Bacteriophages ( a.k.a. Phages) Viruses that target bacteria Virus defining characteristics: parasitic entities Nucleic acid molecules protected by protein

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  • Slide 1
  • Bacteriophages ( a.k.a. Phages) Viruses that target bacteria Virus defining characteristics: parasitic entities Nucleic acid molecules protected by protein coat No metabolism, need host cell for their replication Value: Biological: in bacterial genetics, transduction Research: source of knowledge for cell biology processes Tools for recombinant DNA tech. e.g. Ligase (T4)
  • Slide 2
  • Virus Basic Shapes : Spherical/icosahedralCylindrical/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)
  • Slide 3
  • 7.1 Bacterial Viruses (Bacteriophages/ phages ) common designs
  • Slide 4
  • 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
  • Slide 5
  • Adsorption: Recognition & attachment to host receptor (surface) Deliver of genome into host cytoplasm
  • Slide 6
  • Expression of viral early genes (host take over) Virus genes expressed with help from host Host RNApol Viral mRNA Viral Protein Attacks host DNA
  • Slide 7
  • Host DNA degraded Phage takes over the bacteria & turning it into a virus replication factory
  • Slide 8
  • Many copies of Virus DNA are made 1 st Then the capsids are made
  • Slide 9
  • Viral DNA is Stuffed into Capsid (heads) Final Assembly of the Viral Progeny
  • Slide 10
  • LYSIS PHAGE ASSEMBLY HEAD,TAILS, FIBRES made Bacterial DNA degraded PHAGE DNA synthesis PHAGE injects DNA into bacterium Phage attaches to bacterium INFECTION LYTIC CYCLE
  • Slide 11
  • PHAGE genome inserted into Bacterial chromosome Phage remains dormant prophage PHAGE injects DNA into bacterium Phage attaches to bacterium INFECTION LYSOGENIC CYCLE Host Cell replicates & each daughter cell gets a copy of the prophage INDUCTION: prophage becomes active goes into lytic cycle
  • Slide 12
  • 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
  • Slide 13
  • 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 damaged So, Why not Destroy the phage DNA before it has the chance to take over the bacteria HOW? Cut it up Restriction Endonucleases
  • Slide 14
  • Each bacteria produces its own kind of Restriction Endonuclease Escherichia coli Staphylococcus aureus Haemophilus influenza Eco RI
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