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Viruses of Bacteria

Viruses of Bacteria

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Viruses of Bacteria

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  1. Viruses of Bacteria Chapter 13

  2. General Characteristics of Viruses • Non-living entities • Not considered organisms • Can infect organisms of every domain • All lifeforms • Commonly referred to by organism they infect

  3. General Characteristics of Viruses • Virus architecture • Virus particle called virion • Consists of nucleic acid surrounded by protein coat • Protein coat termed capsid • Capsid composed of capsomers • Virus have different shapes • Isometric • Helical • Complex • Two types of virion • Naked – without envelope • Enveloped – surrounded by lipid membrane

  4. Viral geneome Contains only single type of nucleic acid Either DNA or RNA NEVER BOTH Can be linear or circular Single-stranded or double stranded Replication cycle overview Only multiply inside metabolizing cell Uses host machinery to support reproduction Every virus contains information to make viral proteins, assure replication and move in and out of host cells Viruses live in two phases Extracellular phase Metabolically inert Intracellular phase Metabolically active General Characteristics of Viruses

  5. Virus Interactions with Host Cells • Effect on cells depends on infecting phage • Some phage multiply inside cell producing numerous progeny • Termed productive cycle • Lytic cycle • Phage lyse infected cell • Some phage integrate into host genome • Termed latent cycle • Lysogenic state

  6. Virus Interactions with Host Cells • The six stages of the lytic cycle are: • Adsorption • Penetration • Transcription • Replication • Assembly (or maturation) • Release

  7. Step 1: Attachment The phage attach to specific receptors on the cell wall of E. coli. Bacterial DNA Adsorption (Attachment)

  8. Penetration • Penetration • Entrance of the virus OR its nucleic acid in the host cell • Plant and bacteria viruses inject the nucleic acid into the host through the cell wall • Animal viruses enter the cell whole • Animal cells have no rigid cell wall • Penetration of the virus is through: • Phagocytosis in which the virus is engulfed by the cell • Membrane fusion occurs with enveloped viruses when the viral envelope fuses with the plasma membrane of the host cell • Viruses enter intact but require an uncoating step to release the nucleic acid from the protein coat

  9. Step 1: Attachment The phage attach to specific receptors on the cell wall of E. coli. Bacterial DNA Step 2: Penetration Following attachment, phage DNA is injected into the bacterial cell, leaving the phage coat outside. Penetration

  10. Transcription/ReplicationLytic cycle • Transcription/Replication • Duplication of viral components • During replication: • Virus will inhibit activity of the host DNA • Virus produces enzymes to destroy host DNA • Viral DNA takes over and begins producing proteins • Early viral proteins are synthesized and are associated with the replication of viral nucleic acid • Late viral proteins are synthesized and are associated with the replication of other viral structures

  11. Step 1: Attachment The phage attach to specific receptors on the cell wall of E. coli. Bacterial DNA Step 2: Penetration Following attachment, phage DNA is injected into the bacterial cell, leaving the phage coat outside. Step 4: Replication of Phage DNA and Synthesis of Proteins Phage coat proteins, other protein components, and DNA are produced separately. Host DNA degraded. Phage-induced proteins Step 3: Transcription Phage DNA is transcribed, producing phage mRNA, which is translated to phage proteins. DNA RNA Transcription/Replication

  12. Assembly • Assembly (or maturation) • This stage is the assembling of the replicated viral components into an intact, mature virus

  13. Step 1: Attachment The phage attach to specific receptors on the cell wall of E. coli. Bacterial DNA Step 2: Penetration Following attachment, phage DNA is injected into the bacterial cell, leaving the phage coat outside. Step 4: Replication of Phage DNA and Synthesis of Proteins Phage coat proteins, other protein components, and DNA are produced separately. Host DNA degraded. Step 5: Assembly Phage components are assembled into mature virions. Phage-induced proteins Step 3: Transcription Phage DNA is transcribed, producing phage mRNA, which is translated to phage proteins. DNA RNA + + + Empty head DNA inside head Assembly

  14. Release • Release • Host cell bursts and releases viruses to the outside environment • Viruses are now extracellular • As virus leave the host cell the envelope is picked up • The envelope is made of a portion of the host cell plasma membrane which becomes the lipid envelope of the virus

  15. Step 6: Release The bacterial cell lyses and releases many infective phage. Step 1: Attachment The phage attach to specific receptors on the cell wall of E. coli. Bacterial DNA Step 2: Penetration Following attachment, phage DNA is injected into the bacterial cell, leaving the phage coat outside. Step 4: Replication of Phage DNA and Synthesis of Proteins Phage coat proteins, other protein components, and DNA are produced separately. Host DNA degraded. Step 5: Assembly Phage components are assembled into mature virions. Phage-induced proteins Step 3: Transcription Phage DNA is transcribed, producing phage mRNA, which is translated to phage proteins. DNA RNA + + + Empty head DNA inside head Release

  16. Virus Interactions with Host Cells • Lysogeny • Replication of a temperate virus • This is a non productive cycle • Lysogeny begins like the lytic cycle • Adsorption • Penetration, then; • Incorporation • http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html

  17. Virus Interactions with Host Cells • Incorporation • Viral nucleic acid incorporates onto the host chromosome • This virus is called a prophage • Once incorporated, repressor genes are expressed and repressor proteins are produced • These hide or suppress the viral gene from host immune responses • The viral DNA replicated only when the host cell replicates • This allows for a population of bacterial cells that carry viruses • Cell eventually “pops” off the host chromosome and returns to the lytic cycle

  18. Virus Interactions with Host Cells • Lysogenic conversion • Prophage can confer new properties on cell • Phage DNA not completely suppressed • Genes coding for trait are expressed • Organism displays new trait • Streptococcus pyogenes manufactures toxin resulting in scarlet fever due to lysogenic conversion

  19. Host Ranges of Phages • Number of different bacteria that phage can infected termed host range • Usually limited to single bacterial species for a single phage • Factors limit host range • Two most important • Phage must be able to attach to host receptors • Restriction-modification system host cell must overcome

  20. Host Ranges of Phages • Receptors on bacterial surface • Vary in chemical structure and location • Usually on bacterial cell wall • Sites can be altered by two mechanisms • Receptor sites can be altered by mutation • Lysogenized bacteria can alter cell surface • Results in alteration of receptor site

  21. Host Ranges of Phages • Restriction-modification system • Bacteria have two genes coding for enzymes of restriction-modification system • Restriction enzyme that codes for endonuclease • Cuts small segments of DNA • May recognize viral DNA and cut it • Modification enzyme attaches methyl group to DNA recognized by restriction enzyme • Methylated bases not recognized by restriction enzyme • Protects cells own DNA