Lec. 1 Introduction to Virology PDF

Virology Lec. 1: Introduction Asst. Prof. Dr. Shaimaa R. Al-Salihy Learning Objectives: At the end of this lecture, you will be able to: 1. Define viruses and other infectious agents that do not meet c...

Virology Lec. 1: Introduction Asst. Prof. Dr. Shaimaa R. Al-Salihy Learning Objectives: At the end of this lecture, you will be able to: 1. Define viruses and other infectious agents that do not meet characteristics of cells. 2. Recognize the differences between viruses and bacteria. 3. Understand terms used in virology. 4. Describe the basis of classification of viruses. --------------------------------------------------------------------------------------------------------------------- What’s the meaning of the virus? Viruses are obligate intracellular parasites which means that they replicate (or multiply) only inside a living host cell (they are incapable of independent replication, and can synthesize neither their own energy nor their own proteins). General features of viruses: 1. The smallest infectious agents (ranging between 20-300 nm in diameter). 2. Containing only one type of nucleic acid (either RNA or DNA) as their genome, enclosed within a protective coat of protein called capsid, which may be surrounded by a lipid-containing membrane. 3. They multiply by a complex method (one virus can replicate to produce hundreds of progeny viruses), they do not undergo binary fission or mitosis. 4. They do not possess cellular organelles and have no metabolism (metabolically inert) until they enter the host cell, upon which they absolutely dependent for their replication (Obligate intracellular parasites) 5. They are not susceptible to antibacterial antibiotics. Viral structure and symmetry: The viral core composed of nucleic acid genome (either DNA or RNA). The nucleic acid is enclosed within a protective coat of protein called capsid, which may be surrounded by a lipid-containing membrane called envelope. The complete virus particle called virion. The capsid is made up of morphological units called capsomers. The capsid protects and stabilizes the viral nucleic acid against extracellular enzymes, and facilitates attachment and penetration of the virus upon contact with host cell. In addition to its role in providing structural symmetry. The capsid protein and nucleic acid complex called nucleocapsid. Some viruses are surrounded by lipid-containing membrane, called envelope. It is acquired during viral maturation by a budding process through a cellular membrane. Virus-encoded glycoproteins are exposed on the surface of the envelope. These projections are called peplomers. Figure (1): the typical structure of the virus Symmetry: The arrangement of capsomers gives the virus structure its geometric symmetry. On the basis of capsomers arrangement, the nucleocapsid have the following forms of symmetry: 1. Icosahedral: in which the capsomers are arranged in 20 triangles that form a symmetric figure (icosahedron), with the approximate outline of a sphere, for example, adenovirus. 2. Helical: in which the capsomers are arranged in a hollow coil that appears rod-shaped. The helix can be either rigid or flexible, for example, rabies virus and Ebola virus. 3. Complex: some viruses may not exhibit icosahedral or helical symmetry, but instead may exhibit complex symmetry, for example, Pox virus. * All known human viruses with helical symmetry are enclosed by envelope. ** All known animal viruses with helical symmetry contain RNA genome.  Figure (2): symmetry of the viruses Nomenclature and Classification of viruses: ☸ The nomenclature of viruses, like their classification, is decided by the International Committee on Taxonomy of Viruses (ICTV). The viruses are grouped into: ⮚ Order which are end with the suffix-virales ⮚ families which are named with a suffix-viridae. ⮚ Subfamilies, if any shall end with suffix-virinae ⮚ genera will have suffix of -virus. The prefix may be another Latin word, i.e. an abbreviation derived from some initial letters. The latinized names are written in italics whereas vernacular names are written in roman letters. ☸ Classification of viruses depends on many properties including: 1. Virion morphology: size, shape, type of symmetry, presence or absence of peplomers, and presence or absence of membranes. 2. Virus genome properties, including: a. Type of nucleic acid (DNA or RNA) b. Strandedness (single or double): all DNA viruses are double-stranded except Parvovirus; All RNA viruses are single stranded except Rotavirus.  c. Linear or circular. d. Sense (positive: strand with same polarity as mRNA, or negative polarity: strand complementary of mRNA), e. Segments (number, size). f. Nucleotide sequence and presence of special features (repetitive elements, 5′-terminal cap, or 3′-terminal poly(A) tract) 3. Genome organization and replication, including: a. gene order b. number and position of open reading frames c. strategy of replication (patterns of transcription, translation) d. cellular sites (accumulation of proteins, virion assembly, virion release). 4. Virus protein properties, including: a. number, size, amino acid sequence, modifications (glycosylation, phosphorylation, myristoylation), and functional activities of structural and nonstructural proteins (transcriptase, reverse transcriptase, neuraminidase, fusion activities). 5. Antigenic properties particularly reactions to various antisera. 6. Physicochemical properties of the virion, including molecular mass, buoyant density, pH stability, thermal stability, and susceptibility to physical and chemical agents, especially solubilizing agents and detergents. 7. Biologic properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms, and pathology. Human viruses usually divided into two main types according to the type of viral genome: 1. RNA viruses which include 14 families, which are: Orthomyxoviridae, Picornaviridae, Paramyxoviridae, Retroviridae, flavoviridae, filoviridae, Astroviridae, Calciviridae, Reoviridae, Togaviridae, Arenaviridae, Rhabdoviridae, Bunyaviridae, Coronaviridae. 2. DNA viruses which include 6 families, which are: Poxviridae, Hepadnaviridae, Herpesviridae , Parvoviridae, Papovaviridae, Adenoviridae. * All RNA viruses are single stranded except Rotaviridae (double stranded), and all DNA viruses are double stranded except Parvoviridae (single stranded). * As of 2018, there are, 14 orders, 143 families, 64 subfamilies, 846 genera, and 4,958 species of viruses have been defined.  Figure (3): Classification of the viruses ATYPICAL VIRUS-LIKE AGENTS There are four exceptions to the typical virus as described earlier: 1. Defective viruses: are composed of viral nucleic acid and proteins but cannot replicate without a “helper” virus, which provides the missing function. Defective viruses usually have a mutation or a deletion of part of their genetic material. During the growth of most human viruses, many more defective than infectious virus particles are produced. The ratio of defective to infectious particles can be as high as 100:1. Because these defective particles can interfere with the growth of the infectious particles, it has been hypothesized that the defective viruses may aid in recovery from an infection by limiting the ability of the infectious particles to grow. 2. Pseudovirions contain host cell DNA instead of viral DNA within the capsid. They are formed during infection with certain viruses when the 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. Viroids consist solely of a single molecule of circular RNA without a protein coat or envelope. There is extensive homology between bases in the viroid RNA, leading to large double-stranded regions. The RNA is quite small (molecular weight 1 × 10 5) and apparently does not code for any protein. Nevertheless, viroid replicate, but the mechanism is unclear. They cause several plant diseases but are not implicated in any human disease. 4. Prions are infectious particles that are composed solely of protein (i.e., they contain no detectable nucleic acid). They are implicated as the cause of certain “slow” diseases called transmissible spongiform encephalopathies, which include such diseases as Creutzfeldt-Jakob disease in humans and scrapie in sheep. Because neither DNA nor RNA has been detected in prions, they are clearly different from viruses. Furthermore, electron microscopy reveals filaments rather than virus particles. Prions are much more resistant to inactivation by ultraviolet light and heat than are viruses. They are remarkably resistant to formaldehyde and nucleases. However, they are inactivated by hypochlorite, NaOH, and autoclaving. Hypochlorite is used to sterilize surgical instruments and other medical supplies that cannot be autoclaved. Table (1): Differentiation between viruses and cells Table (2): Properties of naked viruses Table (3): Properties of enveloped viruses ‫غير‬ ‫مستقر‬ - Table (4): features distinguish viroid from viruses Table (5): features of prion