LYTIC CYCLE ( Reproductive cycle of the bacteriophage )or (Replication of bacteriophage )

What is Bacteriophage .............?

Bacteriophage is a type of virus that infect bacteria .

.........( In fact , bacteriophages literally means Bacteria eater because bacteriophages destroy their host cell )

Fig. 1 ( bacteriophage)

- It also called as phages in short form .

What is lytic cycle.......?

The lytic cycle involves the reproduction of viruses using a host cell to manufacture more viruses; the viruses then burst out of the cell.

Fig. 2 ( overview of lytic cycle)

Why is it called lytic cycle?

The lytic cycle is named for the process of lysis, which occurs when a virus has infected a cell, replicated new virus particles, and bursts through the cell membrane.

This releases the new virions, or virus complexes, so they can infect more cells.

Phages of lytic cycle :-

1) Attachment ( adsorption ) :-

- The attachment occurs when the phase particles and bacterial collides with each other .

- The phage particles do not move independently.

- They depend upon the random and unexpected meeting with the right receptors.

- The attachment site on virus attaches itself to a complementary receptor site on bacterial cell wall. ( The attachment is due to formation to weak bond between the virus and receptor site .

2) Penetration :-

- After attachment a small region of the cell wall of bacterium is digested with the help lysozyme enzyme attached to the tail of phage .

- After complete attachment the tail sheet contract and inject the DNA from head of virus through the bacterial membrane .

- The capsid of phage does not penetrate the bacterial membrane .

- The capsid which remain outside the bacterial cell is called ghost .

3) Synthesis of protein and nucleic acids :-

- As soon as viral or phage DNA enter the bacterial cell, it begins controlling , cell metabolism for the synthesis of phage .

- After infection the bacterial RNA synthesis stop and synthesis of phage .

- Two sets of m-RNA are produced as early m-RNA and late m-RNA by phage .

- These m-RNA are responsible for formation of early and late protein .

- The formation of early protein is responsible for breakdown of bacterial DNA and RNA .

- After release of early protein, replication of viral DNA begins.

- After viral DNA replication, release of late protein takes takes places which include coat protein and viral lysozymes or endolysins.

- These product are the part of new virions and protein involved in cell lysis.

4) Virion assembly or maturation :-

- The process of assembly of the phage form its component is called maturation .

- The assembly of the phage takes place in definite sequence.

- The head tail and tail fiber fiber are formed independently and they combine together later on.

- The hexagonal base plates combine first with the tail.

- Then the head capsid which are constructed separately joins with the tail.

- The DNA of the phage is then packed within the heads.

- The whole process of maturation or virion assembly takes about 15 min .

5) Release of virions :-

- the final stage of the viral multiplication is the release of virion after lysis of host cell.

- The breakdown of bacterial cell wall is due to an enzyme called endolysin or lysozyme .

- Bursting takes place within 20 min to 1 hour , it depending on phage and bacterium involved

- The virions are released after bursting the host cell which are capable of infecting a new bacterium.

( molecules of lysozymes remain attached to the tip of the tail of the pages .These later help the phage in drilling the hole through the cell wall of the bacterium for injecting DNA into bacterium.)