The process by which the DNA of the virus combines with the DNA of the host bacteria to form a replicase and remain inside the host cell is called the lysogenic cycle. A virus that exhibits a lysogenic cycle is called a temperate phage. The lysogenic cycle occurs in lambda (λ) phage (P1, M13) viruses. The steps of this cycle are discussed.
1. Infection stage: Lambda (λ) phage is attached to the cell wall of E. coli bacteria. The virus attaches to the receptive spot of the bacterium by tactile fibers. Then the tangential fibers contract. The base plate of the virus is in contact with the bacterial cell wall. The lysozyme enzyme is secreted from the tail of the virus. Lysozyme dissolves mucopeptides in the bacterial cell wall. As a result, a fine pore or duct is formed. This process is called drilling. Virus DNA enters the bacterial cell through pores or tubules. But the capsid coat remains outside.
2. Entry phase of phage DNA into host cell: Viral DNA is transferred from head to tail. The viral DNA then enters through pores or tubules in the cell wall of the host bacteria. But the capsid coat remains outside the host cell.
3. Prophage formation: As soon as the viral DNA enters the host bacteria, the genes of the host cell become activated. Because of this protein and lysozyme cannot be produced. As a result, the capsid envelope surrounding the viral DNA cannot be formed. Endonuclease enzymes cut a specific segment of bacterial DNA. The integrase enzyme joins the viral DNA to the cut in the bacterial DNA. Attached virus DNA is called prophage.
4. Increase in the number of prophages: Prophages attach to the bacterial genome and divide repeatedly and are transmitted to the bacterial cell. Such phage is called lysogenic phage or temperate phage. It can enter the lytic cycle.
Lysogenic virus can infect humans and other animals. For example, Herpes simplex virus. Dr. Siddiq Publications