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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 General Microbiology Course Lecture No. 15 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology
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
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
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
Shape of viruses: • Most viruses are spherical in shape • Brick-shaped virus such as poxviruses • Bullet-shaped virus such as rhabdoviruses (rabies virus)
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
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
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
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