T-even Bacteriophages that attack the bacterium E.coil cause lysis of the cells and are called virulent phages . They show lytic cycle , which is a five step process involving (a) attachment (b) penetration (c ) Biosynthesis (d) maturation and (e ) release
(a) Attachment : - The tail fibres of phages help in attachment to the complementary receptor sites on the bacterial cell wall .
(b) Penetration :- The tail sheath of phage contracts and the tail core is driven in through teh bacterial cell wall . When the tip of the core reaches the plasma membrane , the DNA from the bacteriophage head passes through the tail core through the plasma membrane and enters the bacterial cell . The capsid remains outside the bacterial cell and is called ghost . Thus the phage particle functions like hypodermic syringe and injects its DNA into the bacterial cell .
(c ) Biosynthesis :- When the phage DNA reaches the cytoplasm of the host cell , many copies of phage DNA, enzymes and capsid proteins are synthesized using the cellular machinery of the host cell .
(d) Maturation :- Bacteriophage DNA and capsids are assembled into complete virions .This period of time between the infection by a virus and the appearance of mature virions is called Eclipse period .
(e ) Release :- The plasma membrane of the hst cell gets dissolved or lysed due to viral enzymes , lysozyme . The bacterial cell wall breaks, releasing the newly produced phage particles or varions .
Some Bateriophages such as `lambda ` phages do not cause lysis and death of host cell when they multiply .Instead , the phage DNA, upon penetration into an E.cli cell gets integrated in to the circular bacterial DNA, becomes part of it and remains leatent .Such phages are called temperate phages . The inserted phage DNA is now called prophages .The prophages replicate along with the bacterial genetic material . The prophage remains latent with in the progany cells . In some rare spontaneous events . or when the host cell s exposed to UV light or some chemicals the phage DNA separates from the bacterial genetic material leading to the initiation of the lytic cycle . This lysogenic cycle facilities thee phenomenon of transduction .
