INTRODUCTION TO VIRUSES

Why Study Viruses? Not so good side of viruses  Infect all life forms Useful for  Phage typing of Salmonella  Source of RT, RNA Pol  Baculo and Myxo virus as insecticide  Anti-bacterial and anti-cancer agents  Gene vectors for protein production and gene therapy

General Characteristics Of Viruses What is a virus? Ultramicroscopic, obligate intracellular parasite Alive inside the cell but inert in extracellular space Cannot generate energy for to carry out any biochemical processes Completely dependent on the host cell for its macromolecule synthesis

Why Are Viruses Unique? Virus particles are produced from the assembly of preformed components. A virus particle cannot grow or under go division. Lack genetic information for the generation of metabolic energy.

Pathogenic Agents Smaller than Viruses Viroids: Very small ( nm) circular RNA molecules but have no proteins. Virusoids: Satellite, viroids like molecules, depends on presence and replication of another virus. Prions: Infectious agents consists of a single type of protein with no nucleic acid components.

Viral Existence Virus exists in three ways 1. Extracellular virion: complete virus particle outside the cell 2. Vegetative Phage: Intracellular free NA, autonomous replication 3. Prophage: insert viral NA in bacterial chromosome; no autonomous replication, replication with host genome

Basic Virus Structure Capsid protein Nucleocapsid Naked capsid virus DNA RNA or = + Nucleocapsid Lipid membrane, glycoprotein Enveloped virus +

Basic Structure Of Viruses Virion- A complete infectious virus particle composed of NA and protein Nucleic acid- Either DNA or RNA, but never both. DNA – May be single stranded or ds, RNA - May be ss or ds, or segmented. Capsid- A protein coat that surrounds the NA Envelope- Outer shell consisting of lipid, proteins and carbohydrates

Capsid Symmetry IcosahedralHelical Naked capsid Enveloped Lipid Glycoprotein Matrix

Capsomeres Economy of genetic information Reduces the number of different proteins Genome has to encode if the viral coat is made up of repeating units of a single protein Allows for construction of the virus particles by a process of self assembly into structures held together by non-covalent bonds as occurs in the process of crystallization No need for enzyme-catalyzed reactions for coat assembly Intracellular release of the viral genome only requires dissociation of non-covalent bonds rather than degradation of a protein coat

Geometry of Capsomeres 1. Helical symmetry 2. Icosahedral symmetry 3. Complex structure

Virus Morphology 1. Helical symmetry- long rods, rigid or flexible 2. Icosahedral symmetry- polyhedron with 20 triangular faces and 12 corners. 3. Complex viruses- contain tail, tail fiber, plates and pins etc

Structures Compared

Icosahedral Naked Capsid Viruses Adenovirus Electron micrograph Foot and mouth disease virus Crystallographic model

Helical Naked Capsid Viruses Tobacco mosaic virus Electron micrograph Tobacco mosaic virus Model RNAProtein

Icosahedral Enveloped Viruses Herpes simplex virus Electron micrograph Herpes simplex virus Nucleocapsid cryoEM model

Helical Enveloped Viruses Influenza A virus Electron micrograph Paramyxovirus Electron micrograph

Homework What is Triangulation number?

Properties of Enveloped Viruses Envelope is sensitive to Drying Heat Detergents Acid Consequences Must stay wet during transmission Transmission in large droplets and secretions Cannot survive in the gastrointestinal tract Do not need to kill cells in order to spread May require both a humoral and a cellular immune response

Properties of Naked Capsid Viruses Capsid is resistant to Drying Heat Detergents Acids Proteases Consequences Can survive in the gastrointestinal tract Retain infectivity on drying Survive well on environmental surfaces Spread easily via fomites Must kill host cells for release of mature virus particles Humoral antibody response may be sufficient to neutralize infection

Chemical Composition of Viruses: Proteins Structural proteins:  Provide structural symmetry of the virus particles.  Transfer NA from one host to another.  Protect viral genome.  Participate attachment to the host cell.  Major antigen against which neutralizing Abs are produced.  Some proteins have special functions; e.g., HA of influenza virus- can agglutinate RBC.

Chemical Composition of Viruses: Proteins Non- structural proteins: - Some viruses carry enzymes within the virion- without which the virus cannot be viable. e.g. Retrovirus, influenza virus.

Chemical Composition of Viruses: NA Viral nucleic acid: DNA- ds or ss, circular or linear, size of DNA ranges from 3.2 kb to 375 kb. RNA- ss or ds, ‘+’ sense or ‘-’ sense; segmented or non-segmented; size of RNA ranges from 7 kb to 30 kb. ‘+’ sense RNA genome serve as mRNA ‘-’ sense RNA genome is non-infectious

Chemical Composition of Viruses: Other Viral lipid envelopes: Acquired during budding through cell membrane. Consisting of phospholipids, specificity of which depends on the type of membrane involve in budding Glycosylated proteins are exposed on the outer surface. Unglycosylated proteins are underneath the envelope.

Classification of Viruses Basis for classification A. Primary Characteristics 1. Chemical nature of NA 2. Structure of virion, helical or icosahedral, naked or enveloped. B. Secondary Characteristics 1. Host range 2. Mode of transmission, 3. Specific surface structure

Classification System ICTV- International Committee on Taxonomy of Viruses- authority for classification of viruses. Family Name- viridae; e.g. Retroviridae Sub family Name- virinae; e.g. Lentivirinae Genera- virus; e.g. Human immunodeficiency virus

Baltimore’s System Proposed by David Baltimore Classify all viruses into six groups Based on mode of gene expression mRNA is the ‘plus’ strand RNA Complementary to mRNA is considered as a ‘minus’ strand Strand have the same sequence as mRNA is also a ‘plus’ strand.

Baltimore’s System ss ‘+’ DNA Class-II a ss ‘–’ DNA Class- II b ± DNA Class- I m RNA ss ‘+’ RNA Class- VI ± RNA Class-III ss ‘-’ RNA Class- V ss ‘+’ RNA Class-IV

Reference Field’s Virology Flint’s Medical Microbiology