I NTRODUCTION TO VIRUSES DR NAZIA KHAN ASSISTANT PROFESSOR COLLEGE OF MEDICINE

1. Describe a virus. How is a virus different from viroid and prion 2. Define components of a virus 3. Discuss the classification of virus 4. Discuss briefly the effect of physical and chemical agents on virus with clinical correlation 5. Give an overview of replication cycle of virus and its clinical implication

Viruses are the smallest infectious agents (20 nm to about 300 nm in diameter) contain only one kind of nucleic acid (RNA or DNA) as their genome never both The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-containing membrane. obligate intracellular parasites. cause large number of diseases such as common cold to rabies,AIDS The entire infectious unit is termed a virion. Viruses are inert in the extracellular environment; they replicate only in living cells, being parasites at the genetic level.

A RE VIRUSES BACTERIA VIROIDS AND PRIONS THE SAME ?????

 VIROIDS Small infectious agents that cause diseases of plants. Viroids are agents that do not fit the definition of classic viruses. Viroids replicate by an entirely novel mechanism. viroids have been detected only in plants; none have been demonstrated to exist in animals or humans.  PRIONS Infectious particles composed solely of protein with no detectable nucleic acid. Highly resistant to inactivation by heat, formaldehyde, and ultraviolet light that inactivate viruses. The prion protein is encoded by a single cellular gene. Prion diseases, called "transmissible spongiform encephalopathies," include scrapie in sheep, mad cow disease in cattle, and kuru and Creutzfeldt-Jakob disease in humans. Prions do not appear to be viruses

C OMPONENTS OF A VIRUS Capsid: The protein shell, or coat, that encloses the nucleic acid genome. Capsomere: Morphologic units seen in the electron microscope on the surface of icosahedral virus particles Defective virus: A virus particle that is functionally deficient in some aspect of replication. Envelope: A lipid-containing membrane that surrounds some virus particles Nucleocapsid:The protein-nucleic acid complex representing the packaged form of the viral genome. Structural units: The basic protein building blocks of the coat. They are usually a collection of more than one nonidentical protein subunit. The structural unit is often referred to as a protomer. Subunit: A single folded viral polypeptide chain. Virion: The complete virus particle. In some instances (eg, papillomaviruses, picornaviruses), the virion is identical with the nucleocapsid. In more complex virions (herpesviruses, orthomyxoviruses), this includes the nucleocapsid plus a surrounding envelope. This structure, the virion, serves to transfer the viral nucleic acid from one cell to another.

C LASSIFICATION OF VIRUSES : BASED ON ……. 1. Virion morphology, including size, shape, type of symmetry, presence or absence of peplomers, and presence or absence of membranes. 2..

2. Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome in kilobases (kb) or kilobase pairs (kbp), strandedness (single or double), whether linear or circular, sense (positive, negative, ambisense), segments (number, size), nucleotide sequence, 3. Physicochemical properties of the virion, including molecular mass, buoyant density, pH stability, thermal stability, and susceptibility to physical and chemical agents, especially ether and detergents

4. Virus protein properties, including number, size, and functional activities of structural and nonstructural proteins, amino acid sequence, modifications, and special functional activities (transcriptase, reverse transcriptase, neuraminidase, fusion activities). 5. Genome organization and replication, including gene order, number and position of open reading frames, strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release). 6. Antigenic properties. 7. Biologic properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms, and pathology.

E FFECT OF PHYSICAL AND CHEMICAL AGENTS ON VIRUS 1. Heat & Cold: Icosahedral viruses tend to be stable, losing little infectivity after several hours at 37°C Enveloped viruses are much more heat-labile, rapidly dropping in titer at 37 °C. Viral infectivity is generally destroyed by heating at 50–60 °C for 30 minutes, Viruses can be preserved by storage at subfreezing temperatures, and some may withstand lyophilization and can thus be preserved in the dry state at 4 °C or even at room temperature. Viruses that withstand lyophilization are more heat-resistant when heated in the dry state. Enveloped viruses tend to lose infectivity after prolonged storage even at -90 °C and are particularly sensitive to repeated freezing and thawing. 2. Stabilization of Viruses by Salts: Many viruses can be stabilized by salts like MgCl 2 The stability of viruses is important in the preparation of vaccines. The ordinary nonstabilized oral poliovaccine must be stored at freezing temperatures to preserve its potency. However, with the addition of salts for stabilization of the virus, potency can be maintained for weeks at ambient temperatures even in the high temperatures of the tropics.

3. pH: Viruses are usually stable between pH values of 5.0 and 9.0. Some viruses (eg, enteroviruses) are resistant to acidic conditions. All viruses are destroyed by alkaline conditions. 4. Radiation: Ultraviolet, x-ray, and high-energy particles inactivate viruses. Infectivity is the most radiosensitive property because replication requires expression of the entire genetic contents. Irradiated particles that are unable to replicate may still be able to express some specific functions in host cells. 5. Photodynamic Inactivation Viruses are penetrable to a varying degree by vital dyes such as toluidine blue, neutral red, and proflavine. These dyes bind to the viral nucleic acid, and the virus then becomes susceptible to inactivation by visible light. 6. Ether Susceptibility Ether susceptibility can be used to distinguish viruses that possess an envelope from those that do not

7. Detergents: Nonionic solubilize lipid constituents of viral membranes. Anionic detergents solubilize viral envelopes; in addition, they disrupt capsids into separated polypeptides. 8. Formaldehyde: Formaldehyde destroys viral infectivity by reacting with nucleic acid. Viruses with single-stranded genomes are inactivated much more readily than those with double-stranded genomes. Formaldehyde has minimal adverse effects on the antigenicity of proteins and therefore has been used frequently in the production of inactivated viral vaccines. 9. Antibiotics & Other Antibacterial Agents: Antibacterial antibiotics and sulfonamides have no effect on viruses.

R EPLICATION OF V IRUSES Viruses multiply only in living cells. The host cell must provide the energy and synthetic machinery In order for a virus to replicate, viral proteins must be synthesized by the host cell protein-synthesizing machinery. Therefore, the virus genome must be able to produce a usable mRNA. The unique feature of viral multiplication is that, soon after interaction with a host cell, the infecting virion is disrupted and its measurable infectivity is lost. After the synthesis of viral nucleic acid and viral proteins, the components assemble to form new infectious virions. The duration of the virus replication cycle also varies widely, from 6–8 hours (picornaviruses) to more than 40 hours (some herpesviruses).

G ENERAL S TEPS IN V IRAL R EPLICATION C YCLES 1. Attachment, and Uncoating interaction of a virion with a specific receptor site on the surface of a cell. Receptor molecules differ for different viruses but are generally glycoproteins Ex: HIV binds to the CD4 receptor on cells of the immune system 2. Penetration The virus particle is taken up inside the cell. This step is referred to as penetration or engulfment. In some systems, this is accomplished by receptor- mediated endocytosis, with uptake of the ingested virus particles within endosomes. 3. Uncoating : occurs concomitantly with or shortly after penetration. Uncoating is the physical separation of the viral nucleic acid from the outer structural components of the virion so that it can function 4. Expression of Viral Genomes and Synthesis of Viral Components : specific mRNAs must be transcribed from the viral nucleic acid for successful expression and duplication of genetic information 5. Morphogenesis and Release

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