In Sexuality and Recombination in Bacteria and Viruses, we learnt that when a 'virulent' bacteriophage like T2, T4, SPO1, etc. infects a host bacterium, it entirely depends on the host for its multiplication. It subverts the host's function and utilizes the host machinery for producing a large number of progeny phage particles. The bacterium cell undergoes lysis and dies to liberate a large number of these phage particles, which are each then ready to start another cycle by infecting new bacterial cells. This cycle is known as lytic cycle. However, there are other phages called 'temperate' phages which have a dual existence such that they may either perpetuate through a lytic cycle as above or may take the form of a 'prophage' by integration of their DNA with the DNA of bacterial cell. The latter condition is called 'lysogeny', which provides immunity against infection by further phage particles of the same type, so that there is only one copy of phage per bacterial cell. We know that this prophage may become free, turn virulent and then take the course of lytic cycle as in virulent phages.

When the phage DNA enters the bacterial cell leaving the protein shell outside, it directs the host cell for preferential replication of phage DNA. In doing so, the phage genes are expressed in a particular order. The lytic cycle is divided into (i) 'early infection' which covers the period from the time of infection to the start of replication of phage DNA and (ii) 'late infection' which includes the period from start of phage DNA replication to the final step of 'lysis'.