Fortran

The molecular mechanisms that operate to regulate gene expression in eukaryotes vary greatly from that in prokaryotes. This is basically because of the following reasons: There are six control points of gene regulation in eukaryotes. They are:

The tryptophan operon is required for the synthesis of the amino acid tryptophan. The trp operon is an example of a repressible operon—it is normally on; however, when a molecule called a ‘repressor’ is present, the operon turns off. Structural Genes The tryptophan operon contains five structural genes, namely trpE, trpD, trpC, trpB and trpA, that code for the enzymes […]

When lactose is present in the medium, small amounts of lactose enters inside the bacterial cell via the basal level of permease expression and is hydrolysed to the lactose isomer called allolactose, which acts as an inducer. Allolactose binds to the repressor protein in the inducer-binding site. This binding causes conformational change in the repressor […]

When E. coli cells are exposed to both glucose and lactose as carbon source, the bacterium will metabolize glucose. Although lactose is present from the beginning of the bacterial growth phase, lac enzymes are not produced until glucose in the medium is exhausted. This repression of lac operon by glucose is termed as ‘catabolite repression’ and is mediated by […]

Negative Regulation of Lactose Operon A repressor protein encoded by the i gene prevents the structural genes from being expressed. Close to the promoter is another cis-acting site called operator. The repressor binds to the operator and prevents RNA polymerase from initiating transcription (Figure 7.3). The gene expression is, therefore, turned off. The expression of lac i gene […]

Francis Jacob and Jacques Monod described the operon model in 1961, based on the regulation of lactose metabolism by the intestinal bacterium E. coli. Lactose operon is an ‘inducible operon’, because lactose induces the transcription of the operon. The lac operon is normally off, but when a molecule called an ‘inducer’ is present, the operon turns on. The lac operon […]

There are four different patterns of gene regulations namely negatively inducible, negatively repressible, positively inducible and positively repressible gene regulations (Figure 7.1).   Figure 7.1 Types of gene regulation   Table 7.1 Types of operon

Some substances called ‘inducers’ are capable of inducing or activating the expression of genes. In the presence of such inducers, the genes are transcribed and translated to synthesize the enzymes, which then metabolize the inducer. This phenomenon is called ‘induction’ and the enzymes synthesized are called ‘inducible enzymes’. For example, when E. coli is grown in the […]

Some gene products are required at all time and their genes are expressed at more or less constant level in all cells of a species or organism. The genes for the enzymes that catalyse the steps in a central metabolic pathway such as citric acid cycle fall in this category. These genes are called housekeeping […]

Prokaryotes live in a diversified environment ranging from the human intestine to polluted ponds, rivers, etc. and are thus exposed to different metabolites and molecules. Prokaryotes can survive in such a wide range of ecological conditions because of their ability to ‘switch on’ and ‘switch off’ the expression of specific sets of genes in response […]