The best understood case of a stable switch is the lysis Vs lysogeny decision made by phage m. When phage n infects cells there are two different developmental fates of the phage 1)In the lytic program the phage: replicates DNA, make heads, tails, packages DNA, and lyses host cells 2)In the lysogenic program the phage: integrates DNA and shuts down phage genes. The resulting quiescent phage integrated into the genome is known as a lysogen The decision between these two options must be made in a committed way so the proper functions act in concert. The switch in the case of phage i hinges on the activity of two repressor genes cI and cro. The cI and cro genes have mutually antagonistic regulatory interactions that can be diagramed as follows: 工 cro Lytic genes After an initial unstable period immediately after infection, either cro expression or expression will dominate Mode 1: High cro expression blocks cI expression. In this state, all of the genes for lytic growth are made and the phage enters the lytic program Mode 2: High cI expression blocks cro expression. In this state, none of the genes except for ef are expressed. This produces a stable lysogen n gene regulation, as in good circuit design, stability is achieved by feedback. The result is a bi-stable switch that is similar to a"flip-flop, one of the basic elements of digital electronic circuits Other genes participate in the initial period to bias the decision to one mode or the other. These genes act so that the lytic mode is favored when E coli is growing well and there are few phage per infected cell, whereas the lysogenic mode is favored when cells are growing poorly and there are many phag e per infected ceThe best understood case of a stable switch is the lysis vs. lysogeny decision made by phage λ. When phage λ infects cells there are two different developmental fates of the phage. 1) In the lytic program the phage: replicates DNA, make heads, tails, packages DNA, and lyses host cells. 2) In the lysogenic program the phage: integrates DNA and shuts down phage genes. The resulting quiescent phage integrated into the genome is known as a lysogen The decision between these two options must be made in a committed way so the proper functions act in concert. The switch in the case of phage λ hinges on the activity of two repressor genes cI and cro. The cI and cro genes have mutually antagonistic regulatory interactions that can be diagramed as follows: cI cro Lytic genes – – After an initial unstable period immediately after infection, either cro expression or cI expression will dominate. Mode 1: High croexpression blocks cI expression. In this state, all of the genes for lytic growth are made and the phage enters the lytic program. Mode 2: High cIexpression blocks cro expression. In this state, none of the genes except for cI are expressed. This produces a stable lysogen. In gene regulation, as in good circuit design, stability is achieved by feedback. The result is a bi-stable switch that is similar to a “flip-flop”, one of the basic elements of digital electronic circuits. Other genes participate in the initial period to bias the decision to one mode or the other. These genes act so that the lytic mode is favored when E. coli is growing well and there are few phage per infected cell, whereas the lysogenic mode is favored when cells are growing poorly and there are many phage per infected cell