Virology Lecture Notes PDF

safa amer khalil Lec:3 Virology: is a branch of science that deals with viruses and viral diseases. Viruses are the smallest infectious agents , viruses are obligate intracellular parasite containing genetic material surrounde...

safa amer khalil Lec:3 Virology: is a branch of science that deals with viruses and viral diseases. Viruses are the smallest infectious agents , viruses are obligate intracellular parasite containing genetic material surrounded by protein ,Virus particles can only be observed by an electron microscope. General Properties of Viruses 1.Viruses are smaller than bacteria, they range in size between 20-300 nanometer. (Table- 1-). 2.Viruses contain only one type of nucleic acid, DNA or RNA , but never both. 3.Viruses consist of nucleic acid surrounded by a protein coat. Some viruses have additional lipoprotein envelope. 4.Viruses lack cellular organelles, such as mitochondria and ribosomes. 5.Viruses are obligate cellular parasites. They replicate only inside living cells. 6.Viruses replicate through replication of their nucleic acid and synthesis of the viral protein. 7.Viruses do not multiply in media. 8. Viruses do not undergo binary fission. Table (1) : Comparison between viruses and bacteria property Viruses Bacteria Size 20-300 nm 1000nm (type of nucleic acid) DNA or RNA but not both DNA and RNA Cell wall Envelope present in some present Cell wall viruses Multiplication by negative positive binary fissio 1  safa amer khalil Lec:3 Sensitivity to negative positive antibiotics Crowth in culture Growth only in the living host Grow in culture media cell media Viral Structure Viruses composed of nucleic acid either DNA or RNA, surrounded by a protein coat called the capsid. The capsid is composed of small structural units called capsomeres. The capsid protects nucleic acid from inactivation by the outer physical conditions. The complete infectious virus particle is called virion virus particle = virion capsid + genome = nucleocapsid Basic virus structure DNA or RNA + capsid protein = nucleocapsid = naked virus Nucleocapsid + Lipid membrane, glycoproteins = enveloped virus 2  safa amer khalil Lec:3 Types of symmetry of virus particles Viruses are divided into three groups, based on the morphology of the nucleocapsid and the arrangement of capsomeres. 1 -Icosahedral (Cubic) symmetry e.g. Virus that cause yellow fever and Poliovirus. 2.Helical symmetry e.g. Rabies virus. 3.Complex structures e.g.Poxviruses ,bacteriophage classification of viruses is based on the following characteristics-: 1 -Virion morphology, including size, shape, type of symmetry , presence or absence of enveloped. 2.Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome strand (single or double), whether linear or circular, positive or negative. 3 Virus protein properties, including number, size and functional 3  safa amer khalil Lec:3 activities of proteins, amino acid sequences, and activities reverse transcriptase 4.Biological properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms. Viral Replication: As obligate intracellular parasites, Virus must enter and replicate in living cells in order to “reproduce” themselves. This "growth cycle" involves specific attachment of virus, penetration and uncoating, nucleic acid transcription, protein synthesis, maturation and assembly of the virions and release from the cell by lysis. The typical infectious cycle consists of : Attachment, Penetration, Uncoating, Transcription and Translation , , Assembly, and Release. Attachment: is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. Attachment to the receptor can induce the viral envelope protein to undergo changes that results in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow the virus to enter Penetration: follows attachment: Virions enter the host cell through receptor-mediated endocytosis in non enveloped viruses or membrane fusion in enveloped viruses. This is often called viral entry. 4  safa amer khalil Lec:3 Uncoating: is a process in which the viral capsid is removed, This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid. Transcription and Translation: The viruses must use host cellular machinery to replicate and make functional and structural proteins. Assembly and Release: The process of virion assembly together newly formed viral nucleic acid and the structural proteins to form the nucleocapsid of the virus, Viruses can be released from the host cell by lysis. 5