Transcript Virus defining characteristics

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)
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)
Virus Basic Shapes:
Spherical/icosahedral
Cylindrical/helical
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)
Host RNApol
Viral mRNA
Virus genes expressed with help from host
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
LYTIC CYCLE
INFECTION
Phage attaches to bacterium
PHAGE injects DNA into bacterium
Bacterial DNA degraded PHAGE DNA synthesis
HEAD ,TAILS, FIBRES
made
PHAGE ASSEMBLY
LYSIS
LYSOGENIC CYCLE
INFECTION
Phage attaches to bacterium
PHAGE injects DNA into bacterium
PHAGE genome inserted into Bacterial
chromosome Phage remains “dormant”
prophage
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 damaged
So, 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 coli
Staphylococcus aureus
Haemophilus influenza
Eco RI