When they proposed the operon model, Jacob and Monod suggested that the genetic activity of temperate phages might be controlled by a system analogous to. “Duplex DNA is isolated from bacteriophage lambda (cIind 1 Sam 7). Lambda DNA is base pairs in length. The phage is isolated from the. CERTIFICATE OF ANALYSIS. Lambda DNA/PstI Marker, #SM ( 5×50)µg. Lot: Concentration: µg/µl. Supplied with: 2ml 6X Loading Dye Solution.

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An Introduction to Genetic Analysis. When they proposed the operon model, Jacob and Monod lamgda that the genetic activity of temperate phages might be controlled by a system analogous to the lac operon. In the lysogenic state, the prophage genome is inactive repressed. In the lytic phase, the phage genes for reproduction are active induced. This phage does indeed have an operon-type system controlling its two functional states.

By now, you should not be surprised to learn that this system has proved to be more complex than initially suggested. Furthermore, mutations in the NOfagk P genes prevent most of the genome from being expressed after phage infection, with only those loci lying between N and O being active.

We shall soon see the significance of this observation. The positions of nonlethal fagoo conditionally lethal mutations are indicated, and the characteristic clusters of genes with related functions are shown.

Lambda phage

When wild-type phage particles are placed lanbda a lawn of sensitive bacteria, clearings plaques appear where bacterial cells are infected and lysed, but these plaques are fagp because lysogenized bacteria which are resistant to phage superinfection grow within the plaques. Mutant phages that form clear plaques can be selected as a source of phages that are unable to lysogenize cells. Such clear c mutants prove to be analogous to I and O mutants in E.


For example, conditional mutants for a site called cI are unable to establish a lysogenic state under restrictive conditionssuggesting either a defective repressor or a defective repressor-binding site operator. However, these mutants also fail to induce lysis after superinfecting a cell that has been lysogenized by a wild-type prophageso the operator is functional, because repressor from the wild-type phage is clearly able to prevent the mutant from entering the lytic phase.

Apparently, the cI mutation produces a defective repressor in the lambsa control system. Genetic mapping has revealed two operators, designated O L and O Rlocated on the left and almbda of cIrespectively.

A race begins to determine whether the phage will express its virulent functions and enter the lytic cycle or whether it will repress these functions and establish lysogeny. P L governs the leftward transcript that encodes the antiterminator protein N, plus a faog of lytic functions. P R governs the rightward transcript, including the cro, cII, and Q genes, among others. These transcripts normally terminate at lamvda critical points, before key genes necessary for lytic development can be transcribed.

However, if the N protein is synthesized, it prevents lzmbda of the transcript see Figureand fagoo lytic functions are expressed. In the former case, this blocking prevents synthesis of the N protein and thus also blocks the extension of transcripts from P L and P R. The wavy lines represent mRNA and show the origin, direction, and extent of transcription.


A vertical arrow at the beginning of an mRNA marks the site where more The cI gene has two promoters: Transcription initiated at each promoter requires an activator protein. For P Mthe activator is the cI -encoded repressor itself. Therefore, to synthesize repressor in a cell without preexisting repressor molecules, P E must be used.

P E is activated by the cII product. Thus, cII must be transcribed to establish lysogeny.

Lambda phage: a complex of operons – An Introduction to Genetic Analysis – NCBI Bookshelf

One additional protein is required to establish lysogeny, the integrase, encoded by int. The int gene is transcribed from its own promoter, P I Figurewhich also requires activation by the cII gene product.

The binding of repressor to O R 1 facilitates lambdx binding of a second molecule of repressor to O R 2. Self-regulation of cI repressor levels. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed. Turn recording back on. National Center for Biotechnology InformationU.

Figure Self-regulation of cI repressor levels.

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