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Viruses
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Viruses

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  1. Viruses

  2. What is a virus? • Viruses are not alive • A virus in an obligate intracellular parasite • Requires host cell to reproduce • Can be seen at magnifications provided by the electron microscope (they are microscopic)

  3. Characteristics of Viruses • 1.) Contains a single type of nucleic acid: either DNA or RNA but not both • 2.) Has a protein coat (capsid) surrounding the nucleic acid, some also have a lipid envelope around the capsid • 3.) multiply inside living cells by using the synthesizing machinery of the host cell • 4.) Cause the synthesis of specialized viral structures that can transfer the viral nucleic acid to other cells • 5.) Have a specific host range

  4. Size of a Virus • Usually much smaller than bacteria • must be smaller than the cells they infect: • 20-14,000nm in length

  5. Size Continued

  6. Structure of viruses • Virion = infectious viral particle: completely assembled with a protein coat surrounding the nucleic acid • All viruses are made of at least 2 parts • Inner core of nucleic acid • Enclosed in protein capsid • Some also contain lipoprotein envelope

  7. Structure of viruses • 1.) Nucleic Acids: • Either DNA or RNA, but not both • Single or Double Stranded (SS or DS) • if RNA, it can be plus sense strand (has codons) or minus/antisense (need to make complement sense strand for translation) • If DNA- usually double stranded • Linear or circular • Genome is SMALL • Only a few genes (most have 6-10 genes)

  8. Structure of viruses continued • 2. Capsid – protein coat (protein shell) • Surrounds the nucleic acid • protects the virion in the external environment • Aids in transfer between host cells • Composed of subunits called capsomeres • some capsids have protein-carbohydrate pointed projections called pentons • if pentons are present they are used for attachment to the host cell

  9. Structure of viruses • 3. Envelope (not all viruses) • Function is to protect the virion • some viruses have an envelope around the capsid consisting of lipids, proteins and carbohydrates (cell membrane like) • with envelope = enveloped virus • the envelope may be coded for by the virus or taken from the host cell plasma membrane • some envelopes have carbohydrate-protein complexes called spikes which are used for attachment to the host cell • if a virus does not have an envelope it is called a non-enveloped virus, “naked”

  10. Enveloped virus

  11. Non-enveloped virus Capsomere protein

  12. Morphology/symmetry • The capsid can be distinct and sometimes identifies a particular virus. It is constructed in a highly symmetrical manner • Helical • Cylindrical capsid, hollow • Can be rigid or flexible • Made up of a helical structure of capsomeres with the nucleic acid wound up inside • Examples: Rabies virus, Ebola virus, tobacco mosaic virus (TMV) Rabies Virus

  13. Morphology/Symmetry • Polyhedral • Most are icosahedrons (icosohedral) • 20 equilateral triangle faces and made from capsomeres • 12 corners made form capsomeres called pentons which contain 5 protomers each • Appear spherical • Examples: Adenovirus, Polio virus Polio virus

  14. Morphology/Symmetry • Complex • Several types of symmetry in one virus • Unique shape • Examples: • Bacteriophage: capsid and acessory structure • Pox virus: no clear capsid, just several protein layers around the nucleic acid Glass sculpture of pox virus

  15. Replication • Replication must occur in a host cell (multiply only when inside a living cell) • The viral genome codes for viral structural components and a few viral enzymes needed for processing the viral enzymes • Everything else is supplied by the host: • Ribosomes, tRNA, nucleotides, amino acids, energy etc. • The DNA or RNA of the virus takes control of the host cell' metabolic machinery and new viral particles are produced utilizing the raw materials from the host cell.

  16. Replication • Replication of viruses is studied in great detail in bacteriophages • Bacteriophages are viruses that infect a specific bacteria • Two possible types of infection cycles: • 1.) Lytic cycle (virulent) • Ends with the lysis and death of the host bacterial wall • 2.) Lysogenic cycle • Host cell remains alive, but carries the virus in its genome

  17. Lytic Cycle • 1.)Attachment- phage contacts a bacterium (attachment to host) and uses the tail fibers to attach to proteins on the bacterial cell wall

  18. Lytic Cycle • 2.) Penetration/Entry- the phage injects its DNA into the bacterium • The phage tail releases lysozyme to break down the bacterial cell wall • The sheath contracts to drive the tail core through the weakened cell wall and plasma membrane • The DNA is injected into the bacterium through the tail core • Uncoating- During or before penetration • 3.) Synthesis of new virus particles (Multiplication) • Once inside, host protein synthesis is stopped • Virus has host make proteins and nucleic acid • Virus directs viral nucleic acid replication and transcriptions and translation of viral genes (host’s cell transcription stops) • This results in a pool of viral genomes and capsid parts

  19. Lytic Cycle • 4.) Assembly • “eclipse period” – the time of viral entry • The bacteriophage DNA and capsid spontaneously assemble into complete virons • 5-10 hrs DNA viruses • 2-10 hrs RNA viruses

  20. Lytic Cycle • 5.)Lysis- release of virus and death of host cell • A single virus can give rise to up to 1000 new virus particles from on host cell • Virions will leave bacteria (host) • Lysozyme encoded by viral genes causes the cell wall to break down • The bacteria lyses releasing the virions • Cycle will then repeat with new phages

  21. The Lysogenic Cycle • The lysogenic phage infects the cell, but remains inactive in a stage called lysogeny • 1.) the phage attaches to the host cell and injects DNA • 2.) the phage genome circularizes • At this point, the phage could begin a normal lytic cycle or it can begin the lysogenic cycle/lysogeny

  22. The Lysogenic Cycle • Latency- “dormant” state- unpredictability • Viral DNA/RNA  integrated into DNA of host = hidden DNA=provirus • Can be reactivated in the future • Factors that influence: stress, other viral infections, UV light • Example: fever blisters, chicken pox, HIV 2+ years

  23. Cultivation of Viruses for Study • Embryonated eggs • Refer to handout given in class

  24. Cell Cultures • Refer to handout given in class

  25. Animal Models • Refer to handout given in class

  26. DNA Viruses • Parvoviruses: • Smallest of DNA viruses • ss DNA • Animals: canine parvovirus • Humans: PV B19 contagious erythema infectiosum