Classification of viruses D.pptx .........

MEDICINE

1. Morphology and classification of viruses Dr. Muna M. A. Yousif M.D Clinical Microbiology

2. Overview Viruses are obligate intracellular organisms as they can not synthesis their own energy. Can not be cultured on artificial media only in cell cultures. Viruses are measured in nanometers (one billionth of a meter ) and range in size from 20 to 300nm (0.02 to 0.3 μ m) in diameter. Viruses cannot be seen with a light microscope because the resolution of a light microscope is limited to about 200nm, so a scanning electron microscope is used to view them. Virion is the complete infective form of a virus outside a host cell, with a core of RNA or DNA and a capsid.

3. Comparison of viruses and bacteria Property Viruses Cells Type of nucleic acid DNA or RNA Both DNA and RNA Proteins Few Many Lipoprotein membrane In enveloped viruses only Present Ribosomes Absent Present Mitochondria Absent In eukaryotic cells Enzymes None or few Many Multiplication by binary fission or mitosis No Yes

4. Structure of viruses The basic structure of the virus or the core is composed of genetic material (nucleic acids made of either DNA or RNA). The DNA or RNA maybe linear or circular, single stranded or double stranded. Surrounding the nucleic acids is an outer coat made of protein (capsid). The capsid is made of units called capsomers The arrangement of capsomers determines the architecture of the virus or the nucleocapsid (made of the capsid and the nucleic acid). C apsomer arrangement maybe: Icosahedral Helical Complex

5. Icosahedral Looks like a sphere but it actually has 20 triangles and 12 coroners fused together in a spherical shape. Examples of viruses with an icosahedral structure are the poliovirus, rhinovirus, and adenovirus

6. Helical The virus particle is elongated or pleomorphic (not spherical) and the nucleic acid is spiral. Examples of viruses with a helical nucleocapsid are influenza and measles viruses.

7. Complex These virus structures have a combination of icosahedral and helical shape The Poxvirus one of the largest viruses in size has a complex structure with a unique outer wall and capsid .

8. Structrure of viruses cont .’d The nucleocapsid of many viruses is surrounded by an envelope Viruses with no envelopes are called naked viruses The outer layer of the virus envelope may contain surface spikes or projections called glycoproteins The envelope of the virus is derived from the host cell membrane when the virus is exiting the cell via budding, and the infectivity of these viruses is mostly dependent on the envelope. E xamples of enveloped viruses are the influenza virus, Hepatitis C and HIV.

9. Viral structure

10. Comparison of enveloped and non-enveloped viruses

11. Nomenclature Family name ends in – viridae Genus name ends in –virus Example: Family: Herpesviridae Genus: Varicella virus Common name: Chickenpox virus Disease: Chickenpox

12. Viral growth curve One virion replicates inside an infected cell to give hundreds of virions in approximately 10 hours. In this way viruses can spread rapidly from one cell to another. First the virus disappears (but its nucleic acid continues to function and to accumulate within one cell ) The time during which no virus is found inside the cell is known as the eclipse period The eclipse period ends with appearance of the virus

13. Viral growth curve cont .’d

14. Viral replication Adsorption Penetration Uncoating Viral genome replication Assembly and release

15. Steps in Viral Replication Attachment . This is the first step in viral replication. Surface proteins of the virus interact with specific receptors on the target cell surface Penetration. Enveloped viruses (e.g., HIV, influenza virus) penetrate cells through fusion of the viral envelope with the host cell membrane . Non-enveloped viruses penetrate cells by translocation of the virion across the host cell membrane or receptor mediated endocytosis of the virion with accumulation of viruses in cytoplasmic vesicles. Uncoating (disassembly). The viral genome is uncoated by removing the capsid proteins, allowing the genome (nucleic acids)to be available for transcription and viral multiplication.

16. Steps of viral replication cont .’d 4. Genome replication. Viruses must use host cellular machinery to replicate and make functional and structural proteins. The first step in viral gene expression is mRNA synthesis. Different viruses follow different pathway depending on the nature of their nucleic acids and the part of the cell in which they replicate DNA viruses generally replicate in the nucleus and use host cell DNA- dependent RNA polymerase to synthesis their mRNA Most RNA viruses replicate in the cytoplasm where they do not have access to the host cell DNA- dependent RNA polymerase . They therefore carry their own polymerase within the virus particle

17. Steps of viral replication cont . ’d Assembly and Release . Progeny virus particles are assembled by packaging the viral nucleic acid within the capsid proteins. Virus particles are then released from the cell by either of two processes Non-enveloped viruses are release by rupture of the cell membrane and release of the mature particles Enveloped viruses are released by budding through the outer cell membrane (except herpesviruses which acquires their envelope from the nuclear membrane rather than from the outer cell membrane)

18. Overview Classification of viruses is based on chemical and structural criteria. Two important components of viruses used in classification are: Nucleic acids (DNA or RNA) Capsid (size, symmetry) and whether it is enveloped or non-enveloped

19. C lassification of DNA viruses Virus family Envelope present Capsid symmetry Size (nm) DNA structure Medically important virus Parvovirdae No Icosahedral 22 SS, linear Parvovirus B19 virus Polyomaviridae No Icosahedral 45 DS, circular JC, BK virus Papillomavirdae No Icosahedral 55 DS, circular Human papilloma virus Adenoviridae No Icosahedral 75 DS, linear Adenovirus Hepadnaviridae Yes Icosahedral 42 DS, incomplete circular Hepatitis B virus Herpesviridae Yes Icosahedral 100 DS, linear HSV, VZV, CMV, EBV Poxviridae Yes Complex 250X400 DS, linear Smallpox virus, Molluscum contagiosum virus

20. DNA viruses There are 7 families of DNA viruses. Four of them are non-enveloped, icosahedral viruses (Parvovirus, Polyomavirus , Papillomavirus and Adenovirus) Two of them are enveloped icosahedral (hepatitis B virus and herpesvirus ) Poxvirus is the largest virus, has an envelope and a complex internal symmetry.

21. Non-enveloped DNA viruses Parvovirus Polyomavirus Papillomavirus Adenovirus

22. Enveloped DNA viruses Hepadnaviruses Herpesviruses

23. RNA viruses The genome of all RNA viruses consists of single-stranded RNA except that of the reovirus family, which have double-stranded RNA genome. RNA viruses fall into four groups with different strategies for synthesizing mRNA Single-stranded RNA with positive polarity (their RNA has the same base sequence as that of mRNA). These viruses use their RNA genome directly as mRNA. Example: poliovirus Single-stranded RNA of negative polarity (their RNA has a base sequence that is complementary to the mRNA). Here an mRNA must be transcribed by using the negative strand as a template. Because the cell does not have an RNA polymerase capable of using RNA as a template, the virus carries its own RNA-dependent RNA polymerase. Negative polarity RNA viruses are either segmented (e.g., influenza virus) or non-segmented(e.g., measles virus)

24. 3. RNA viruses with double-stranded RNA as its genome (e.g., rotavirus). Because the cell has no enzyme capable of transcribing their RNA into mRNA, the virus carries its own polymerase. 4. Single-stranded RNA of positive polarity that is transcribed into double-stranded DNA by an RNA-dependent DNA polymerase (reverse transcriptase) which is carried by the virus. This DNA copy is then transcribed into viral mRNA by the regular host cell RNA polymerase. Example retroviruses (HIV)

25. Summary of classification of RNA viruses Virus family Envelope present Capsid symmetry Size (nm) RNA structure Medically important virus Picornaviridae No Icosahedral 28 SS linear, nonsegmented ,, + ve polarity Poliovirus, rhinovirus, HAV Hepeviridae No Icosahedral 30 SS linear, nonsegmented ,, + ve polarity Hepatits E virus Calcivirdae No Icosahedral 38 SS linear, nonsegmented ,, + ve polarity Norwalk virus Reoviridae No Icosahedral 75 DS linear, 10 or 11 segments Rotavirus Flaviviridae Yes Icosahedral 45 SS linear, nonsegmented ,, + ve polarity Yellow fever virus, West Nile virus, den gue virus Togaviridae Yes Icosahedral 60 SS linear, nonsegmented ,, + ve polarity Rubella virus Retroviridae Yes Icosahedral 100 SS linear, nonsegmented ,, + ve polarity HIV, human T-cell leukaemia virus

26. Classification of RNA viruses cont .’d Virus family Envelope present Capsid symmetry Size (nm) RNA structure Medically important virus Orthomyxoviridae Yes Helical 80-120 SS linear, 8 segments, negative polarity Influenza virus Paramyxoviridae Yes Helical 150 SS linear., n onsegmented , negative polarity Parinfluenza , measles, mumps, RSV Rhabdoviridae Yes Helical 75X180 SS linear., n onsegmented , negative polarity Rabies virus Filoviridae Yes Helical 80 SS linear, nonsegmented , negative polarity Marburg, Ebola Coronaviridae Yes Helical 100 SS linear, n onsegmented , positive polarity Coronavirus Arenaviridae Yes Helical 80-130 SS linear. 2 segments, negative polarity LCM, Lassa fever virus Bunyaviridae Yes Helical 100 SS linear. 3 segments, negative polarity Rift valley virus Californa encephalitis virus Deltaviridae Yes Helical 37 SS circular, closed circle, negative polarity Hepatits D virus

27. RNA viruses There are 15 families of RNA viruses. 4 are non-enveloped, icosahedral RNA viruses ( picornavirus , hepevirus , calcivirus and reovirus ) 3 are enveloped icosahedral viruses ( retrovirus , flavivirus and togavirus ) The remaining 8 are enveloped helical RNA viruses ( orthomyxoviruses , paramyxoviruses , rhabdovirus , filovirus , coronavirus, renavirus , bunyavirus and deltavirus )

28. Non-enveloped RNA viruses Picornavirus Hepevirus Calcivirus Reovirus

29. Enveloped RNA viruses Flavivirus (Icosahedral) Togavirus (Icosahedral) Retrovirus (Icosahedral) Orthomyxovirus (Helical) Paramyxovirus `(Helical) Rhabdovirus (Helical) Filovirus (Helical) Coronavirus (Helical) Arenavirus (Helical) Bunyavirus (Helical) Deltavirus (Helical)

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