Infection consists of four steps: -

I. Insertion of phage genome in to the cell. The DNA molecule from the phage passes into the bacterial host cell and becomes a closed circle.It’s the preliminary stage .First phage attached the host cell then insert it’s genome DNA by a syringe like method.

II. Early Protein Synthesis. A brief period of mRNA synthesis by the host cell is necessary in order to synthesize a repressor protein coded by the phage DNA. This protein inhibits the synthesis of the specific mRNA that encodes the lytic functions. The crucial regulatory test here is how fast a critical level of the specific repressor can be synthesized. The outcome determines whether the phage will undergo a lytic or a lysogenic cycle. If sufficient repressor is present, it blocks the transcription of all other phage genes. As a consequence, none of the virion proteins is made, and the cell does not lyses.

III. Integration of Viral DNA. The phage DNA joins with the DNA in the bacterial chromosome and is inserted into it by the action of a phage-coded DNA-insertion enzyme (coded by the int gene). Integrated in this way, the viral genome is now called a prophage.Phage DNA is inserted at a certain position in the E. coli chromosome, between the gal (galactose) and bio (biotin) genes. During this insertion, the phage DNA forms a circle, followed by physical breakage and rejoining of phage and host DNA. Other temperate phages have their own sites of integration on the bacterial chromosome. However, some temperate phage, such as phage Mu, have no specific site for insertion and may be able to insert multiple copies of their DNA at various sites in a single bacterial chromosome.

IV.Attainment of Lysogeny. The bacterial host cell remains alive and continues to grow and multiply, despite having a prophage integrated among its own genes. The phage genes replicate as part of the bacterial chromosome.Continued production of the repressor maintains the integrated prophage condition in the lysogenic cells. If at any time the repressor is inactivated (e.g., by a protease enzyme induced by exposure to ultraviolet light), the phage operons become depressed and start functioning, and the phage enters the lytic cycle and destroys its host cell. Thus a single repressor gene decides the fate of both the bacterial cell and the phage.