Question

1. If   a   lambda   cI   mutant   infected   Escherichia   coli,   what   lifecycle   would   you   expect   to   take   place   after   infection?   Explain   your   answer.   

2. Briefly   describe   how   the   bacteriophage   lambda   decides   on   undergoing   lysis   or   lysogeny.   

3. How   can   lysis   occur   if   a   lambda   phage   has   already   formed   a   stable   lysogeny?  

4. In   generalized   transduction,   only   1   or   2   progeny   phage   may   function   as   transducing   particles.   On   the   other   hand,   in   specialized   transduction,   all   the   progeny   phage   are   transducing   particles.   With   the   aid   of   a   diagram,   explain   this   statement.      

5. Is   the   bacteriophage   carrying   donor   bacterial   DNA   (transducing   particle)   capable   of   producing   progeny   phage?   

Answer 1

1) lamba cI protein is known as repressor protein, which repress the lytic cycle and cause stable incorporation of phage genome in to bacterial genome. This cI repressor help in formation of prophase. If this cI protein is mutated then after entry of phage in to E coli cI will not synthsized, there will be production of cro protein and which cause lytic phase always after infection.

2) main factor which which decide that after infection phage either go to lytic or lysogenic phase is nutritional status of E coli or surrounding medium.

If enough food is present- then E coli will divide rapidally, so here phage will so lytic phase and try to infect more and more E coli without having negative effects  

But if food is not enough , and E coli is deprived of food- then E coli will not divide rapidally so these E coli can not supports large no of phage in to them . So here phage will not show lysis here phage genome will present in form of lysogen of prophase. In this way phage ensure its survival with it much affecting E coli. When the condition will return normal it again enters in to lytic phase.

3) if lambda phage is present in form of stable progeny, then presence of any stress signal may cause switching from lysogenic to lytic phase.

In this stress condition there is activation of protease Rec A. This Rec A cleave the cI repressor molecules. So in absence of repressor molecules, bacteria is not able to keep repression of lyric gene expression. And there will be syntesis of cro protein which supports lytic phase.

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