The lac-operon is actually a series of adjacent genes and regulatory elements in one small part of the E. coli circular chromosome.
Bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA transcript. In this case, when lactose is required as a sugar source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated: lacZ, lacY, and lacA. The gene product of lacZ is β-galactosidase which cleaves lactose, a disaccharide, into glucose and galactose. The protein that is formed by the lacI gene is known as the lac repressor. The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor (lac repressor) unless some molecule (lactose) is added. 1 The lac operon consists of three structural genes, and a promoter, a terminator, regulator, and an operator. 2  The three structural genes are: lacZ, lacY, and lacA. The protein that is formed by the lacI gene is known as the lac repressor. The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor (lac repressor) unless some molecule (lactose) is added. 1 A promotor is the sequence of DNA where RNA polymerase attaches when a gene is to be transcribed. 2  An operator is a short sequence of DNA where an active repressor binds, preventing RNA polymerase from attaching to the promotor--transcription therefore does not occur. A group of transcription factors binds to a promoter adjacent to a gene; the complex attracts and binds RNA polymerase, but transcription may still not begin. Transcription activators are often involved in controlling transcription in eukaryotes. 1  Different combinations may regulate different genes. An inducer (allolactose or an analog) binds to the repressor and prevents its binding to the operator, thereby releasing the repression and allowing transcription of the lac operon. a) Most mutations in the operator, the binding site for repressor, lead to lower affinity for the repressor and hence less binding. An up mutation would make the lac operon no longer dependent on the positive regulation by the cAMP‑CAP complex (when the operon is induced). A down mutation would not allow expression even in the de‑repressed state (presence of inducer) and hence would show a non‑inducible phenotype. 15.3 a) Choice (c) is correct. Overview of the lac Operon. 1  Gene is normally off. 2  There is no transcription because a repressor binds to the control site. 3  When lactose is present, it inactivates the repressor, allowing transcription to begin. 4  When lactose is used up, the repressor is again free to bind to DNA, and halt transcription. 5  Glucose must be absent.  When glucose is present, the CRP does not bind to the promoter and expression of the lac operon remains low even if lactose is present. However, when glucose is absent, the CAP binds to the promoter region and promotes transcription. Glucose does not directly regulate the binding of CRP. A cAMP-CRP complex is entailed. Structure and Mechanism. The lac operon consists of three structural genes, Z, Y and A, which code for the enzymes necessary to catabolize lactose and a regulator gene, I, which codes for a regulator protein. The regulator protein regulates the operon. The lac-operon is actually a series of adjacent genes and regulatory elements in one small part of the E. coli circular chromosome. Lac-Operon components Definitions: P strands for promoter; it is the site where RNA polymerase attaches in order to transcribe mRNA. When the copies created heterozyous cells (I + , I -) normal regulation was observed. This indicated that the I gene codes for something that can move and interact with the operators of both copies of the lac-operon present in these cells. One of these mutations can occur in the repressor genes such that the protein loses affinity for the operator, and causes the continual production of the inducible enzymes regardless of if lactose was present or not. Therefore, the lac operon becomes constitutive. Structure and Mechanism. The lac operon consists of three structural genes, Z, Y and A, which code for the enzymes necessary to catabolize lactose and a regulator gene, I, which codes for a regulator protein. The regulator protein regulates the operon. In the lac operon, allolactose binds to the repressor protein, allowing gene transcription, while in the trp operon, tryptophan binds to the repressor protein effectively blocking gene transcription. In both situations, repression is that of RNA polymerase transcribing the genes in the operon. Also unlike the lac operon, the trp operon contains a leader peptide and an attenuator sequence which allows for graded regulation. It is an example of repressible negative regulation of gene expression.  lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available. The protein that is formed by the lacI gene is known as the lac repressor. The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor (lac repressor) unless some molecule (lactose) is added.
https://en.wikipedia.org/wiki/Lac_operon https://en.wikipedia.org/wiki/Lac_operon http://classes.midlandstech.edu/carterp/Courses/bio101/chap15/chap15.htm http://www.bx.psu.edu/~ross/workmg/PartFourAnswers.htm http://www.uvm.edu/~dstratto/bcor101/lac1.htm http://userpages.umbc.edu/~lrowan1/lactoseoperon.html http://www.tamu.edu/faculty/magill/gene603/PDF%20versions/regulation1.pdf http://userpages.umbc.edu/~lrowan1/lactoseoperon.html https://en.wikipedia.org/wiki/Trp_operon https://en.wikipedia.org/wiki/Lac_operon
which gene mutation is constitutive mutation gene regulation lac operon