BACTERIOPHAGES

     Bacteriophage, normally called phage, is viruses that infect prokaryote. They are obligate intracellular parasites that are capable of existence as phage particles outside the host cell but can only reproduce inside the cell.

    They consist of a nucleic acid genome surrounded by a protein coat called a capsid.

     Phages possess the ability to infect a prokaryote and redirect the cell to synthesize phage components.

STRUCTURE OF BACTERIOPHAGE:

    • Phage, like viruses, have number of shapes

    • The genome can be DNA or RNA, single stranded or double stranded, circular or linear.

    • Some phage are very small such as E.coli, single-stranded RNA phage, MS2, whose genome contain information for only for proteins, while others are large likeT4 whose genome encodes around 135 different proteins.

PHAGE LIFE CYCLE:

            • A life cycle of phage starts by the adsorption of the phage onto the specific receptor on the bacterial cell surface followed by the penetration of the genetic material into the host.

            • Enzymes encoded by the phage genome are synthesized, which is followed by nucleic acid replication.

      • Finally, phage capsid proteins are made and assembled into a new phage coat, at the same time packaging a copy of the genome.

      • The phage are released, normally by lysis, into the surrounding medium.

LYSOGENIC LIFE CYCLE:

              • Some phage, a typical example being phage λ, instead of starting the lytic cycle on entry into the cell, produce repressor protein that stop the phage replication.

       • The phage enters a state call lysogeny in which genome is replicated at the same time as the host chromosome and is passed from one generation to the next.

       • the phage may be induced from this lysogenic state spontaneously and enter into the lytic cycle.

       • In order to replicate with the host, most lysogeny phage integrate into the chromosome to form a prophage but some like, P1, exist in the cytoplasm in a form similar to plasmids.