Medical Virology Lecture 1 PDF

Virology Lecture(( 1)) Dr.Emad Abul-Eis Medical virology : Virology is the study of viruses and virus-like agents, including, but not limited to, their taxonomy, disease-producing properties, cultivation, and genetics Virus A virus is an infe...

Virology Lecture(( 1)) Dr.Emad Abul-Eis Medical virology : Virology is the study of viruses and virus-like agents, including, but not limited to, their taxonomy, disease-producing properties, cultivation, and genetics Virus A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. A virus cannot replicate alone; instead, it must infect cells and use components of the host cell to make copies of itself. Characteristics of a Virus:  They reproduce at a spectacular rate, but only in live host cells.  They can be transformed.  They are acellular, i.e., they have no cytoplasm or cellular organelles.  They do not conduct any metabolism on their own and must replicate using the metabolic machinery of the host cell. Virion: is the physical particle in the extra-cellular phase which is able to spread to new host cells; complete intact virus particle. The whole virus particle is called (Virion). General Properties of Viruses 1. Viruses are smaller than bacteria, they range in size between 20-300 nanometer(nm). 2. Viruses contain only one type of nucleic acid, either DNA or RNA, but never both. 3. Viruses consist of nucleic acid surrounded by a protein coat(capsid). Some viruses have additional lipoprotein envelope. 4. Viruses lack cellular organelles, such as mitochondria and ribosomes. 5. Viruses are obligate cellular parasites. They replicate only inside living cells. 6. Viruses replicate through replication of their nucleic acid and synthesis of the viral protein. 7. Viruses do not multiply in chemically defined media. 8. Viruses do not undergo binary fission. 1 Comparison between viruses and bacteria : No property Viruses Bacteria 1 Size 20-300 nm 1000nm 2 Genome DNA or RNA but not DNA and RNA (type of nucleic acid) both 3 Cell wall Envelope present in Cell wall some viruses 4 Ribosomes No Ribosomes Ribosomes 5 Multiplication by binary - + fission 6 Sensitivity to antibiotics - + 7 Growth in culture Growth only in the Grow in chemical culture media living host cell media The structure of viruses: 1-Viral nucleic acid: it is genetic materials responsible for virulence The viral nucleic acid is located internally and can be either single or double- stranded RNA or DNA. The nucleic acid can be either linear or circular. The DNA is always a single molecule, the RNA can exist either as a single molecule or in several pieces (segmented). Some RNA viruses are positive polarity and others are negative polarity. Positive polarity is defined as an RNA with same base sequence as the mRNA. (positive strand RNA) Negative polarity has a base sequence that is complementary to the mRNA. (Negative strand RNA). 2- Capsid : consist of many small protein units (Capsomeres ) The protein shell, or coat, that encloses the nucleic acid genome, the function: A.) Protection of N.A from environment B.) Organ of attachment of the virus to specific receptors on the host cell surface in non-envelope virus ( naked virus). C.) Responsible for viral symmetry as shown below figure (3). 2 3- Capsomeres : Morphologic units seen in electron microscope. Each capsomere, consisting of one or several proteins. Naked viruses are composed of nucleic acid + capsid (nucleocapsid). ( Figure -1-) Figure(1)Naked virus composition 4- Viral Envelope: The envelope is a lipoprotein membrane composed of lipid derived from the host cell membrane during release by budding and protein that is virus- specific. Furthermore, there are frequently glycoproteins in form of spike-like projections on the surface, which attach to host cell receptors. Envelope sensitive to lipid solvent ,detergent Matrix protein mediates the interaction between the capsid proteins and enveloped The presence of an envelope confers instability on the virus. Nucleic acid +capsid + envelope = enveloped Viruses (Figure (2). Figure(2) illustrate the difference between Enveloped virus and Naked virus. 3  Types of symmetry of virus particles: Viruses are divided into three groups, based on the morphology of the nucleocapsid and the arrangement of capsomeres. 1- Icosahedral (Cubic) symmetry Composed of 12 corner, has 20 faces (each an equilateral triangle) with the approximate outline of a sphere. All DNS except pox virus and some of RNA Virus. 2. Helical symmetry The virus particle is elongated or pleomorphic (not spherical), and the nucleic acid is spiral. Caposomeres are arranged round the nucleic acid. Most of RNA viruses e.g. Rabies virus. 3. Complex structures = difference The virus particle does not confirm either cubic or helical symmetry e.g. Poxviruses. Icosahedral Helical Complex Pox (brick shape) Bacteriophage Figure(3) Types of symmetry of virus particles. Reaction to physical and chemical agents: 1.Heat and cold Viral infectivity is generally destroyed by heating at 50-60oC for 30 mint., Viruses can be preserved at -90 oC or -196oC (liquid nitrogen) for longtime. 2.PH Viruses can be preserved at physiological PH (7.3). 3.Ether susceptibility: Ether susceptibility can be used to distinguish viruses that possess an envelope from 4  those that do not. 4. Detergents: Nonionic detergents solubilize lipid constituents of viral membranes. The viral proteins in the envelope are released Anionic detergents also solubilize viral envelopes ,in addition, they disrupt capsids into separated polypeptides. 5. Salts Many viruses can be stabilized by salt in concentrations of 1 mol/L. e.g. MgCl2, MgSO4, Na2SO4. 6. Radiation Ultraviolet, X-ray, and high-energy particles inactivate viruses 7. Formaldehyde Destroys viral infectivity by reacting with nucleic acid. 8. Antibiotics Antibacterial antibiotics have no effect on viruses. Classification of viruses:. 1- Virion morphology, including size, shape, type of symmetry, presence or absence of enveloped. 2. Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome, strandedness (single or double), whether linear or circular, positive or negative sense (polarity), segments (number, size). 3.Physicochemical properties of the virion, including 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 non-structural proteins, amino acid sequences, and special functional activities (transcriptase, reverse 5. Genome organization and replication , including gene order, strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release). 5  6. Antigenic properties 7. Biological properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms, and pathology. Viruses can be divided into several classes, including: A.) By symptomology : Old classification like neurotropic , hepatotropic, enterotropic B.) Hierarchical: Based on families or subfamilies  DNA or RNA.  Viral replication strategy  Symmetry of capsid  Presence or absent of an envelope C.) Baltimore classification The Baltimore classification is the most commonly used for studying the system of virus classification. This system was developed by an American biologist David Baltimore in the 1970s, for which he was awarded the Nobel Prize. Viruses were divided into six groups based on the their nucleic acid and m-RNA production. — 1- ds-DNA viruses. — 2- ss-DNA viruses. — 3- ds- RNA viruses. — 4- ss-RNA viruses with positive strands( positive polarity). — 5- ss-RNA viruses with negative strands(negative polarity). — 6- ss-RNA viruses associated with the enzyme reverse transcriptase. — — Universal system of virus taxonomy: — Families – on the basis of virion morphology, genome structure and strategies of replication. — Virus family names have the suffix – viridae for example Herpesviridae — Genera – based on physicochemical or serological differences. — Genus names carry the suffix – virus for example Herpesvirus. 6 1. Viruses range in size from: a.1-100 nm b. 25-300 nm c. 10-100 μm d.400-1000 nm e. 1-10 μm 2. A structural component that is found in all viruses is: a.The envelope b. DNA c. Capsid d. Tail fibers e. Spikes 3. A chemical component that is found in all viruses is: a.Protein b. Lipid c. DNA d. RNA e. Glycoproteins 4. A common polyhedral capsid shape of viruses is a : a.Pentagon b. Cube c. Icosahedral d. Pyramid e. Sphere 5. What is Virology? a) Virology is the study of bacteria b) Virology is the study of viruses c) Virology is the study of fungi d) Virology is the study of algae 6. Viruses outside their host cells survive as ____________ a) virions b) algae c) protozoa d) bacteria 7. 5. Which of the following phase determines the specificity of the virus? a) Uncoating b) Release c) Attachment d) Penetration 8. Which of the following is the most common capsid shape of the virus? a) Cone b) Icosahedron c) Cube d) Rod 7