viruses of bacteria chapter 13. general characteristics of viruses non-living entities not...
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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
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
General Characteristics of Viruses
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
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
The six stages of
the lytic cycle are: Adsorption Penetration Transcription Replication Assembly (or
maturation) Release
Virus Interactions with Host Cells
Adsorption (Attachment)
Bacterial DNA
Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.
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
Penetration
Bacterial DNA
Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.
Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.
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
Transcription/Replication
Bacterial DNA
Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.
Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.
RNA
Phage-inducedproteins
Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.
DNA
Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.
Assembly
Assembly (or maturation) This stage is the assembling of the replicated
viral components into an intact, mature virus
Assembly
Bacterial DNA
Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.
Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.
RNA
Phage-inducedproteins
Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.
DNA
Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.
Step 5: AssemblyPhage components areassembled intomature virions.
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DNA insidehead
+ +
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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
Release
Bacterial DNA
Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.
Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.
RNA
Phage-inducedproteins
Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.
DNA
Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.
Step 5: AssemblyPhage components areassembled intomature virions.
Emptyhead
DNA insidehead
+ +
+
Step 6: ReleaseThe bacterial celllyses and releasesmany infective phage.
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/007255678
1/student_view0/chapter17/animation_quiz_2.html
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
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
Virus Interactions with Host Cells
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
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
Host Ranges of Phages
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