microbiology - Virus introduction, reproduction

Virus introduction, reproduction

1. Viruses MICROBIOLOGY

2. Simple, ultra microscopic, sub cellular no protoplasm, cell or cell organelles, energy generating Cannot grow only replicate Formed of proteins and DNA or RNA Not free living but host specific, obligatory parasites, infect other organisms Multiply only in living cell not cultured in artificial or synthetic media otherwise inert Genetic material DNA or RNA never both, so two types DNA virus and RNA virus Viral nucleic acid- single stranded or double stranded, coating- capsid , subunits- capsomeres , capsid & nucleic acid form nucleocapsid “Eclipse phase ”- lifecycle during which virus particles disintegrates to its molecular constituents towards end progeny formed Size: largest – pox viruses 350nm Smallest – 20nm diseases in foot and mouth Characteristics

3. Spherical : arbovirus , causes influenza parotitis , japanese encephalitis, measles 10-150nm Rod-shaped : TMV, Potato blight virus 300nm long15nm wide Cuboidal virus : vaccinia viruses, adenovirus, reovirus Spermatozoid virus : bacterial virus Form and shape

4. Are virus living or non living?

5. Possess genetic material and proteins have inherent potential for replication, propagation and mutation Border line of living and non living No cell, cell organelles, energy metabolism outside living cell inert cannot grow only multiply crystallised to dormant dry inert form Living

6. Sporadic eg . Mumps Endemic eg . Rabies Epidemic eg . Measles Pandemic eg . Influenza Viral diseases

7. Classification of virus Based on natural host Phytoviruses , Zooviruses , Mycoviruses , Phycoviruses , Bacterial viruses, Mycoplasma viruses Not satisfactory. Why ? Aspects considered : Kinds of nucleic acids Strandedness of nucleic acids(SS or DS) Capsid morphology (Size, symmetry, no. of capsomeres ) presence or absence of envelop Range of host 1962 – lwoff , Horne and Tournier – LHT System of classification Based on physical and chemical nature, range of natural host

8. ICTV (International Committee on Taxonomy of Virus) Three major aspects : 1.Kind of nucleic acid present (DNA or RNA) 2. Strandedness of the nucleic acid (single or double) 3. Pre. Or abs. of additional envelope 8 Clusters of classification Enveloped and single-stranded DNA viruses Non Enveloped and single-stranded DNA viruses Enveloped and double-stranded DNA viruses Non Enveloped and double-stranded DNA viruses Enveloped and single-stranded RNA viruses Non Enveloped and single-stranded RNA viruses Enveloped and double-stranded RNA viruses Non Enveloped and double-stranded RNA viruses

9. Classification based on Icosahedral viruses ( eg . Adenoviruses) Helical viruses ( eg . TMV) Complex virus ( eg . Bacteriophage ) Enveloped virus ( eg . HIV) Non enveloped viruses ( eg . Adenoviruses) Lytic cycle eg . T4 bacteriophage Lysogenic cycle eg . Lambda phage Morphology Replication strategy

10. Animal viruses eg . HIV Plant viruses eg . TMV Bacteriophages eg . T4 phage Archaea viruses eg . Sulfolobus spindle-shaped virus Respiratory viruses (RSV, influenza virus) Gastrointestinal virus eg . Rotavirus Neurological virus eg . Rabies virus Systemic viruses eg HIV, Ebola virus Host range Disease Association

11. Baltimore classification (first defined in 1971) seven groups Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals and discriminate viruses depending on their mode of replication and genome type. Depend on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication. Baltimore classification

12. I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses , Poxviruses) II: ssDNA viruses (+)sense DNA (e.g. Parvoviruses ) III: dsRNA viruses (e.g. Reoviruses ) IV: (+) ssRNA viruses (+)sense RNA ( e.g. Picornaviruses , Togaviruses ) V: (−) ssRNA viruses (−)sense RNA ( e.g. Orthomyxoviruses , Rhabdoviruses ) VI: ssRNA -RT viruses (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses) VII: dsDNA -RT viruses (e.g. Hepadnaviruses )

13. Architecture of viruses Structural organization Varies depending on the type of virus Main components of virus * Viral genome : Genetic material either dsDNA ( herpesviruses , poxviruses , adenoviruses) or ssRNA (HIV, Corona viruses), linear , circular , segmented. Contains only exons or coding sequences * Viral capsid : A protein shell that encloses the genome, providing protection and structural support capsid made of smaller units called capsomeres . Capsid and nucleic acid core together constitute : Nucleocapsid Three major functions: - protect nucleic acid from inactivation by nucleases and other degrading sub. -introduces the viral genome -determine antigenic specificity * Helical :TMV, Icosahedral , Complex : eg . Pox viruses, Filamentous * Viral Envelope : A lipid bilayer membrane, derived from host cell, that surrounds the capsid and helps with entry into host cells - protect virus from degradation by enzymes -Facilitates a smooth entry of the virus into the host cell -confers on virus chemical, biological and antigenic or infectious properties * Nucleocapsid : combination of genome and capsid

14. Occur by Duplication or self replication Occurs in living host cell Governed by genetic information contained in nucleic acid Uses synthetic machinery of their host Six sequential steps in virus multiplication cycle - Adsorption - Penetration - Uncoating - Biosynthesis - Maturation - Release Viral multiplication

15. Attachment of virus to specific surface receptors or receptor site of host cells Cell type-specific e g . Hepatitis virus absorb only on liver cells e g . Influenza virus receptor sites are formed of glycoprotein e g . Polio viruses formed of lipoprotein Transmembrane passage of whole virus or its nucleic acid into host cell. Bacterial cell –rigid and impervious cell wall so whole bacterial virus cannot penetrate into it- nucleic acid alone introduced into bacterium- leaving other components behind by Complex mechanism Complex mechanism – with the help of tail fibres and lytic enzyme of phage- enzyme make hole-viral genome is injected into bacterium Animal cell lack rigid cell wall therefore whole virus can enter by simple mechanism called viropexis 1. Adsorption 2. Penetration

16. Viral capsid is striped off, releasing nucleic acid into host cell. Occurs after complete penetration into cell effected by action of lysosomal enzymes of host cell poxvirus – uncoating is completed in two steps First step: outer part of capsid removed by lysosomal enzymes in the phagocytic vesicle Inner capsid and nucleic acid are released to cytoplasm Second step: Inner capsid is removed by enzymatic action, liberating DNA 3. Uncoating

17. 4. Biosynthesis Synthesis of viral nucleic acid, capsid proteins Involves replication, transcription and translation of viral genome Major steps -Integration of viral genome into host cell genome - Transcription of mRNA - Translation of mRNA - Replication of viral nucleic acids - Synthesis of structural ‘late protein’ which contain components of daughter virions

18. * Assembling of viral components (viral nucleic acids and viral protein) to form daughter virions - take place in nucleus ( Hebrpes viruses, adenovirus) or cytoplasm ( picoma viruses, pox viruses) of host cell - when occur in Nucleus- envelope derived from nuclear membrane - when occur in cytoplasm- envelope derived from plasma membrane * liberation of progeny virions from host cell -Bacterial virus – take place by lysis of infected bacterium -Animal virus – no lysis of host cell – host cell unaffected-go on dividing – daughter cell release virions - exception –poliovirus causes damage to host cell released by cell lysis 5. Maturation 6. Release

19. 1 . dsDNA Viruses (Double-stranded DNA Viruses) - Replication: - These viruses have double-stranded DNA genomes. - Replication occurs in the host cell’s nucleus. - The viral DNA is replicated using the host cell’s DNA polymerase enzymes. - The replication process is similar to the host’s DNA replication. - Transcription: - Transcription of viral genes also occurs in the nucleus. - Host RNA polymerase II transcribes viral DNA into mRNA. - The mRNA is then translated into viral proteins in the cytoplasm. - **Examples:**- Herpesviruses - Adenoviruses - Poxviruses (replicate in the cytoplasm) Replication and transcription of virus

20. 2 . ssDNA Viruses (Single-stranded DNA Viruses) - **Replication:** - These viruses have single-stranded DNA genomes. - After infection, the ssDNA is converted into a double-stranded form by host DNA polymerase. - The double-stranded DNA is then used as a template for replication. - **Transcription:** - Transcription occurs after the ssDNA is converted to dsDNA . - Host RNA polymerase transcribes the dsDNA into mRNA. - The mRNA is translated into viral proteins. - **Examples:** - Parvoviruses

21. Discover by Twort 1915 In detail by De Herelle 1917 Large abundant in enteric bacteria Four major groups - ssDNA , dsDNA , ssRNA , dsRNA

22. Lytic and Lysogenic cycle