I read that the DNA segment of lambda phage integrated in host DNA could switch between lysogenic state where cI represses the expression of Cro and lytic state where Cro expression takes over and self produces more.

  1. But how exactly does the high production of Cro kill its host?
  2. The virus has a DNA segment shielded with a protein coat. While entering host it gets attached to host membrane and releases DNA inside without damaging host. Then how does host get killed when virus goes outside it which happens during lytic phase and doesn't happen when it got inside?
  3. And if the virus has to go outside host, it needs protein coating. How is that produced? Is it produced only during lytic phase or also during lysogenic phase?

2 Answers 2


Have a look at the wikipedia page on lambda-phage (it is nicely illustrated and referenced).

See this map of the lambda-phage genome (Source).

enter image description here

  1. Cro (Controller of Repressor's Operator) does not kill the host. It is a transcription factor that represses cI. cI, in turn, represses Cro and other pro-lytic genes (expressed from the PL promoter), thereby forming a positive feedback loop with Cro (some people also use the term "double-negative" feedback loop which in my opinion is incorrect).

  2. During infection the virus just injects its linear DNA into the host via a porin protein (a sort of membrane channel). But at the peak of the lytic cycle many virus particles are formed inside the host cell (not just DNA but fully packed viruses). The viral proteins R and S cause breakdown of the cell membrane and the cell wall, which facilitates cell lysis.

  3. As mentioned in the previous point, the virus is fully packaged while inside the host. This happens only during the lytic phase. During lysogeny, the viral DNA gets integrated with the host genome and remains dormant until triggered by some kind of stress such as UV (which actually leads to activation of SOS response protein RecA which by degrading cI tilts the balance towards the lytic cycle).

  • $\begingroup$ I thought that a positive feedback loop is when a gene product either directly, or indirectly, stimulates its own activity, essentially an amplification mechanism. I don't understand how cI and cro together form a positive feedback loop; doesn't each one shut down the other one? $\endgroup$
    – mdperry
    Sep 11, 2015 at 0:42
  • $\begingroup$ @mdperry the sign of the loop is the product of all the individual interactions (or edges). So negative times negative is positive. By repressing its repressor Cro is indirectly activating its own synthesis (same for cI ). $\endgroup$
    Sep 11, 2015 at 5:06
  • $\begingroup$ The viral replication happens only due to the host reproduction, so one cell divides into two, each of the cells have only one viral dna segment integrated in their dna. At any time one host has only viral dna. Then even in the lytic phase only one virus will get out of host which should not burst it. Am I right or does virus replicate in a single host into many copies of itself? If so, how does it perform? $\endgroup$
    – Pavan
    Sep 11, 2015 at 5:22
  • $\begingroup$ @Pavan No viral replication is not dependent on the host. See this section of the wikipedia article. $\endgroup$
    Sep 11, 2015 at 5:41
  • 1
    $\begingroup$ @AlwaysConfused Yes. The link doesn't seem to work. I don't know the original source of this image. Unfortunately, I am not able to find a better (easy to understand but yet descriptive) replacement for this image. $\endgroup$
    Aug 28, 2018 at 14:09

After infection of the bacterial host, but before initiation of DNA replication of the viral genome, the phage has to decide between two mutually exclusive fates:

  1. A lytic infection that will lead to the death of the host by lysis, and a simultaneous burst of phage growth.


  1. A lysogenic infection where the viral genome integrates into the host's chromosome, and remains dormant, like a parasitic passenger.

Upon entering the host cell both the lambda repressor cI protein AND the cro protein are synthesized from viral transcripts. Essentially, whichever one "wins" will throw that switch. If cro levels are high enough they will shut off transcription of the cI repressor, leading (eventually) to cell lysis. If cI levels are high enough they will shut off transcription of the cro gene leading to integration and lysogeny.

This explanation is a bit simplified; there are other players that influence how the switch gets "thrown."

In a lysogen, just about the only phage protein being made is the cI repressor, and it keeps all the early genes shut off.

In the general viral life-cycle, early genes are transcribed before DNA replication of the viral genome, and late genes are transcribed after DNA replication of the viral genome. So typical early gene products are regulators, and DNA replication and recombination proteins. Once there are many, many copies of the viral DNA typical late gene products are the structural proteins that make up the viruses protein capsids, and enzymes required for packaging the DNA into the phage pro-heads.


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