Virus Study (PDF)

THE VIRUSES  Introduction  Virus, Latin word meaning poison or venom  Viruses are simple, acellular entities.  Can reproduce only within living cells because they are obligate intracellular parasites.  Introduction  Most possess only one type o...

THE VIRUSES  Introduction  Virus, Latin word meaning poison or venom  Viruses are simple, acellular entities.  Can reproduce only within living cells because they are obligate intracellular parasites.  Introduction  Most possess only one type of nucleic acid, either DNA or RNA.  Virus is a unique group of infectious agents.  Viruses are quite different from prokaryotic and eukaryotic microorganisms; they are studied by virologists.  Introduction  A complete virus particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein.  Some viruses have additional layers that can be very complex and contain carbohydrates, lipids, and additional proteins.  Introduction  Viruses can exist in two phases: extracellular and intracellular.  In extracellular phase, possess few if any enzymes and cannot reproduce independent of living cells.  In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete virus particles or virions are released.  How viruses are different?  Viruses differ from living cells in at least three ways:  (1) their simple, acellular organization  (2) the presence of either DNA or RNA, but not both, in almost all virions  (3) their inability to reproduce independent of cells and carry out cell division as prokaryotes and eukaryotes do. Generalized structure of virus  HISTORY The great epidemics of A.D. 165 to 180 and A.D. 251 to 266, which severely weakened the Roman Empire and aided its decline, may have been caused by measles and smallpox viruses. Smallpox had an equally profound impact on the New World.  HISTORY Progress in preventing viral diseases began years before the discovery of viruses. Early in the eighteenth century, Lady Wortley Montagu, wife of the English ambassador to Turkey, observed that Turkish women inoculated their children against smallpox. Later in the century an English country doctor, Edward Jenner, began inoculating humans with material from cowpox lesions. He published the results of 23 successful vaccinations in 1798. Although Jenner did not understand the nature of smallpox, he did manage to successfully protect his patients from the dreaded disease through exposure to the cowpox virus.  HISTORY Louis Pasteur used the term virus for any living infectious disease agent. The development of the porcelain bacterial filter by Charles Chamberland in 1884 , one of Pasteur’s collaborators and inventor of the autoclave, made possible the discovery of what are now called viruses.  HISTORY STUDY OF TOBACCO MOSAIC DISEASE: In 1892 Russian Pathologist Dimitri Ivanowski study this disease. Tobacco mosaic disease was the first studied with Chamberland’s filter.  He published studies showing that leaf extracts from infected plants would induce tobacco mosaic disease even after filtration removed all bacteria. However, he attributed this to the presence of a toxin.  HISTORY Martinus Beijerinck, working independently of Ivanowski, published the results of extensive studies on tobacco mosaic disease in 1898 and 1900. The filtered sap of diseased plants was still infectious, he proposed that the disease was caused by an entity different from bacteria, what he called a filterable virus. He observed that the virus would multiply only in living plant cells, but could survive for long periods in a dried state.  HISTORY IDENTIFICATION OF HOOF AND MOUTH DISEASE VIRUS: In 1898 foot and mouth disease virus was identified. Friedrich Loeffler and Paul Frosch in Germany found that the hoof-and-mouth disease of cattle was also caused by a virus rather than by a toxin It shows that a virus could be transmitted among animals as well as plants.  HISTORY In 1900 Walter Reed began his study of the yellow fever disease whose incidence had been increasing in Cuba. Reed showed that this human disease was due to a virus that was transmitted by mosquitoes. Mosquito control soon reduced the severity of the yellow fever problem. Thus by the beginning of the 20th century, it had been established that viruses were different from bacteria and could cause diseases in plants, livestock, and humans. In1911, Peyton Rous reported that a virus, now known as the Rous sarcoma virus, was responsible for a malignant muscle tumor in chickens. These studies established that some malignancies are caused by viruses.  HISTORY IDENTIFICATION OF BACTERIOPHAGES: In 1915 Frederick Twort reported that bacteria also could be attacked by viruses. Twort isolated bacterial viruses that could attack and destroy micrococci and intestinal bacilli. In 1917 bacteriophages were identified by FRENCH Canadian Scientist FELIX D`HERELLE. He noted that when a virus suspension was spread on a layer of bacteria growing on agar, clear circular areas containing viruses and lysed cells developed. By the early 1930s viruses were assumed as living microorganism.  HISTORY In 1935 tobacco mosaic virus was crystallized. The chemical nature of viruses was established when Wendell Stanley announced in 1935 that he had crystallized the tobacco mosaic virus (TMV) and found it to be largely or completely protein. A short time later Frederick Bawden and Norman Pirie managed to separate the TMV virus particles into protein and nucleic acid. Thus by the late 1930s it was becoming clear that viruses are complexes of nucleic acids and proteins able to reproduce only in living cells In1940 polio virus was identified. In 1941 with the invention of electron microscope, virologists visualize viruses including TMV.  Size of viruses Virion ranges in size from about 10 to 400 nm in diameter. The smallest viruses are a little larger than ribosomes. Poxviruses are largest, about the same size as the smallest bacteria and can be seen in the light microscope. Most viruses, however, are too small to be visible in the light microscope and must be viewed with scanning and transmission electron microscopes. STRUCTURE OF VIRUSES Unique features of viruses They have no organelles, no cytoplasm and no nucleus. They are composed of two basic constituents i.e. i. core of nucleic acid ii. coat of proteins The genetic material of almost all viruses is composed of either DNA or RNA. It is mostly coiled or folded that helps in maintaining small size. Nucleic acid of virus is a linear or circular molecule. CAPSID: The protein coat of virus is called as “capsid” that provides. shape and symmetry. Capsid also provides protective layer for the viral genome because the construction of its amino acids resists temperature, pH and other environmental fluctuations. CAPSOMERE are protein subunits that form capsid. NUCLEOCAPSID: The capsid having genetic material is called as “nucleocapsid”.Some viruses consist only of a nucleocapsid. The nucleocapsid is composed of a nucleic acid, usually either DNA or RNA, held within a protein coat called the capsid, which protects viral genetic material and aids in its transfer between host cells. SPIKES: In some viruses capsid proteins are transformed into “spikes” that help in the attachment of virus to the host cell and facilitates penetration. VIRION: A completely assembled and infectious virus outside its host cell is known as virion.  ENVELOPED VIRUSES: NAKED VIRUSES: Viruses that are composed of The nucleo capsid of many viruses only nucleocapsids are known as are surrounded by a flexible “naked viruses”. membrane known as “ envelope”,Such viruses are referred as enveloped viruses COMPOSITION OF ENVELOPE: The envelope is composed of lipids and proteins as it is acquired from the host cell during replication Unique to each type of virus. The viruses can lose their infectivity if the envelope is destroyed or symmetry of capsid is changed.  Animal virus envelopes usually arise from host cell nuclear or plasma membranes. In contrast, envelope proteins are coded for by virus genes and may even project from the envelope surface as spikes, which are also called peplomers. SHAPES OF VIRUSES Viruses are grouped according to their nucleocapsid symmetry.. There are three main shapes in which viruses are arranged: 1. HELICAL VIRUSES 2. ICOSAHEDRAL VIRUSES 3. COMPLEX VIRUSES 1) HELICAL SHAPED VIRUSES:  Viruses that exist in helix shape are said “helical shaped” viruses.  The helix is a tightly wound coil resembling a corkscrew or spring. EXAMPLE: Rabies and tobacco mosaic virus (TMV). 2. Icosahedral viruses:  (Icos=20, edros=sides) viruses having this type of shape have 20 triangular faces with 12 corners.  EXAMPLE: Herpes simplex and polio virus 3.Complex viruses: A few viruses have a combination of helical and icosahedral symmetry, a construction described as complex symmetry. EXAMPLE: Bacteriophages & small pox 3.Complex viruses: Bacteriophages, for example, have an icosahedral head with a collar and tail assembly in the shape of a helical sheath. Poxviruses, by contrast, are largest viruses with a brick like shape, an envelope with an unusual appearance and a complex capsid symmetry. HOST RANGE OF VIRUSES  Host refers to the organisms, the virus can infect and it is also based on virus’ capsid structure. Viruses can infect almost any cellular organism. Most of the viruses have narrow host range e.g. specific bacteriophage that effects narrow range of bacterial species. Small pox virus can effect only human beings. Polio virus can also effect only human beings and their primates. Rabies viruses have broad host range as these can infect humans and most warm blooded animals.  TISSUE TROPISM Many viruses only infect certain cell type or tissue within a multicellular plant or animal, even within its host range this term is known as tissue attraction or tropism. EXAMPLE: Host range for the human immunodeficiency virus (HIV) is a human. HIV viruses affect the T helper cells of WBC group because spikes are attached to these cells. Rabies virus affects the nervous system because its envelope just recognize the proteins of this system. Therefore, a virus’ host range and tissue tropism are linked to infectivity. CLASSIFICATION OF VIRUSES  Viral nomenclature  The measles virus and poxviruses, for example, are named after the disease they cause.  The Ebola and Marburg viruses after the location from which they were originally isolated.  Epstein-Barr virus after the researchers who studied it.  Others are named after morphologic factors—the coronaviruses (corona = “crown”) have a crown-like capsid.  Picornaviruses (pico = “small”; rna = “ribonucleic acid”) are very small viruses with an RNA genome.  CLASSIFICATION OF VIRUSES  Based on chemical and morphological criteria.  Two major components used in classification are 1. Nucleic acid (molecular weight and structure) 2. Capsid (size, symmetry, enveloped or not)  Viruses can be classified according to their genome and strand type:  DNA viruses  RNA viruses  DNA viruses  They may have single or double stranded genomes and that may be linear or segmented.  DNA polymerase replicates the genome by making DNA to DNA copies.  Most of the viruses replicate in host cell nucleus except poxvirus which replicates in cytoplasm, it may be due to ability to carry its own DNA polymerase gene.  Examples of DNA viruses  Parvoviruses  Hepadnaviruses  Polyomaviruses  Herpes viruses  Papillomaviruses  Poxviruses  Adenoviruses Classification of DNA viruses  RNA VIRUSES A large number of viruses contain either ssRNA or dsRNA genomes, which are replicated by direct RNA-to-RNA copying. The genomes can be linear or segmented Some single stranded viruses such as picornaviruses and coronaviruses have their genome in the form of mRNA and such RNA viruses are termed as +ve stranded viruses. ssRNA viruses that consists of complimentary strands for mRNA are called as –ve stranded. For example, orthomyxoviruses and paramyxoviruses.  RNA viruses like retroviruses are replicated indirectly through a DNA intermediate (RNA-to-DNA-to-RNA).Each virion contains two copies of + strand RNA. During the infection process, a DNA intermediate will be formed using a reverse transcriptase enzyme carried within the virion. As a rule, RNA virus genomes are smaller than DNA virus genomes and depend more heavily on host cell proteins and enzymes for replication.  Examples of RNA viruses  Picornaviruses (smallest  Paramyxoviruses RNA viruses)  Rhabdoviruses  Hepeviruses  Filoviruses  Caliciviruses  Coronaviruses  Reoviruses  Arenaviruses  Flaviviruses  Bunyaviruses  Togaviruses  Deltaviruses  Retroviruses  Orthomyxoviruses Classification of RNA viruses VIRAL REPLICATION  Introduction  A virus invades a living host cell a thousand or more times its size  Hijacks the metabolism of the cell to produce copies of itself  Often destroys the host cell when new virions are released.  Bacteriophages undergo a lytic or lysogenic cycle of infection. Viral replication includes following steps: 1. Host cell invasion 2. Hijacking the metabolism 3. Production of genome copies 4. Destruction of host cells  REPLICATION OF BACTERIOPHAGES Bacteriophages T2,T4, and T6 are in this group. They are large, complex, naked DNA virions with the characteristic head and tail of bacteriophages They contain tail fibers, which function similar to spikes on animal viruses and identify what bacterial species the phage will be able to infect.  The T-even phages are virulent viruses, meaning they lyse the host cell while carrying out a lytic cycle of infection. Why viruses invade other cells? The nucleic acid in a phage contains only a few of the many genes needed for viral synthesis and replication. It contains genes for synthesizing viral structural components, such as capsid proteins, and for a few enzymes used in the synthesis But it lacks the genes for many other key enzymes, such as for nucleic acid synthesis. Therefore, its dependence on the host cell is substantial. LYTIC CYCLE OF BACTERIOPHAGE Lytic cycle of bacteriophages in E.Coli consists of following steps: 1. Attachment 2. Penetration 3. Biosynthesis 4. Maturation 5. Release  ATTACHMENT It includes following steps: 1. Random collision of virulent phages with bacterial cells. 2. Attachment of phage’s tail fiber with complimentary receptor site on cell wall of bacterium. 3. The attachment consists of weak chemical union between phage and receptor site of bacterium. 4. Sometimes bacterium flagellum or pilus act as receptor site.  PENETRATION 1. After attachment, the tail of phage release the lysozyme that dissolves the attached portion of cell wall. 2. Tail core drives through the cell wall after the contraction of tail sheath. 3. DNA is ejected through the hollow tail core from the cell membrane into the cell cytoplasm of bacteria. 4. This process takes less than two seconds and capsid remains outside.  BIOSYNTHESIS After entering into cytoplasm phage DNA uses bacterial nucleotide and enzymes to produce multiple copies of genome and capsid. mRNA molecules of phage DNA appears in the cytoplasm and biosynthesises the phage enzymes along with capsid proteins and amino acids.  MATURATION Once the phage parts are made, they are assembled into complete virus particles. The enzymes encoded by the viral genes guides the assembly in step by step certain fashion. In one area of the host cytoplasm, phage heads and tails are assembled from protein subunits; in another area, the heads are packaged with DNA; and in a third area, the tails are attached to the heads.  RELEASE Mature phage particles burst out from the ruptured bacterial cell. For some phages, lysozyme, encoded by the bacteriophage genes late in the replicative cycle, degrades the bacterial cell wall. The mature bacteriophages are set free to infect more bacterial cells.  Lysogenic cycle TEMPERATE PHAGES:  Some viruses undergo lysogenic cycle in which viruses do not cause cell lysis and viral DNA interacts with the bacterial chromosome as a prophage. Such bacteriophages are called as temperate phages.  For example, lambda (λ) phage also infects E. coli but may not immediately cause cell lysis.  Lysogenic cycle  As the bacterial cell undergoes DNA replication and binary fission, the prophage is copied and vertically transferred to daughter cells as part of the replicated bacterial chromosome. Thus, as cells divide, each daughter cell is “infected”; that is, it contains the viral genome as a prophage.  Such binary fissions can continue for an undefined period of time. Usually at some point, the bacterial cells become stressed (e.g., lack of nutrients, presence of noxious chemicals). This triggers the prophage to excise itself from the bacterial chromosomes and switch to a lytic cycle, lysing the bacterial cells as new λ phage are released. ANIMAL VIRUS REPLICATION Like bacteriophages, animal viruses cause more damage to the animal cell as they cause infection. For productive infection, virus has to undergo same stages of replication like bacteriophages but there are some differences in this process. Attachment: In animal viruses, there are spikes that are distributed over the capsid and are responsible for the determination of host cell range e.g. HIV & adenovirus.  PENETRATION Some virus like HIV and adenoviruses requires a second receptor known as co-receptor that help the virus to penetrate into the cytoplasm. Unlike bacteriophages, in animal virus not only the DNA is entered into the cytoplasm but the whole nucleocapsid can taken up into the cytoplasm. For viruses like HIV, the viral envelope fuses with the plasma membrane and releases the nucleocapsid into the cytoplasm. Some viruses enters into the cell by endocytosis. The capsid disassembles from the genome in a process called uncoating and the genome is transported to the site where transcription or replication will occur. BIOSYNTHESIS & MATURATION DNA viruses provides the genetic code for enzymes that synthesize viral parts. Although the poxviruses replicate entirely in the host cell cytoplasm. Most of the DNA viruses employ a division of labor: DNA genomes are synthesized in the host cell nucleus, and capsid proteins are produced in the cytoplasm The proteins are then transported to the nucleus and join with the nucleic acid molecules for maturation. Adenoviruses and herpesviruses follow this pattern. RNA viruses show different pattern of biosynthesis and maturing. +ssRNA viruses act as mRNA, following uncoating, RNA immediately supplies codes for protein synthesis. -ssRNA as influenza virus, use their RNA as a template to synthesize a complimentary +ve strand of RNA with the help of RNA dependant polymerase which is present in the virus. This synthesized +ssRNA is used as a mRNA for protein synthesis as well as to form –ssRNA genome. In the final step of maturation envelop proteins are synthesized in the form of spikes and depending upon the virus it is incorporated into nuclear, or cytoplasmic membrane.  RELEASE In the final stage, enveloped viruses either: 1. Push through the plasma membrane, forcing a portion of the membrane ahead of and around the virion, resulting in an envelope. Or 2. A membrane enclosed virus fuses with the plasma membrane and releasing the virion. This is known as budding e.g. herpes virus. This process need not necessarily kill the cell during release But with the case of naked viruses they leave the cell membrane when it is ruptured and cell death occurs.  VIRUSES ALSO PRODUCE LATENT INFECTION Most of the RNA viruses cause productive infection. But many DNA and retroviruses can cause a “latent infection” which is characterized by the suppression of a viral genes i.e. the viral gene remains dormant.  For example, some herpesviruses, such as herpes simplex virus-1 (HSV- 1), can generate a productive or latent infection.  No viral parts are produced for months or even years until some stress events “reactivates” the viral dsDNA and a new productive infection will be started. In an infected sensory neuron, HSV-1 undergoes latency as the viral dsDNA enters the neuron’s cell nucleus and circularizes and becomes integrated into the DNA of one chromosome. This integrated viral genome is referred to as a provirus and represents a unique and stable association between the viral DNA and the host genome. The “proviruses” will be divided everytime with cell division so it will be replicated along with host genome and will be present in all progeny cells. As a provirus and as a part a host genome it is protected from the attack of antiretroviral drugs. At any time suitable environment is available and these proviruses are reactivated to produce a latent infection. DETECTION OF VIRUSES  Introduction A prompt identification often is necessary for selecting possible antiviral therapy. Diagnosis of viral diseases like cold and flu usually does not require any lab confirmations. In some cases where viral infection leave their identification mark on infected individuals also don’t require laboratory evaluations e.g. measles that is characterized by the “Koplik spots”  Introduction KOPLIK SPOTS: Bright red patches with white pimple like centers on the lateral mouth surfaces. Measles is accompanied by Koplik spots.  Swollen salivary glands are associated with mumps.  Teardrop-like skin lesions with chickenpox infection. Koplik spots Teardrop-like skin lesions  Virus detection is not straightforward and the proper identification is needed on relating a particular virus to a particular disease. There are certain diseases that require proper identification and detection of virus for specific disease. CYTOPATHIC EFFECT: Clinical laboratory diagnosis of viral disease is carried out by using light microscopy for examining the cell obtained from infected body tissue or fluid. When viruses replicate in host cells, often a noticeable deterioration or structural change occurs. This is called a cytopathic effect (CPE). Viruses often cause changes in cell structure e.g. the condition “infectious mononucleosis “ is characterized by a large number of lymphocytes with a “foamy look” highly vacuolated cytoplasm.  SYNCYTIA:  Paramyxoviruses cause host cell to fuse together into multinucleate giant cell called as “Syncytia”. NEGRI BODIES: Viruses sometime also produce cell inclusions (usually lifeless, often temporary, constituent in the cytoplasm of a cell) e.g. the brain tissue of a rabid animal contain a cytoplasmic nucleoprotein inclusion that’s called as “Negri bodies” Cells from the herpes infected patient contain nuclear granules.  Sometime viral identification is carried out by the presence of viral antibodies in the patient’s serum. Sometimes viruses are compared by directly observing the structure of unknown viruses from electron microscope and comparing it with known virus.  RIVERS POSTULATE:  Viral cultivation do not follow the koch’s postulates for their growth.  In 1937 Thomas M. Rivers expanded Koch’s postulates to include viruses.  He proposed that filtrates of the infectious material isolated from the diseased host shown not to contain bacterial or other cultivatable organisms must produce the same disease as found in the original host; or, the filtrates must produce specific antibodies in appropriate animals. CULTIVATION OF VIRUSES  INOCULATION OF FERTILIZED EGG The most common method to cultivate viruses is to inoculate them into the fertilized chicken egg by drilling a hole into the egg shell and introducing the viral suspension into the egg. But now a days this method is only used for the preparation of influenza virus vaccines. CELL CULTURES: The most common method for cultivating and detecting viruses is to infect cell cultures. To prepare the culture, animal cells are separated from a tissue with enzymes and suspended in a solution of nutrients, growth factors, pH buffers, and salts, such cultures are termed as “Primary cultures”. Cells from such cultures adhere to the bottom of dish and reproduce in the form of a single layer called a monolayer. CELL LINE: Different cells from the primary cell culture are isolated enzymatically and such isolated single type of cell is called as “ cell line”. Cell line is a cell culture developed from a single cell and therefore consisting of cells with a uniform genetic makeup. The type of cell culture used will depend on the virus species to be cultivated in the monolayer. Viruses are then introduced into the culture.  PLAQUE: Viral plaque is a visible structure formed by virus propagation within a cell culture. “Plaques” formation on the monolayer by the viruses is used to detect them. A plaque is a clear zone within the cloudy “lawn” of bacterial cells or monolayer of animal cells.  Plaque The viruses infect and replicate in the cells, thereby destroying them and forming plaques.  PHAGE TYPING: The viruses replicates and infect the cells and produce plaque. This type of identification is called as “Phage typing”. Phage typing is a method that uses bacteriophages for detecting and identifying single strain of bacteria. Therefore Viruses can be “grown” in various types of tissue culture and detected by the formation of plaques. CANCER AND VIRUSES  What is a cancer? Cancer = “crab”; a reference to the radiating spread of cells, which resembles a crab. Every year more than 7 million people die due to cancer, worldwide. In USA death count is 557000 anuually. Cancer is uncontrolled mitosis of a single cell. The cell that escapes the cell cycle controlling factors and forms a cluster of cells. This cluster yields a clone of abnormal cells i.e. referred to tumor.  What is a cancer? Tumors are the result of uncontrolled cell divisions. Body responds to a tumor by surrounding it by a capsule of connective tissue. Such tumors are designated as “benign tumors” and these are not life threatening normally. Additional changes can occur to the tumor cells that release them from the specific boundaries. Tumor cells bound less firmly than normal cells and fail to stop dividing when come in contact with one another. They may break out of the capsule and metastasize, a spreading of the cells to other tissues of the body. Such a tumor now is described as malignant. There are a series of changes that are responsible for the conversion of healthy cell into tumor or cancer cells.  How can such a mass of cells bring illness and misery to the body?  By their sheer numbers, cancer cells invade and erode local tissues,  Interrupt normal functions, and choke organs to death by robbing them of vital nutrients.  Therefore, cancer patient will commonly experience weight loss even while maintaining a normal diet.  VIRUSES ASSOCIATION WITH HUMAN TUMORS WHO estimates that 60-90% of human cancers are related to carcinogens that may be physical and chemical agents and cause cellular changes. About 20% of all human tumors are associated with viruses. Commonly known chemical carcinogens are hydrocarbons found in cigarette smoke, asbestos, nickel, some pesticides ,dyes and environmental pollutants. Physical agents include UV light and X-rays.  Oncogenic viruses In 1911 “ Peyton Rous” demonstrated that sarcoma* was caused in chicken by the virus. A number of viruses are isolated from the human cancers and when these viruses are transferred to other animals and cell cultures, an observable transformation of normal cells to tumor cells takes place. Epstein-Barr virus causes the Burkitt lymphoma, a tumour of jaws.  Oncogenic viruses Cervical cancer is the second most common cancer in the women under age 35, some human papilloma viruses (HPV) and their sub types are related with this cancer. In one study, 71% of the women developing cervical cancer had HPV present in their PAP smear.* Fortunately, vaccine called “Gardasil” provides almost 100% protection against two most common HPV strains that cause 70% cervical cancers and two other strains that are responsible for 90% genital warts.  Oncogenic viruses transform infected cells into cancerous cells Cancer usually involves many cell changes and oncogenic viruses play an important role in some of these steps that disable the control of normal cell growth. For example in case of Hepatitis B & C long term inflammation of liver due to virus can eventually leads to tumour formation. Some viruses have some genes whose protein products disturbs the cellular growth control. When HPV infect a cell, abnormal growth starts, because HPV produce such proteins that prevent the infected cell from committing suicide, leading to cancer.  Some oncogenic viruses carry “oncogenes” that has activity to transform a cell. In 1969 researchers suggested that oncogenes normally reside in the DNA of the cell. In late 1970’s it was discovered that oncogenes exist in certain viruses. Later it was found that this gene is a part of every living cell. This gives genetic basis of cancer. It appeared that the oncogenes were not viral in origin but part of the genetic endowment of every living cell. PROTO-ONCOGENES: Researchers proposed that normal genes called “proto- oncogenes” are the precursor of oncogenes. Proto-oncogenes may have important role as regulator of growth and mitosis. Proto-oncogenes also play important role in cell metabolism as growth regulators. Proto-oncogenes can be converted to oncogenes by radiations, chemical carcinogens, chromosomal breakage or virus.  How viruses trigger the transformation of proto-oncogenes? Viruses such as retroviruses when enter into the cell they become a provirus. 1. The integration of pro-viruses will be adjacent to a proto-oncogene, that alters the normal growth control and tumor may result. 2. In addition,when viral replication is triggered, provirus not only replicates its own DNA but also few neighbouring host genes. Thus, when new viruses are produced, the viral DNA contains the proto-oncogenes as a part of its genome. Such captured proto-oncogenes are known as “V-oncogenes”.  V-oncogenes These v-oncogenes are under the control of virus not the host cell. Now these proto-oncogenes or V-oncogenes can influence the cellular growth and mitosis. V-oncogenes may provide genetic sequence for the growth factors that stimulate uncontrolled cell development and reproduction.  POWER OF VIRUSES Treatment of genetic diseases and many forms of cancer and other diseases with viruses is called as “virotherapy”. as viruses are the ideal cellular killers they kill the infected cells. One potential way is to use viruses against cancer. Some cancer killing viruses are being developed in which some causes cancer cell to commit suicide while some make alert the immune system to cancer danger. Viral gene therapy back draws. In 2002, French scientist used virus inserted gene to cure four young boys suffering from non-functional immune system, the therapy worked but virus inserted gene disturbed other cellular function and leukemia develops in two boys. So the major problem with virotherapy is immune system response as it attacks on the viruses. This may destroy the virus before reaching the target. To overcome this situation researchers have developed “vaccinia virus and adenovirus coated with an extracellular envelop” that makes the cell invisible to immune system and making able to reach the target.  VIRUS LIKE AGENTS When viruses were discovered, scientists believed they were the ultimate infectious particles. It was difficult to conceive of anything smaller than viruses as agents of disease in plants, animals,and humans. However, the perception was revisedas scientists discovered new disease agents—the subviral particles referred to as virus-like agents.  VIROIDS Viroids are tiny fragments of nucleic acid known to cause diseases in crop plants.  In the 1960s,Theodore O. Diener and colleagues investigating a suspected viral disease, potato spindle tuber (PST), which results in long pointed potatoes, shaped like spindles. Nothing would destroy the disease agent except an RNA degrading enzyme. In 1971,the group postulated that a fragment of single stranded RNA was involved. Diener called the agent a viroid meaning “virus-like.” Now more than 25 viroids have been identified and largest among these viroids is about one-fifteenth of the size of smallest virus.  PRIONS Prions are infectious proteins and they lack nucleic acid. In 1986 a mysterious disease “mad cow disease” was spread in cattle , that was responsible for weight loss, aggressiveness, lack of co-ordination and eventual death. In 1990 several people die due to a brain disorder, symptoms of which resemble with mad cow disease.  Symptoms included dementia, weakened muscles, and loss of balance. It appeared the disease agent was transmitted from cattle to humans.  In 1980s, Stanley Prusiner and colleagues isolated an unusual protein from scrapie-infected tissue, which they thought represented the infectious agent. Prusiner called the proteinaceous infectious particle as prion.  The sequencing of the protein led to the identification of the coding gene, called PrP. The PrP gene is primarily expressed in the brain. This led Prusiner and colleagues to propose the protein-only hypothesis, which predicts that prions are composed solely of protein and contain no nucleic acid. The protein-only hypothesis further proposes there are two types of prion proteins. Normal cellular prions (PrPC) are found on the surfaces of brain cells while abnormal, misfolded prions, as found in scrapie (PrPSC), have a different shape  Besides mad cow disease, similar neurologic degenerative diseases have been discovered and studied in other animals and humans. These include scrapie in sheep and goats, wasting disease in elk and deer, and Creutzfeldt-Jakob disease in humans. All are examples of a group of rare diseases called transmissible spongiform encephalopathies (TSEs) because, like mad cow disease, they can be transmitted to other animals of the same species and possibly to other animal species, including humans, and the disease causes the formation of “ sponge-like” holes in brain tissue.  References  Alcamo, fundamentals of Microbiology, 9th edition.  Prescott, Harley, Microbiology, 15th edition.