bacteriophages ( a.k.a. phages) viruses that target bacteria virus defining characteristics:...
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Bacteriophages ( a.k.a. Phages) Viruses that target bacteria
Virus defining characteristics: parasitic entitiesNucleic acid molecules protected by protein coatNo metabolism, need host cell for their replicationValue:Biological: in bacterial genetics, transductionResearch: • source of knowledge for cell biology processes• Tools for recombinant DNA tech. e.g. Ligase (T4)
Virus Basic Shapes:
Spherical/icosahedral Cylindrical/helical
STRUCTURAL FEATURES OF VIRUSES:Genome:•Either DNA or RNA only•Circular or linear genome•Single or doubles stranded•Codes for Virus structural and replicative elements
Capsid:•Proteins in different arrangements•Enclosing genome, contact point for host receptor•May have an envelope (rare in phages)
7.1 Bacterial Viruses (Bacteriophages/ phages ) common designs
General Events in a Phage Lifecycle
Adsorption:• Recognition & attachment to host receptor (surface)• Deliver of genome into host cytoplasm• Expression of viral early genes (host take over)
Replication:• Many copies of viral genome produced first• Production of capsid and other viral elements
Assembly:Capsid assembly, packaging of viral genome into capsid
Release:Host lysis (usually) release of virions
Adsorption:Recognition & attachment to host receptor (surface)Deliver of genome into host cytoplasm
Expression of viral early genes (host take over)
Virus genes expressed with help from host
Host RNApol
Viral mRNA
Viral Protein Attacks host DNA
Host DNA degraded
Phage takes over the bacteria
& turning it into a virus replication factory
Many copies of Virus DNA are made 1st
Then the capsids are made
Viral DNA is Stuffed into Capsid (heads)
Final Assembly of the Viral Progeny
LYSIS
PHAGE ASSEMBLY
HEAD ,TAILS, FIBRES
made
Bacterial DNA degraded PHAGE DNA synthesis
PHAGE injects DNA into bacterium
Phage attaches to bacteriumINFECTION
LYTIC CYCLE
PHAGE genome inserted into Bacterial chromosome Phage remains “dormant” prophage
PHAGE injects DNA into bacterium
Phage attaches to bacteriumINFECTION
LYSOGENIC CYCLE
Host Cell replicates & each daughter cell gets a copy of the prophage
INDUCTION: prophage becomes active
goes into lytic cycle
Transduction: transfer of bacterial DNA between bacterial cells mediated by phage, not benefice for phage
GENERALIZED: random bacterial DNA pieces
E.g. Phage T4
SPECIALIZED: bacterial DNA transferred by phage is always from same location in chromosome
E.g. Phage Lambda
HOW DOES BACTERIA PROTECT THEMSELVES FROM A PHAGE ATTACK?
Phage DNA is injected into bacterial cytoplasm, but …
at that point phage DNA is most vulnerable and could be damagedSo, Why not …Destroy the phage DNA before it has the chance to
take over the bacteria HOW? Cut it up Restriction Endonucleases
Each bacteria produces its own kind of Restriction Endonuclease
Escherichia coliStaphylococcus aureusHaemophilus influenza
Eco RI