Viral Classification & Structure PDF

Batterjee Medical College Tile Viral Classification & Structure Name :Prof. Dr. Manal El Said Designation: Head of Microbiology Department Department: Microbiology Batterjee Medical College Topic/Subject Comparison of Viruses and...

Batterjee Medical College Tile Viral Classification & Structure Name :Prof. Dr. Manal El Said Designation: Head of Microbiology Department Department: Microbiology Batterjee Medical College Topic/Subject Comparison of Viruses and Cells Virus Structure Virus Growth Curve Classification of DNA Viruses Classification of RNA Viruses Viral replication AMBOSS Cases References Introduction Viruses are not cells. Obligate intracellular parasites (reproduce only within cells). Not capable of independent replication Synthesize neither their energy nor their proteins Very small (not seen by light microscope). Comparison of Viruses and Cells Comparison of Viruses and Cells Property Viruses Cells Type of nucleic acid DNA or RNA but not both DNA and RNA Proteins Few Many Lipoprotein membrane present Cell membrane in some Envelope viruses present in all cells Ribosomes Absent Present Mitochondria Absent Present in eukaryotic cells Enzymes None or few Many Multiplication by binary No Yes fission or mitosis Virus Structure Virus Size & Shape Viruses range in size from (~20 nm to ~300 nm). Most viruses appear as spheres or rods in electron microscope. Virus Structure Viral Nucleic Acids Viruses contain either DNA or RNA, but not both. DNA & RNA genomes either single-stranded or double-stranded. Some RNA viruses e.g. influenza virus & rotavirus have segmented genomes. All viruses have one copy of their genome i.e., haploid (except retroviruses, which have two copies i.e., diploid.) Virus Structure Capsid All viruses have a protein coat called capsid that covers & protects viral genome from nucleases Capsid is composed of repeating subunits called capsomers virus symmetric appearance f Nucleocapsid Capsid & the enclosed nucleic acid Nucleic acid Virus Structure Nucleocapsid Viral nucleocapsids have two forms of symmetry: 1-Icosahedral (enveloped or naked) : capsomers are arranged in 20 triangles that form symmetric figure (icosahedron) with approximate outline of sphere. (all DNA viruses are icosahedral except poxvirus, which has a complex capsid) 2-Helical (enveloped): capsomers are arranged in hollow coil that appears rod-shaped Virus Structure Viral Proteins 1- Surface Proteins Mediate attachment to host cell receptors Targets of antibody which inhibits viral replication. Some viruses produce antigenic variants of their surface proteins evade host defenses. 2- Internal proteins They are DNA or RNA polymerases Virus Structure Viral Proteins 3-Matrix protein Mediates interaction between viral nucleocapsid proteins & envelope proteins. 4- Superantigens Proteins Epstein-Barr virus & Cytomegalovirus produce superantigens non-specific polyclonal activation of T cells Virus Structure Viral Proteins 5- Regulatory Proteins Some viruses e.g. herpes simplex virus & cytomegalovirus contain regulatory proteins in a structure called tegument, located between the nucleocapsid & envelope. Regulatory proteins include transcription & translation factors that control either viral or cellular processes. Virus Structure Viral Envelope Lipid bilayer contains viral glycoproteins & lipid derived from host cell Envelope is acquired as virus exits from cell in process called budding. Virus Structure Viral Envelope Property Enveloped Viruses Naked Viruses Virulent Less virulent More virulent Transmission Direct Indirect via blood & body fluids e.g. fecal–oral route Sensitivity to heat & Sensitive Resistant dryness Organic solvents e.g. Rapid inactivation (presence Resistance alcohol & detergents of Lipid bilayer) Viral Growth Curve Virus disappears & virus particle is no longer present (Solid line dropping to x axis) Viral nucleic acid continues to function & begins to accumulate within cell (Dotted line). Eclipse period: Time during which no virus is found inside cell & ends with appearance of virus (Solid line). Latent period: Time from onset of infection to appearance of virus extracellularly Classification of DNA Viruses Virus Family Envelope Capsid DNA Structure Medically Important Viruses Present Symmetry Parvovirus No Icosahedral SS, linear B19 virus Polyomavirus No Icosahedral DS, circular, JC virus, BK virus supercoiled Papillomavirus No Icosahedral DS, circular, Human papilloma virus supercoiled Adenovirus No Icosahedral DS, linear Adenovirus Hepadnavirus Yes Icosahedral DS, incomplete Hepatitis B virus circular Herpesvirus Yes Icosahedral DS, linear Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein- Barr virus Poxvirus Yes Complex DS, linear Smallpox virus, molluscum contagiosum virus 1SS = single-stranded; DS = double-stranded. Classification of RNA Viruses Virus Family Envelope Capsid Symmetry RNA Structure Medically Important Present Viruses Picornavirus No Icosahedral SS linear, Poliovirus nonsegmented, Rhinovirus positive polarity hepatitis A virus Hepevirus No Icosahedral SS, linear Hepatitis E virus non-segmented positive polarity Calicivirus No Icosahedral SS linear, Norwalk virus nonsegmented, positive polarity Reovirus No Icosahedral DS linear, 10 or 11 Rotavirus segments Flavivirus Yes Icosahedral SS linear, Yellow fever virus, nonsegmented, dengue virus positive polarity West Nile virus, hepatitis C virus Togavirus Yes Icosahedral SS linear, Rubella virus nonsegmented, positive polarity  Classification of RNA Viruses Virus Family Envelope Capsid RNA Structure Medically Important Viruses Present Symmetry Retrovirus Yes Icosahedral SS linear, 2 identical HIV strands (diploid), positive human T-cell leukemia virus polarity Orthomyxovirus Yes Helical SS linear, 8 segments, Influenza virus negative polarity Paramyxovirus Yes Helical SS linear, nonsegmented, Measles virus, negative polarity mumps virus, respiratory syncytial virus Rhabdovirus Yes Helical SS linear, nonsegmented, Rabies virus negative polarity Filovirus Yes Helical SS linear, nonsegmented, Ebola virus negative polarity Marburg virus Classification of RNA Viruses Virus Family Envelope Capsid Symmetry RNA Structure Medically Important Present Viruses Coronavirus Yes Helical SS linear, Coronavirus nonsegmented, positive polarity Arenavirus Yes Helical SS circular, 2 segments Lymphocytic with cohesive ends, choriomeningitis virus negative polarity Bunyavirus Yes Helical SS circular, 3 segments California encephalitis with cohesive ends, virus, hantavirus negative polarity Deltavirus Yes Uncertain SS circular, closed circle, Hepatitis delta virus negative polarity Viral Replication Events of the Viral Growth Cycle Early events 1. Attachment 2. Penetration 3. Uncoating Middle events 4. Gene expression & Genome replication Late events 5. Assembly 6. Release Viral Replication 1-Attachment Virus can only infect cells possessing surface “receptors” specific to particular virus species. Virus attached to a cell with: -Capsid (Naked viruses) -Envelope proteins (Enveloped viruses) Viral Replication Receptors used by viruses Cytomegalovirus : integrins Epstein-Barr virus: CD21 Human immunodeficiency virus: CD4, CXCR4, CCR5 Parvovirus B19: P antigen on erythrocytes Rabies virus: nicotinic acetylcholine receptor Rhinovirus: ICAM-1 Viral Replication 2-Penetration Viruses penetrate into cell by viropexis (pinocytosis). In enveloped viruses, envelope fuse with cell membrane, releasing virus into cytoplasm. Non Enveloped Virus Enveloped Virus Viral Replication 3- Uncoating Release of nucleic acid from capsid. A low pH within vesicle favors uncoating Activated by cellular enzymes & contribution from cell membranes (except smallpox virus) Case Problem -1: AMBOSS An 82-year-old woman is brought to the physician by her daughter because of a 3-day history of a runny nose, headache, and cough. Testing of nasal secretions is performed to identify the viral strain. Electron microscopy shows Rhinovirus. Binding to which of the following is responsible for the virulence of this virus? a. P antigen b. CD21 c. CCR5 d. ICAM-1 e. CXCR4 f. Sialic acid residues g. Integrin Case Problem -2: AMBOSS The occupational health department at a hospital implements new safety precautions to prevent laboratory-acquired infections. One of the new precautions includes disinfecting the microbiology laboratory benches with 70% ethanol before and after use. This measure is most likely to be effective in preventing the transmission of which of the following viruses? a. Enveloped viruses b. Naked viruses c. Nucleocapsid d. Matrix proteins References 1. Levinson W (2016): Review of Medical Microbiology & Immunology, 14th Ed., McGraw-Hill Companies, Inc. ISBN: 978-0-07-181812-4, 2. AMBOSS Library https://next.amboss.com/us/questions/TABN6QWdX/5/article/WM0Pn g#Zd5f80c0b0f3a51e415ac24e6e49a8b77 https://next.amboss.com/us/questions/Cc55qeQkX/16 https://next.amboss.com/us/questions/Cc55qeQkX/17 Batterjee Medical College Thank You