General properties of viruses 1-They are very small in size, from nm 2-They contain one kind of nucleic acid (RNA or DNA) as their genome 3-They are metabolically inert because they do not possess enzyme systems necessary for the synthesis of new viral material (ribosomes) 4-They are obligate intracellular parasites as they replicate inside living cells 5-They are only seen by electron microscope

Viral Structure Each virus particle or virion is composed of: A protein coat  capsid A nucleic acid  core Many viruses are naked but some are enveloped

Viral capsid It is the protein coat surrounding the nucleic acid It is composed of small protein subunits arranged symmetrically around the nucleic acid called capsomeres

General structure: A. nonenveloped; B. enveloped viruses.

Viral nucleic acid and viral Envelope Viruses contain either DNA or RNA but not both Most DNA viruses are double stranded some are single stranded Most RNA viruses are single stranded some are double stranded Virus envelope Many viruses are surrounded by a lipid or lipoprotein envelopes which may be covered by spikes (glycoproteins)

Virus Symmetry Viruses have three types of symmetry Cubical symmetry These viruses resemble a crystal and are called icosahedral virus Example: Adenoviruses Helical symmetry In which the particle is elongated Most helical viruses are enveloped Example: Influenza virus Complex symmetry In which the viruses are complicated in structure Example: Poxviruses and Bacteriophage

10 5 BASIC TYPES OF VIRAL STRUCTURE HELICAL ENVELOPED HELICAL ENVELOPED ICOSAHEDRAL COMPLEX ICOSAHEDRAL Adapted from Schaechter et al., Mechanisms of Microbial Disease nucleocapsid icosahedral nucleocapsid nucleocapsid helical nucleocapsid lipid bilayer glycoprotein spikes = peplomers

Virus particle = virion

13 Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997

14 Growth on artificial media Division by binary fission Contain DNA and RNA Contain protein synthesis machinery Contain muramic acid Sensitive to antibiotics Bacteria often yes yes yes often yes Viruses never no Either DNA or RNA no* no no * The arenavirus family appears to ‘accidentally’ package ribosomes, but these appear to play no role in protein synthesis.

Classification of Viruses Classical virus classification schemes have been based on the consideration of major properties of viruses 1- The type of nucleic acid which is found in the virion (RNA or DNA, single stranded or double stranded) 2-The symmetry and shape of the capsid (Cubic, helical, complex) 3- The presence or absence of an envelope (enveloped, naked)

4- The size of the virus particle 5- Antigenic properties 6- Biologic properties, including natural host range, mode of transmission, vector relationship, pathogenicity, and tissue tropism

DNA IcosahedralComplex Pox viruses Naked Enveloped Papilloma virus Herpes viruses Hepatitis B Classification of Viruses

18 HERPESVIRIDAE HEPADNAVIRIDAE ENVELOPED PAPILLOMAVIRIDAE POLYOMAVIRIDAE (formerly grouped together as the PAPOVAVIRIDAE) CIRCULAR ADENOVIRIDAE LINEAR NON-ENVELOPED DOUBLE STRANDED PARVOVIRIDAE SINGLE STRANDED NON-ENVELOPED POXVIRIDAE COMPLEX ENVELOPED DNA VIRUSES Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed All families shown are icosahedral except for poxviruses

a Classification of Viruses

20 FLAVIVIRIDAE TOGAVIRIDAE RETROVIRIDAE ICOSAHEDRAL CORONAVIRIDAE HELICAL ENVELOPED ICOSAHEDRAL PICORNAVIRIDAE CALICIVIRIDAE NONENVELOPED SINGLE STRANDED positive sense BUNYAVIRIDAE ARENAVIRIDAE ORTHOMYXOVIRIDAE PARAMYXOVIRIDAE RHABDOVIRIDAE FILOVIRIDAE SINGLE STRANDED negative sense REOVIRIDAE DOUBLE STRANDED RNA VIRUSES ENVELOPED HELICALICOSAHEDRAL NONENVELOPED Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

Viral replication Viruses multiply only in living cells Steps of viral replication 1- Adsorption Virus attaches to the cell surface 2- Penetration (Entry) Enveloped viruses: receptor mediated endocytosis 3- Uncoating Uncoating is the physical separation of viral nucleic acid from the outer structural components

4- Transcription of mRNA 5- Synthesis of viral components This involves the synthesis of viral proteins and viral genomes 6- Assembly New virus particles are assembled by packaging of the genome into capsid 7- Release Virus may be released due to cell lysis, or, if enveloped, may bud from the cell

Viral replication

24 ADSORPTION

25 ADSORPTION

26 PENETRATION herpesviruses, paramyxoviruses, HIV

27

28 PENETRATION - ENVELOPED VIRUSES from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 1998

29 PENETRATION NON-ENVELOPED VIRUSES entry directly across plasma membrane:

30

31 Smallpox virus cytoplasmic assembly and maturation F. A. Murphy, School of Veterinary Medicine, University of California, Davis.

32 HIV budding and maturation Hsiung, GD et al., Diagnostic Virology 1994 p204 (D. Medina)