1. Viruses are the smallest infectious diseases (ranging from 20-300 nm) They are obligatory intracellular parasites without own metabolism (being parasites at the genetic level) Virus genomes consist of DNA or RNA Viruses are completely dependent on their host cells for machinery of energy production and synthesis of macromolecules. Virus progeny is assembled from newly synthesized components Defining properties of viruses

2. envelope (Lipids+Proteins) capsid (Protein coat) genome (DNA or RNA) -The entire infectious unit is termed a virion - Function of the outer shell (capsid, envelope): envelopment and protection of the Genome attachment to the host cell Components of a virion

3. (Spike) - Nucleocapsid: Pr-nucleic acid complex - Capsomers: Morphologic units, represents clusters of polypeptides - Matrix: protein layers in some viruses - Spikes: coded by virus

4. Helical virus structures –example: tobacco mosaic virus (TMV) Icosahedral symmetry Virus capsids are symmetric formations

5. Classification of viruses 1) By genome RNA viruses single strand double strand DNA viruses single strand double strand 2) Virion Physicochemical3) Virion morphology 4) Biological properties

6. General steps in viral replication cycles 1- Attachment, penetration and uncoating 2- Expression of viral genome and synthesis of viral components 3- Morphogenesis and release

7. Viral replication – basic principles Host cell genome replication Protein synthesis Virus release virus-assembly and maturation virus entry Uncoating Protein synthesis is directed exclusively by the cellular machinery

8. Attachment -The first step in infection of a cell is attachment to the cell surface, interaction of a virion which a specific receptor site on the surface on the cell. - Receptor molecules differ for different viruses but are generally glycoproteins. - Receptor binding is believed to reflect configurationally homologies between a virion surface and a cell surface component. - The presence or absence of receptors plays an important determining role in cell tropism and viral pathogenesis. Cell receptor Viral glycoproteins

9. Penetration After binding, particle taken up inside the cell. Enveloped viruses (A) Entry by fusing with the plasma membrane. Some enveloped viruses fuse directly with the plasma membrane. Thus, the internal components of the virion are immediately delivered to the cytoplasm of the cell (B) Entry via endosomes at the cell surface. Some enveloped viruses require an acid pH for fusion to occur and are unable to fuse directly with the plasma membrane. Non-enveloped viruses Non-enveloped viruses may cross the plasma membrane directly or may be taken up into endosomes. They then cross (or destroy) the endosomal membrane

10. Uncoating, Expression of viral genome and synthesis of viral components - Uncoating is the physical separation of the viral nucleic acid from capsid -Specific mRNAs must be transcribed from the viral nucleic acid for successful expression and duplication of genetic information. - Then, virus use cell components to translate the mRNA. Host cell genome replication Protein synthesis Uncoating

11. Morphogenesis and release -Newly synthesized viral genomes and capsid polypeptides assemble together to form progeny viruses. -Virus may be released due to cell lysis, or, if enveloped, may bud from the cell. Budding viruses and do not necessarily kill the cell. -Thus, some budding viruses may be able to set up persistent infections. Virus release virus-assembly and maturation

12. Reaction to physical and chemical agents - Heat and cold: Heating: Icosahedral and non-envelope viruses are more resistant than helical and enveloped viruses Destroy by heating at 56-60 ⁰ C in 30 minutes Losing a little activity at 37⁰C in 3-4 minutes Preserve at 4⁰C and less -Radiation: ionic (β, γ, X, …) and nonionic (UV) - Formaldehyde: Combination with (A, G, C) -Cationic, anionic (SDS: sodium dodecyle sulfate ) and nonionic (Triton X100) detergents - PH and ether susceptibility - Photodynamic Inactivation