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