Microbiology 2 - Structure of Viruses (PDF)

Microbiology Structure of viruses Microbiology| Structure of viruses Contents : Virus Structure 3 Symmetry of Nucleocapsid 11 Properties of Naked Viruses 18 Envelope 20 Properties of Enveloped Viruses 22 Microbiology| Structure of viruses Virion : The complete infectious unit of virus particle. Stru...

Microbiology Structure of viruses Microbiology| Structure of viruses Contents : Virus Structure 3 Symmetry of Nucleocapsid 11 Properties of Naked Viruses 18 Envelope 20 Properties of Enveloped Viruses 22 Microbiology| Structure of viruses Virion : The complete infectious unit of virus particle. Structurally mature, extracellular virus particles. Function: transfer the viral nucleic acid from one cell to another. Microbiology| Structure of viruses Viral core : The viral nucleic acid genome, In the center of the virion. Control the viral heredity and variation, responsible for the infectivity. Microbiology| Structure of viruses Genome (Chemical Composition of Viruses) : The genome of a virus can be either DNA or RNA (classification property). DNA-double stranded (ds): linear or circular. Single stranded (ss): linear or circular. RNA- ss: segmented or non-segmented ss: polarity+(sense) or polarity –(non-sense). ds: linear (only reovirus family) Microbiology| Structure of viruses DNA Double-stranded Linear Single Circular RNA Single-stranded Linea r Circular Double-stranded Linear Sin Multi Multi Multi Single Single Single gle ple ple ple Single-stranded Linear (circular) (+) Sense (-)Sense Multi Multi Single Single ple ple Microbiology| Structure of viruses In addition to the properties of the genome the are other properties have been used as a basis for the classification of viruses. 1. Virion morphology: including size, shape, type of symmetry. 2. Virus protein properties: including number, size, and functional activities of structural and nonstructural proteins, amino acid sequence. 3. Antigenic properties. 4. Biologic properties: including natural host range, mode of transmission, vector relationships. Microbiology| Structure of viruses Viral Capsid : The protein shell, or coat, that encloses the nucleic acid genome. Functions: a. Protect the viral nucleic acid. b. Participate in the viral infection. c. Share the antigenicity Microbiology| Structure of viruses Nucleocapsid : The core of a virus particle consisting of the genome plus a complex of proteins. Or: The protein–nucleic acid complex representing the packaged form of the viral genome. The term is commonly used in cases in which the nucleocapsid is a substructure of a more complex virus particle. complex of proteins = Structural proteins + Non- Structural proteins (Enzymes & Nucleic acid binding proteins) Microbiology| Structure of viruses Structural units : The basic protein building blocks of the coat. They are usually a collection of more than one nonidentical protein subunit. The structural unit is often referred to as a protomer. Microbiology| Structure of viruses Symmetry of Nucleocapsid HELICAL CUBIC /ICOSAHEDRAL COMPLEX Microbiology| Structure of viruses In cases of helical symmetry protein subunits are bound in a periodic way to the viral nucleic acid, winding it into a helix. The filamentous viral nucleic acid–protein complex (nucleo-capsid) is then coiled inside a lipid-containing envelope. This arrangement results in rod-shaped or filamentous virions: can be short and highly rigid, or long and very flexible. The genetic material, in general, single-stranded RNA, but ssDNA in some cases. Microbiology| Structure of viruses Helical California Encephalitis Virus Mumps Virus Coronavirus Parainfluenza Virus Hantavirus Rabies Virus Influenza Virus (Flu Virus) Respiratory Syncytial Virus (RSV) Measles Virus (Rubeola) Microbiology| Structure of viruses Cubic (Icosahedral) symmetry : icosahedral is a solid figure with twenty plane faces, especially equilateral triangular ones. Most animal viruses are icosahedral or near-spherical with icosahedral symmetry. A regular icosahedron is the optimum way of forming a closed shell from identical sub-units. The icosahedron has 20 faces (each an equilateral triangle), 12 vertices, and fivefold, threefold, and twofold axes of rotational symmetry. Microbiology| Structure of viruses Many viruses, such as rotavirus, have more than twelve capsomers and appear spherical but they retain this symmetry. Capsomers at the apices are surrounded by five other capsomers and are called pentons. Capsomers on the triangular faces are surrounded by six others and are called hexons. There are exactly 60 identical subunits on the surface of an icosahedron. To build a particle size adequate to encapsidate viral genomes,viral shells are composed of multiples of 60 structural units. Microbiology| Structure of viruses Icosahedral Adeno-associated Virus (AAV) Norwalk Virus Epstein-Barr Virus (EBV) Adenovirus Papilloma Virus (HPV) Hepatitis A Virus (HAV) B19 Polio virus Hepatitis B Virus (HBV) Coxsackievirus - A Rhinovirus Hepatitis C Virus (HCV) Coxsackievirus - B Rubella Virus Hepatitis Delta Virus (HDV) Cytomegalovirus (CMV) Saint Louis Encephalitis Virus Hepatitis E Virus (HEV) Eastern Equine Encephalitis Virus (EEEV) Varicella-Zoster Virus (HHV3) Echovirus Western Equine Encephalitis Virus (WEEV) Herpes Simplex Virus 1 (HHV1) Herpes Simplex Virus 2 (HHV2) Human Immunodeficiency Virus (HIV) Human T-lymphotrophic Virus (HTLV) Yellow Fever Virus Microbiology| Structure of viruses Complex Virus Structures : A well known example is the tailed bacteriophages such as T4. The head of these viruses is cubic with a triangulation number of 7. This is attached by a collar to a contractile tail with helical symmetry. Some virus particles do not exhibit simple cubic or helical symmetry but are more complicated in structure. Ex. poxviruses are brick shaped, with ridges on the external surface and a core and lateral bodies inside. Microbiology| Structure of viruses Properties of naked viruses : 1. Stable in hostile environment. 2. Not damaged by drying, acid, detergent, and heat. 3. Released by lysis of host cells. 4. Can sustain in dry environment. 5. Can infect the GI tract and survive the acid and bile. 6. Can spread easily via hands, dust, fomites, etc. 7. Can stay dry and still retain infectivity. 8. Neutralizing mucosal and systemic antibodies are needed to control the establishment of infection. Microbiology| Structure of viruses Naked viruses (Non Enveloped) Adeno-associated Virus (AAV) Adenovirus B19 Coxsackievirus - A Coxsackievirus - B Echovirus Hepatitis A Virus (HAV) Hepatitis E Virus (HEV) Norwalk Virus Microbiology| Structure of viruses Envelope (Chemical Composition of Viruses) : A lipid-containing membrane that surrounds some viral particles. It is acquired during viral maturation by a budding process through a cellular membrane, Viruses-encoded glycoproteins are exposed on the surface of the envelope. Not all viruses have the envelope. Microbiology| Structure of viruses Viral Glycoproteins (attached to the envelope) : They are virus encoded. Function: Attach the virus particle to a target cell by interacting with a cellular receptor. Involved in the membrane fusion step of infection. The glycoproteins are also important viral antigens. Involved in the interaction of the virus particle with neutralizing antibody. Microbiology| Structure of viruses Properties of enveloped viruses : Labile in dry , arid environment. Damaged by drying, acid, detergent, and heat. Pick up new cell membrane during multiplication. Insert new virus-specific proteins after assembly. Virus is released by budding. Microbiology| Structure of viruses Consequences of Properties for enveloped viruses : Must stay moist. Must not infect the GI tract for survival. Must be transmitted in the protective, droplets, secretions, blood and body fluids. Must reinfect another host cell to sustain. Humoral and cell-mediated immunity are needed to control the infection. Microbiology| Structure of viruses Icosahedral California Encephalitis Virus Herpes Simplex Virus 2 Rotavirus Coronavirus (HHV2) Rubella Virus Cytomegalovirus (CMV) Human Immunodeficiency Saint Louis Encephalitis Virus Eastern Equine Encephalitis Virus (EEEV) Virus (HIV) Smallpox Virus (Variola) Epstein-Barr Virus (EBV) Human T-lymphotrophic Virus Vaccinia Virus Hantavirus (HTLV) Varicella-Zoster Virus (HHV3) Hepatitis B Virus (HBV) Influenza Virus (Flu Virus) Venezuelan Equine Encephal. Vir. (VEEV) Hepatitis C Virus (HCV) Molluscum contagiosum Western Equine Encephalitis Hepatitis Delta Virus (HDV) Papilloma Virus (HPV) Virus (WEEV) Herpes Simplex Virus 1 (HHV1) Polio virus Yellow Fever Virus Rhinovirus