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NAJRAN UNIVERSITY College of Applied Medical Sciences

NAJRAN UNIVERSITY College of Applied Medical Sciences. General Microbiology Course Lecture No. 15. By. Dr. Ahmed Morad Asaad Associate Professor of Microbiology. General properties of viruses. Definition: Viruses are obligate intracellular parasites

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NAJRAN UNIVERSITY College of Applied Medical Sciences

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  1. NAJRAN UNIVERSITY College of Applied Medical Sciences General Microbiology Course Lecture No. 15 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

  2. General properties of viruses • Definition: • Viruses are obligate intracellular parasites • They are the smallest infectious agents (20 – 300 nm in diameter) • They contain one kind of nucleic acids (either DNA or RNA) in their genome • The nucleic acid is encased in a protein coat and the entire unit is called the virion • Some viruses are enveloped in a lipid envelope

  3. Viruses are classified on the basis of several properties: • Nucleic acid type: RNA viruses or DNA viruses. Single-stranded or double-stranded • Size and morphology • Susceptibility to physical and chemical agents • Host, tissue and cell tropism • Symptomatology

  4. Size: • 3 methods are used to determine viruses size: • 1- Filtration through filters of known pore sizes • 2- Determination of sedimentation rate in ultracentrifuge • 3- Using electron microscopy • The unit of measurement is nanometer • According to the sized, viruses may be: • Small viruses (20 – 30 nm) such as Polio and Coxackie viruses • Medium-sized viruses (75 – 150 nm) such as influenza and measles viruses • Large viruses (200 – 300 nm) such as chicken pox virus

  5. Shape of viruses: • Most viruses are spherical in shape • Brick-shaped virus such as poxviruses • Bullet-shaped virus such as rhabdoviruses (rabies virus)

  6. Structure of viruses: • The viral particle (virion) consists of a nucleic acid core (virus genome) surrounded by a protein coat (capsid) • 1- Viral nucleic acid: • Viruses contain a single kind of nucleic acids either DNA or RNA that encode the genetic information necessary for viral replication • The genome may be single-stranded or double-stranded

  7. Functions of viral nucleic acids: • It is the essential infectious component of the virion • It encodes the genetic information necessary for viral replication • It is the site of genetic material which carries the heritable characteristics of the virus • The type of nucleic acid and strandedness are major characters for classifying viruses

  8. 2- Capsid: • It is the protein coat surrounding the viral genome • It is made of tiny protein subunits called capsomeres • The nucleic acid + its protein coat = nucleocapsid • Functions: • It protects the nucleic acid from damage in external environment • It facilitates the attachment of the virus to host cells in the process of infection • It represents the viral antigen

  9. 3- Envelope: • It is a lipoprotein membrane composed of lipid layer (from the host cell) and a protein (from the virus) • They are frequently glycoproteins in the form of spike-like projections to attach to host cell receptors for viral entry into host cell • Another protein, the matrix protein, mediate the interaction between the capsid proteins and the envelope

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