Virology Lecture 1 PDF

Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah Size & Shape of Viruses (Virion) Viruses are defined as obligate intracellular parasites -they cannot live outside a cellular host and replicate only inside the living cells of other organisms. Virus...

Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah Size & Shape of Viruses (Virion) Viruses are defined as obligate intracellular parasites -they cannot live outside a cellular host and replicate only inside the living cells of other organisms. Viruses are complexes consisting of protein and an RNA or DNA genome. They lack both cellular structure and independent metabolic processes. They replicate solely by exploiting living cells based on the information in the viral genome. It is the smallest infectious agents and can infect all types of life forms, from animals and plants to bacteria and archaea. Size ranges from (20nm-300nm in diameter). The shape of virus particles is determined by the arrangement of the repeating subunits that form the protein coat (capsid) of the virus. Viral Structure I-Viral Nucleic acid The genome consists of single nucleic acid [contains one kind of nucleic acid as their genome (DNA or RNA)] which stores all the viral information required by the virus for its multiplication. Though the viruses possess single nucleic acid, they show a large diversity in this respect. 1. Some are DNA, other RNA; 2. Some are double stranded (ds), others are single stranded (ss), 3. Some are linear, others circular; 4. The DNA is always a single molecule (double or single); RNA can exist either as a single molecule or in several pieces. 5. Some have plus polarity others have minus polarity. A nucleic acid that encodes the information for making proteins needed by a virus is called a positive sense nucleic acid. This strand serves as a direct template for subsequent transcription and translation. RNA (+)- containing viruses (picornaviruses, togaviruses) use positive-sense nucleic acid as a messenger RNA for protein synthesis. This RNA is considered to possess the infectivity, being able to induce the infectious process directly after virus penetration. 1 Page A nucleic acid made up of bases complementary to those of a positive sense nucleic acid is called a negative sense nucleic acid. Negative or minus strand means the nucleic acid chain complementary the positive one. In that case viral protein formation is impossible without preliminary synthesis of positive chain on the negative strand template. For this purpose (–) RNA viruses contain RNA polymerase. It catalyzes the complementary RNA (+) synthesis within infected cells. The latter serves as mRNA  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah II-Viral Capsid & Symmetry The capsid is the “shell” of virus-coded protein that encloses the nucleic acid and is more or less closely associated with it. The combination of these two components (Capsid and DNA) is often termed the nucleocapsid, especially if they are closely associated as in the myxoviruses. The capsid is made up of subunits, the capsomers, the number of which varies but is specific and constant for each viral species. These (capsomeres) are spherical or cylindrical structures composed of several polypeptides. The capsid protects the nucleic acid from degradation by nucleases (to protect the genome from external harmful factors or agents). In all except enveloped viruses, it is responsible for the attachment of the viruses to the host cell. Viral Symmetry The arrangement of capsomers gives the virus structure and its geometric symmetry. Viral nucleocapsids have two forms of symmetry: a. Icosahedral: It appears as sphere. e.g., Herpesviruses, Adenoviruses b. Helical: In which the capsomeres are arranged in a hollow coil that appears rod-shaped. e.g., Influenza viruses All human viruses that have a helical nucleocapsid are enclosed by an outer membrane called an envelope while icosahedral nucleocapsid can be either enveloped or nonenveloped (naked). c. Complex Symmetry: Complex structural patterns and these viruses possess a capsid that is neither purely helical, nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall, such as bacteriophages and the smallpox virus This tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell 2 Page Structure of T4 Bacteriophage  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah III-Viral Envelope: Some virus families have an outer envelope consisting of a lipoprotein membrane lipid bilayer surrounding the viral capsid, such viral lipid envelopes are derived in part from modified host cell membranes during particle formation and release (budding) from the infected cell and protein that is virus specific. They may help viruses avoid the host immune system. In general, the presence of an envelope confers instability on the virus. Enveloped viruses are more sensitive to heat, drying, detergents, and lipid solvents such as alcohol and ether than are nonenveloped (nucleocapsid) viruses. They are transmitted by direct contact via blood and body fluids, whereas naked viruses are more stable and can survive longer in the environment and can be transmitted by indirect means such as the fecal–oral route. Depending on the virus, the envelopes of the viruses may or may not be covered by spikes. The spikes are glycoprotein-like projections on the outer surface of the envelope which attach to host cell receptors during the entry of the virus into the cell, and are highly antigenic which have enzymatic and/or adsorption and/or hemagglutinating activity. Both the outer capsid and envelope proteins of viruses are glycosylated and are important in determining the host range and antigenic composition of the virion. Most spikes act as viral attachment protein (VAP) which capable of binding to structures on target cells. The VAP that binds to red blood cells is called hemagglutinin. The ability of certain viruses, such as influenza virus to agglutinate red blood cells is due to the presence of these hemagglutinins. The process is called hemagglutination and it forms the basis of hemagglutination inhibition test used in the viral serology. 3 Page Viral (Virion)Structure  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah Component of the basic virion (Diagram showing the structural difference between enveloped and naked viruses) IV- Viral Proteins: Viral proteins can be broadly divided into two groups: a. Structural proteins: are the proteins present in the coat of the virus 1. Envelope protein: All class of enveloped viruses contain glycoprotein which differ from virus to virus, for example, one glycoprotein in rhabdoviruses, two glycoproteins in paramyxoviruses and four in orthomyxoviruses. 2. Nucleocapside protein: The helical capsids contain single type of protein (TMV) and icosahedral capsid contains several types of protein (adenovirus contain 14 protein type). 3. Core protein: Protein found in the nucleic acid is known as core protein. 4. Matrix protein mediates the interaction between the capsid protein and the envelope. 5. glycoproteins in the form of spike like projections on the surface, which attach to host cell receptors during the entry of the virus into the cell. 4 Page  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah Structural Proteins of viruses have several important functions: 1. Facilitate transfer of the viral nucleic acid from one host cell to another. 2. Protect viral genome against inactivation by nucleases 3. Participate in the attachment of the virus particle to susceptible cell. 4. Provide the structural symmetry of the virus particle. 5. Determine the antigenic characteristics of the virus. b. Non-structural proteins (functional proteins): Viruses require a number of different enzymes depending on genome type and mode of infection. In several virus species, enzymes are a component of the virus particle, therefor Enzymes: Are essential for the initiation of the viral replicative cycle when the virus enter host cell. a. RNA polymerase: which is carried by negative sense RNA virus that is needed to copy the first mRNA. b. Reverse transcriptase in retroviruses that makes a DNA copy of the viral RNA. c. Some viruses contain regulatory proteins in the virion in a structure called the tegument which is located between nucleocapsid and the envelope. These regulatory proteins include transcription and translation factors that control either viral or cellular processes. Herpes simplex and cytomegalovirus have well characteristics tegument. Most of non-structural viral proteins are expressed inside the cells in the course of viral infection. Viral proteins possess antigenic properties, reacting with specific antibodies and immune cell receptors. Besides its own proteins, during maturation the virus may capture the proteins of the host cells. For instance, HIV harbors cellular protein cyclophilin up to 30% of its weight; without this protein HIV particles are non-infectious. Virus Classification/Nomenclature: The taxonomic system used for viruses is based on the following morphological and biochemical criteria: 5 Page 1. Morphology: size, shape, presence of envelope, etc. 2. Physicochemical properties: thermal stability, detergent stability, molecular mass, etc. 3. 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).  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah 4. Genome organization and replication, including strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release). 5. Proteins: number, size, sequence, etc. 6. Antigenic properties: serological relationships. 7. Biological properties: Host range, mode of transmission, pathogenicity, tissue tropisms, geographic distribution, etc. Universal system of virus taxonomy: The International Committee on taxonomy (Nomenclature) of Viruses (ICTV) produced the following the hierarchical system for viral classification, starts at the level of order and follows as thus, with the taxon suffixes given in italics: Orders.. (virales), Families..(-viridae), Subfamilies.. (-virinae), Genera.. (-virus), Species.. (virus). There are the next viral groups according to Baltimore classification: group I – double-stranded DNA viruses; group II – single-stranded DNA viruses; group III – double-stranded RNA viruses; group IV – single-stranded RNA viruses – positive-sense RNA; group V – single-stranded RNA viruses – negative-sense RNA; group VI – positive-sense single-stranded RNA viruses that replicate through a DNA intermediate. group VII – double-stranded DNA viruses that replicate through a single-stranded RNA intermediate. 6 Page Viral classes: Baltimore classification of viruses  Lecture 1/ Virology /7Semester / Dept. of (MLT) Dr.Sheylan S. Abdullah Atypical Virus –like agents: 1. Defective viruses: are viruses composed of viral N.A and proteins but cannot replicate without a "helper" virus, which provides the missing function. There is usually had a mutation or deletion of a part of their genetic material. 2. Pseudovirions: contain host cell DNA instead of viral DNA within the capsid.they are formed during infection with certain viruses when host cell DNA is fragmented and pieces of it are incorporated within the capsid protein. Pseudovirions can infect cells, but they do not replicate 3. Viriods: single molecule of circular RNA without a protein coat or envelope. Affects plants. 4. Prions: are infectious particles that are composed solely of protein. 5. Virion: is the physical particle in the extracellular phase which is able to spread to new host cells; complete intact virus particle. 23/ Sep./2024 7 Page