BACTERIOPHAGE

                                                       



Bacterophage  :  these  are  viruses  that  attack  and  infect  bacteria  cell.  Like  other  type of  viruses  bacteria phage  vary a lot  in  their  shape  and  genetic  makeup.  Phages were first discovered in  1915  by  Fredric  Twort   in  Egghead  and  in 1917  by  Felix  d’ here’ll in  France  .  d here’ll  named  them  as  bacteria phage  which  means  eaters of  bacteria . In order  to  survive  bacteria phage  requires  bacteria environment. They  basically  become  living when they  invade a  host  bacterium, Example. Temperate  phages cmu and Lambda filamentous and spherical phage.

STRUCTURE  OF  BACTERIOPHAGE
The  virile  of T-even  phage  is   binary  or  tadpole  like  structure  with   a polyhedral  head  connected  to  a  helical  tail through  a  short  collar.  The head composed  of  nucleic  acid  material.  Phage  genomes    can  consist  either  DNA or RNA and  can  contain  as  few  as   four  genes or  as  many  as  several  hundred  ( 50nmlong ). The  tail has  an  inner  hollow  tube  called  core,  surrounded  by  a  contractile  sheath  which  consist  of  24 annular mugs. The distal  end of  the tube is  connected to  a   hexagonal  basal plate with  spike  or  tail  spin  at  each  Conner six  long, flexible also  arise   from   the  basal  plate   which  help  in   adsorption  to  bacteria.

LYTIC CYCLE
Lytic  is  from  Greek  word  lyein , meaning  to  “break  down”. The host cell is  destroyed  during  a lytic cycle.  Once inside the host  cell (phage  DNA) a virus  genes  are  expressed  and  the  substances  that   are  produced  take  over  the host  cell genetic  material.  The  viral  genes uses its  own  enzymes,  raw  materials  and  energy  to  make  copies  of  viral  genes  that  along  with   protein  are  assembled  into  new  virus  which  burst  from  the  host  cell  killing  it.  The new viruses can then infect and kill other host cells. This process is what is called a lytic cycle. Hence  lytic  cycle  is  the  bacteria phage  life  cycle  in  which  the  phage  nucleic  acid  enters  the  host  cell (bacteria  cell),  infect  the  cell  and  cause  death  of  the host  cell.  The processes occur as illustrated below.

The  bacteriophage  approaches  the  bacterium  host  cell  (attachment); phage  approaches  bacteria  and  tail  fibers  fit  into  receptor  sites on  bacterial cell  surface.



The  bacteriophage  inject  its   nucleic  acid  (DNA) into  the  bacterial  cell;  tail  fibers bend  to  anchor  the  pins  and  base plate  to  the  cell  surface,  tail  sheath  contracts,  forcing  hollow  spike  into  cell,  enzyme  lysozyme  in  base plate aids  process.  DNA thus   injected into cell.





Phage  DNA codes  for  production  of  phage  enzymes  using  protein  synthesizing  machinery  (ribosome  of  the host )




Phage  inactivates  host  DNA and  phage  enzymes  breaks  it  down,  phage DNA takes  over  cell  machinery


   

The  hosts  metabolic  machinery  makes  viral  nucleic  acid  (DNA) and protein [replication]; phage DNA replicates itself  and  codes for  new  protein coat.



New  virus  particles assembly  (assembly ); new phage  particle  made by  spontaneous assembly of  protein  coats around  phage  DNA;  lysozyme is  made phage  DNA




The host cell breaks, open and releases new virus particles (LYSIS and RELEASE ), cell lysis i.e. bursting assisted by action of  lysozyme; About 200-1000 phages released. Phages infect further bacteria.




LYSOGENIC CYCLE
Lysogenic cycle this is a replicating cycle in which the virus’s nucleic acid is integrated into the host cells chromosomes. The viral nucleic acid is integrated into the genetics materials of the host cell and replicates with it for a while before entering a lytic cycle i.e. the viruses attaches to the host cells plasma membrane and its nucleic acid enter the cell. However in a lysogenic cycle, instead of immediately taking over the  hosts genetic material, the viral DNA is integrated into the host cell’s chromosomes. Viral DNA that is integrated into the host cell’s chromosomes is called a PROVIRUS. A provirus may not affect the functioning of its host cell ,which continues to carry out its own metabolic activity. However, every time the host cell reproduces, the provirus is replicated along with the host cell’s chromosomes. Therefore every cell that originated from an infected host cell has a copy of the provirus. The lysogenic phase can continue for many years. The processes occurs in the following stages.

i)Attachment and entry of phage nucleic acid the bacteriophage; the bacteriophage approaches the bacteria host cell and inject its nucleic acid (DNA) into the bacteria cell. (This is as it is in the lytic  cycle in first and  second stage)




(ii)The viral DNA intergrate with host DNA (provirus formation) the phage DNA attach itself to the host DNA and become part and parcel of the host DNA. The viral nucleic acid (DNA) is the called provirus when it becomes part of the hosts chromosomes.



(iii)Cell division, although the provirus is inactive, it replicates along with the host cell’s (DNA) chromosomes



(iv)When provirus is activated its chromosomes leaves the host DNA and virus nucleic acid (DNA) and proteins are made start its lytic life cycle and cause death to bacteria cell.



REFERENCE
SARITHA A. (2013). A text book of CBSE biology for class ix first Education. India. Nirja  publisher and printer.
DJ.TYLOR, N.P.O GREEN and G.W. STOUT (2012) Biological science 182 third Edition .Britain Cambridge university press
Alton Biggs, Kathleen Gregg, W.C. Hagins, kapicka and linda lundgren (2002). The……of life united state of America.
           

Prepared by... GSK, TEE, GKH, & MMZ