Jacob-Monod model

Modifications to Model - The following three major modifications to the Jacob-Monod model occurred as the system was subjected to further analysis ... 

While a gene cluster might be an operon, it can be referred to as an operon in the strict sense of the Jacob-Monod model only if certain predictions of the model are met [1]. The evidence that the leu gene cluster is, in fact, an operon is as follows ... 

Other than the apparent arrangement of the genes in an operon, there is no evidence that any other features of the Jacob-Monod model are involved in expression of the leu genes. Although the cluster is repressed... 

The only other comparison that has been made between the azV mutants and the wild type is a very preliminary examination of the disc gel electrophoretic patterns of several ammonium sulfate-precipitated fractions of crude extracts. Thus far, no protein that might be a repressor has been found missing in the azV extracts. We are therefore left with the rather operational interpretation that the azl gene may be a genetic element required for multivalent repression. Whether that element is or is not the structural gene for the postulated aporepressor of the Jacob-Monod model is, of course, impossible to state at this time. 

Richmond [77] discovered apparently one-step mutants made from the inducible wild type, which were constitutive, made 0.01 to 0.1 % as much enzyme as the full magnoconstitutive strains, and yet complemented a constitutive i mutant in the diploid state, suggesting that they carried a functional gene. These mutants mapped between the i and p genes, and no clear explanation was possible in terms of the Jacob-Monod model if the mutants had lost both inducibiUty and the ability to express the structural gene normally as a result of a single mutation. 

Neither possibility 1 nor possibility 2 is compatible with the Jacob-Monod model. That is hardly surprising for why shouldn t such very much more complicated higher organisms be equipped with additional... 

Let us call to mind the key enzyme of phenylpropane metabolism, phenylalanine-ammonium-lyase or PAL (page 122). Its synthesis can be induced by light in a variety of plants, including cucumber seedlings. One of the products resulting from PAL activity, / -coumaric acid, can repress the synthesis of the enzyme. The nature of this repression is complex. In one sense it appears to be end product repression in accordance with the Jacob-Monod model, but there is also circumstantial evidence for the participation of a newly formed protein. However it may be, the accumula-... 

Bottom-up systems biology does not rely that heavily on Omics. It predates top-down systems biology and it developed out of the endeavors associated with the construction of the first mathematical models of metabolism in the 1960s [10, 11], the development of enzyme kinetics [12-15], metabolic control analysis [16, 17], biochemical systems theory [18], nonequilibrium thermodynamics [6, 19, 20], and the pioneering work on emergent aspects of networks by researchers such as Jacob, Monod, and Koshland [21-23]. 

On the basis of the studies of his group on the mechanisin of action of ecdysone. and of the Jacob and Monod model of induction and repression in bacteria, Karlson proposed in 1963 a general hypothesis on the mechanism of action of hormones (Fig. 3). According to this hypothesis, hormones would act on their target tissues by ululating the activity of certain genesL This model is now believed to be too simple. It does not explain immediate effects of hormones, nor some delayed... 

More than 30 years ago Jacob and Monod introduced the Escherichia coli lac operon as a model for gene regulation. The lac repressor molecule functions as a switch, regulated by inducer molecules, which controls the synthesis of enzymes necessary for E. coli to use lactose as an energy source. In the absence of lactose the repressor binds tightly to the operator DNA preventing the synthesis of these enzymes. Conversely when lactose is present, the repressor dissociates from the operator, allowing transcription of the operon. 

The behavior of the various mutations we have just discussed led Jacob and Monod to propose a model for the regulation of protein synthesis. The genetic elements of this model consist of a structural gene or genes, a regulator gene, and an operator locus (fig. 30.6). 

Schematic model illustrating the operon hypothesis. This diagram is modified from the original proposed by Jacob and Monod, who thought i gene repressor was an RNA rather than a protein, (a) The i gene encodes a repressor that binds tightly to the operator o locus, thereby preventing transcription of the mRNA from the z, y, and a structural genes. (b) When inducer is present, it combines with repressor, changing its structure so it can no longer bind to the operator locus. Inducer also can remove repressor already complexed with the o locus.

It was Jacob and Monod in 1961 who proposed the operon model for the regulation of transcription. The lac operon is a good example of how operons work (Fig. 1). The operon model proposes three elements ... 

The operon model was described in 1961 by F. Jacob and J. Monod to account for the molecular and genetic mechanisms involved in bacterial enzyme indue-... 

Figure 26.2 The operon model, as proposed in 1961 by Jacob and Monod. 

Influential model of Jacob and Monod ( ) shown in Figure 1. that control Is exercised by a repressor protein that affects the Initiation of transcription. For many years this was the favorite paradigm of biologists concerned with the normal processes of homeostasis, growth and differentiation, as well as their pathological manifestations—Infectious diseases. Inborn genetic errors and cancer. By almost any criterion this has been one of the most fruitful Ideas in modern biology. 

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