "toxin' secreted by them passed through the filter and make the filtrate, infectious. Within six years after the experiment of Ivanowski, M. Beijerinck (1898) confirmed by repeating, the same experiments and found that the tobacco mosaic disease was cause not by any pathogenic bacteria, or toxin but rather by some new type of pathogenic agents which he called "Contagium vivium fluidum', (infectious living fluid) and referred it subsequently as a 'virus‟ (poison). He also said that the viruses, multiply only inside the living cell, GENERAL CHARACTERISTICS OF VIRUSES,  All viruses are obligate parasites (Obligate: is being restricted or fixed to a particular type of, behaviour),  They multiply only within their living hosts cells and remain metabolically inert outside the host cell.

Page 2 :  They are ultramicroscopic and can only be viewed with electron microscope (the smallest known, virus is merely 0.002 um in diameter, while the largest ones are typically about 0.8 um diameter).,  Viruses are actually nucleoproteins or nucleocapsids. The proteinaceous coat (capsid) surrounds the, nucleic acid, which forms the central core of the Virus particles.,  The viral genetic material, the nucleic acid, may either be DNA or RNA. The two nucleic acids are, never present together in given virus.,  These are the nucleic acids of the viruses which are infectious, and not protein coat.,  Viruses are usually minute that they can easily pass through a filter which can hold back even the, smallest bacteria.,  Viruses are easily transmitted from infected host to the healthy host through various agencies, (vector).,  Viruses are so effective that even their smallest amount can cause infection on the host successfully.,  Viruses can easily be crystallized as nucleoprotein molecules (nucleic acid and protein coat).,  Viruses are considered to be host-specific and represent obligate parasitism even at genetie level.,  Since the viruses have no metabolic activities of their own and utilize the metabolism or host cells,, antibiotic have no effect on them., , , , , , , , Biological (Animate) status of viruses, The status of viruses as living agents/organisms remain obscure as has been depicted in the five, kingdom Classification of living organisms by Whittaker. One is equally sure about their being, organism or even living being of any kind as about their being non-living. An intense controversy, exists between biologists who consider virus as organisms and biochemists who consider them as, molecules. Nonetheless, viruses possess the properties some of which are traditionally associated, with organisms, while others characterize non-living molecules. Therefore, till date there are two, separate schools of thought that are still debating about the status of viruses i.e. organisms concept, and molecular concept., 1. Animate (living) characteristic of viruses (organism concept), Biologists have regarded viruses as organisms on the following four important counts., Viruses multiply within the host cells and give rise to the same genetic types i.e. they reproduce their, typical progenies., They undergo mutation like living organisms., Viruses show extremely specific intracellular parasitism, a characteristic of the living organisms and, can be easily transmitted from host to host., Viruses can be easily reconstituted with the help of their already separated components (i.e. from, their individual nuclei acid and protein or capsids components), the reconstituted virus may undergo, enzymatic changes in vitro., 2.Inanimate (non-living) characteristics of viruses (molecular concept)., Viruses are not 'cells‟ because they have no cytoplasm, nucleus, membranes, ribosomes,, enzymes etc. necessarily present in a cell. They lack the ability to grow independently and, completely depend upon other living organisms tor their very existence. Viruses do not have an, energy production system of their own. They can easily be crystallized, precipitate and remain as, inert chemicals outside the host cell. These non-living characteristics of viruses enable one to, hesitate accepting them as organisms. The biochemists proclaim that the uniform size, shape,, chemical composition and crystallizability are the properties of viruses that they share with, molecules. They give the following arguments in support of the molecular concept, ,  Size of some viruses falls within the range of size of protein and nucleic acid molecules.,  Building blocks of the viral protein of the protein coats (capsid) from protein monomers, (capsomeres) follow the same general principles of chemistry as in the synthesis of protein molecules,  Chemical and physical homogeneity of virus particles can be compared with the homogeneity of, protein molecules.

Page 3 :  Cystallization is a physical phenomenon based on the formation of configurations as of minimum, energy level. Viruses do seem and behave as molecules during their crystallization., What are the virus are then? What is their biological status? These questions remain, controversial and no consensus has yet resulted. Since the viruses are very important and play, significant role in the biological system, A. Lwoff (1953) once remarked that a „virus is virus‟- it is, neither a living organisms nor a non-living chemical but something between. According to Salle, (1975) viruses “appear to fall in between chemical molecules and living cells”, MORPHOLOGY or SYMMETRY OF VIRUSES, Viruses, so far studied, usually fall into three main symmetry groups on the basis of electron, microscopic studies. These groups are:, 1. Icosahedral (or cubic) symmetry: Many of the Viruses (e.g. herpes virus, polio virus,, polyomavirus, adenovirus, turnip yellow mosaic virus) have icosahedral symmetry. They, resemble small crystal and appear approximately spherical in electron micrographs and their, capsomeres (the bulding block of protein coat or the capsid) are arranged to form an Icosahedron., An icosahedron is a regular polyhedron with 20 face formed by equilateral triangles and 12, intersecting corners., 2. Helical symmetry: Some viruses (e.g. TMV, Bacteriophage MI3, Influenza virus, measles virus, and rabies virus) resemble long rods and show helical symmetry. In these viruses the capsomeres, are arranged in a helix around a single rotation: The capsomeres curve into a helix because they, are thicker at one end than the other. The helical capsids may be naked (e.g. TMV) or surrounded, by a loose membranous envelop (e.g. Influenza virus, Mumps Virus). Plant viruses with helical, symmetry usually appear rod-shaped in electron micrographs., 3. Enveloped Viruses, The capsid of some viruses is covered by an envelope. Enveloped viruses are roughly spherical, When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical, or enveloped polyhedral viruses’, An example of an enveloped helical virus is the influenza virus, An example of an enveloped polyhedral (icosahedral) virus is the herpes simplex virus, 4. Complex Symmery: This group of viruses shows complex or uncertain symmetries that result, in complicated morphology. Viruses of this group can be divided into two subgroups as follows:, A) That illustrated by most bacteriophage that have a head of the shape of bipyramidal hexagonal, prism attached to a tail consisting of a helical contractile sheath surrounding a central hollow, core and, B) That illustrate by, pox viruses that do, not possess clearly, identifiable capsids, but have several, coats around the, nucleic acid

Page 4 : STRUCTURAL COMPONENT OF VIRUSES, Viruses in extracellular state are called virions or virus particles. Virions represent the, complete assembly of the infectious virus and vary widely in size ranging from 0.002 μm to, 0.03μm. A Virion consists of a nucleic acid core surrounded by a protein coat (Called capsid)., Few viruses have more complex structure beside nucleocapsids they also have membraneous, envelope. Such virions are called enveloped viruses in which nucleocapsids is enclosed in a, membrane that are acquired by the viruses from their counter host cell., VIRAL NUCLEIC ACID (VIRAL GENETIC MATERIAL), We all know that all cells have double-stranded DNA as their genetic material. But in, contrast, all viruses possess either DNA or RNA as their genetic material, and it can be either, single-stranded or double-stranded. However, there is a third group of viruses that use both DNA, and RNA as their genetic material but at different stages of their reproductive cycle. This third, group is exemplified by retroviruses which contain RNA as genetic material but replicate through, DNA intermediate, and Hepadnaviruses (human hepatitis B virus) which contain DNA as genetic, material but has an RNA intermediate in replication., Despite the diversity in genetic material, however, all viruses obey the central dogma of, molecular biology i.e. all genetic information flows from nucleic acid to protein. In addition, all, viruses use the host cell translational machinery (ribosome, t-RNA, amino acids etc.) and so no, matter what the genome structure of the virus, messenger RNA (m-RNA) must be generated that, can translate on the host ribosomes., NUCLEOCAPSID PROTEIN, The protein coat of virus is called capsid. The viral capsids are made up totally of proteins of, identical subunits (Capsomeres) which enclose nucleic acid and help protecting it from nucleases, action (enzyme produce by the host cell that destroy nucleic acids). The helical capsids contain, single type of protein and icosahedral capsid contains several types of protein., THE ENVELOPES, Some animal and plant viruses have outer membranous covering called as envelope which is, acquired from the cell cytoplasmic or nuclear membrane as they penetrate the host cell or organelles, depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrate-protein, complexes that project from the surface of the envelope., CORE OR INTERNAL PROTEINS, Proteins associated with viral nucleic acid are called core proteins., Beside the core protein viruses also carries with them some enzymes which are responsible, for different activities in viruses. Some examples of viral enzymes are RNAase and reverse, transcriptase in retroviruses, protein kinase in Herpes and adenoviruses, DNA dependent RNA, polymerase in poxvirus., Tobacco Mosaic virus (TMV), Tobacco mosaic virus is a historically important (The first virus discovered) and thoroughly studied, plant virus, which cause mosaic disease in tobacco plants and other members of the Solanaceae such as, tomato, potato etc. W. M. Stanley (1955) for the first time isolated this virus in its crystallize form and he, was awarded Nobel Prize for this work., A. Structure, TMV is simple rod-shaped helical virus consisting of centrally located stranded RNA (5.6%), enveloped by a protein coat (94.4%). The rod is considered to be 3000 Å in diameter in length and, about 180 Å in diameter. The protein coat is technically called „capsid‟. R. Franklin estimated 2,130, protein sub-units, namely, capsomeres in a complete helical rod and 49 capsomeres on every three, turns of a helix. Thus there would be about 130 turns per rod of TMV. The diameter of RNA helix is, about 80 Å and the RNA molecule lies about 50 Å inward from the outer surface of the rod. The

Page 5 : central core of the rod is about 40 Å in diameter. Each capsomere is a grape like structure containing, about 158 amino acids and having a molecular weight of 17,000 dalton as determined by Knignt. The, RNA is little more in length (about3,300 Å ) slightly protruding from one end of the rod., The ssRNA molecule consists of about 7300 nucleotides and the molecular weight of RNA, molecule being about 25,000dalton., , Life-cycle (replication), Plant viruses such as TMV penetrate and enter the host cells and their replication completes, within such infected host cells. Inside the host cell, the protein coat dissociates and the viral nucleic, acid (RNA) becomes free in the cell cytoplasm. Although the sites for different steps of viral, multiplication and formation of new viruses have not yet been determined with absolute certainty,, the studies suggest that after becoming free in the cell cytoplasm the viral-RNA move into the, nucleus (probably into the nucleolus). The viral RNA induces the formation of specific enzymes, called „RNA polymerases‟, in the presence of which the single-stranded viral RNA synthesizes an, additional RNA strand called "replicative RNA”. This RNA strand is complimentary to the viral, genome and serves as a template for producing new RNA strands which are copies of the parental, viral-RNA. The new viral RNAs are released from the nucleus into the cytoplasm and serve as, messenger-RNAs (mRNAS). Each mRNA, in cooperation with ribosomes and t-RNA of the host cell, directs the synthesis of protein sub-units (Capsomeres)., After the desired protein sub-units (capsomeres) have been produced the new viral nucleic acid is, considered to organize the protein subunits around it resulting in the formation of a complete Virus particle,, the virion. No lysis of the host cell, as seen in case of virulent bacteriophages, takes place. The host cells, remain alive and viruses move from one cell to the other causing systemic infection. When transmitted by, some means (by vectors or through wounds), the viruses infect other healthy plants., , BACTERIOPHAGES (BACTERIAL VIRUSES), The viruses that attack bacteria are called bacteriophages. The first clue regarding these Viruses was, given by Twort (1915) in England who observed that the bacterial colonies were lysed and this lytic affect, spread from one colony to the other within a short time. He speculated that the lysis of bacterial colonies, would have been due to a virus. de Herelle (1917) repeated Twort's experiments at Pasteur Institute (Paris),, rediscovered Twort's findings, named the phenomenon as Twort-de Herelle-phenomenon and coined the, term "bacteriophage (i.e. bacteria eater). Since then numerous bacteriophages have been discovered and they, are the most extensively studied

Page 6 : The bacteriophages are commonly called as phages, they possess dsDNA, ssDNA, dsRNA, ssRNA, as genetic material. Three common forms are known viz. tailed, cubic and filamentous bacteriophages. The, tailed bacteriophages form the largest group and are much studied, they have been named as T-phages, particularly T-even phages (T2, T4, T6 and so on) and T-odd phages (T1, T5, 15 and so on). However on the, basis of their interaction with the host bacterium, the phages are divided into two groups: virulent and, temperate phages. Virulent phages are those that normaly Iyses (destroy) the attacked host/ bacterial cells,, there is probably no other alternative strategy for their multiplication. This characteristic growth of virulent, bacteriophages is called lytic cycle (e.g. T-even phages). Contrary to the virulent strains, the temperate, phages adopt two alternative mode for their multiplication, (a) they may enter a lytic cycle and behave like, virulent phage or (b) they may integrated themselves into the bacterial chromosome thus resulting in a, 'lysogenic cycle', in the integrated (lysogenic) state the phage is describe as prophage (e.g. λ-phage)., T-EVEN BACTERIOPHAGES (dsDNA VIRULENT PHAGES), Structure:, , The T-even phages are characterized by the presence of a hexagonal head about, 900Å wide. It consists of dsDNA molecule protected by a protein coat made up of numerous facets. The, DNA molecule, measuring about 52,000 Å in length, is coiled and packed inside the head. The head is, attached with a cylindrical tail consisting of a hollow core surrounded by protein sheath. The hollow central, core measures about 80-100Å in diameter and is considered continuous from the head to the end of the tail, forming a channel through which the nucleic acid moves into invade the host cell being infected. The protein, sheath is spirally coiled and is connected to a thin disc like structure called „collar' at the base of the head, and to a hexagonal‟ end plate' at the end of the tail. The protein sheath of the tail is capable of contracting in, the longitudinal direction. At the six corners of the hexagonal plate there are small spikes to which long, fibres called "tail-fibres are connected. The tail fibres are organs of attachment to the wall of the host, bacterial cell., LAMBDA (λ) PHAGE (dsDNA TEMPERATE PHAGES), Structure, Lambda phage (λ) phage contain a double stranded (ds) circular DNA of about 17mm in length, packed in protein head of capsid. The head is icosahedral, 55nm in diameter consisting of 300-600, capsomeres (subunits) of 37,500 dalton molecular weight. Tne capsomeres are arranged in clusters of 5 and, 6 subunits i.e. pentamers and hexamers. The head is joined to a non-contractile 180mm long tail by a, connector. There is a hole in capsid through which passes this narrow neck portion expanding into knob like, structure inside. The tail possesses a thin tail-fibre (25nm long) at its end which recognizes the hosts. Also, the tail consists of about 35 stacked discs or annuli. Unlike T-even phage it is simple structure devoid of the, tail sheath.

Page 7 : Life-cycle of T-even bacteriophages( Lytic Cycle), The infection cycle of T-even bacteriophage lasts about 20minutes, culminating in lysis (bursting, open) of the host cell, E. coli. The whole process can be classified into, (i) adsorption or infection, (ii), penetration or injection, (iii) the eclipse or latent period, (iv) maturation, and (v) lysis or release., (i). Adsorption: The virus particle on coming into contact with the host bacterial cell get attached or, adsorbed unto the receptors on the bacterial cell wall with the help Virus proteins. These receptors, determined whether the cells are susceptible to infection. If the receptor site is altered, the host may become, resistant to virus infection., (ii) Penetration or injection: After the tail fibres get adsorbed, an enzyme system is supposed to make a, "pore or 'hole in the cell wall of the host. This enzyme-system is believed to constitute the phage-lysozyme, which is synthesized during the multiplication of the parent phage that causes the previous host to lyse. The, tail fibres that attached to the host cell bend and bring the end-plate in contact with the host cell. Now the, protein sheath of the tail longitudinally contracts pushing the central tubular core through the pore inside the, wall of the host cell and the phage DNA molecule is released or injected into the cytoplasm. After the DNA, is injected, the protein coat becomes of no use., (iii). The eclipse or latent period: When the phage DNA is released, it first make the host cell immune to, infection by genetically similar phage particle, Secondly, it immediately takes charge of the cell machinery, and suppresses all cellular activities such as synthesis of cellular DNA, RNA and Proteins etc. This is the, parasitism of a virus at the genetic level. This suppression is short lived and the cell machinery of protein, synthesis starts functioning under the control of viral-DNA in the place of the cellular-DNA. New, messenger-RNA molecules are synthesized very rapidly and a series of new enzymes, namely 'early proteins, is synthesized. Some of the early proteins are used as enzymes for the viral-DNA synthesis. The newly, synthesized viral DNA molecule direct the formation of new type of proteins namely the late proteins., Majority of the late proteins are viral coat proteins, whereas some are lysozyme. The viral coat proteins, constitute the sheath of the phage and the phage-lysozyme later help in the injection process., (IV)Maturation or assembly: Assembly of the various components to constitute a new phage particle, within the host cell is called maturation. Head and tail formation start separately; the protein components, aggregate around the DNA and form the head of the phage. End-plate is formed later. Hundreds (about 200), of new phage particles are produced from each bacterium by the time of lysis., (V). Lysis or release: After the production of new bacteriophages, the host bacterial cell bursts open and the, phage particles are released. Bursting open of the host bacterial cell is called lysis.

Page 8 : Lysogenic life cycle, The lambda (λ) is first adsorbed on the host wall surface at the receptor site and its DNA is injected, into the bacterial cytoplasm. But in this case, the viral-DNA instead of starting lytic cycle gets inserted into, the bacterial DNA and remains as prophage for many generations i.e. the virus remain in lysogenic state for, many bacterial generations (lysogenic cycle). However under certain conditions, the inserted viral-DNA, (prophage) may get dissociated from the bacterial DNA and start functioning as virulent phage culminating, in the lysis of the host cells (lytic cycle). Such conversion of the temperate phage (prophage) into virulent, phage is referred to as 'induction', which can be achieved artificially by exposing the infected bacterial cells, with UV-radiation or with H,O (hydrogen peroxide).