Guide To Learn
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 […]
Protein synthesis inhibitors serve two major purposes (Figure 6.25) (Table 6.1). First, they have been very useful scientifically in elucidating the biochemical mechanisms of protein synthesis. Second, some of these inhibitors affect prokaryotic but not eukaryotic protein synthesis and thus are medically important antibiotics. Streptomycin Streptomycin is an aminoglycoside antibiotic that affects the function of […]
Some proteins such as low-density lipoprotein (LDL), the iron-carrying protein transferrin, peptide hormones and circulating proteins, which are destined to be degraded, are imported into certain cells from the surrounding medium. These proteins bind to receptors on the outer face of the plasma membrane. These receptors are concentrated in the invaginations of the membrane called […]
The nucleus is separated from the cytoplasm by two membranes which form the ‘nuclear envelope’. The nuclear membrane acts as barrier that prevent the free passage of molecules between the nucleus and the cytoplasm. The transport of macromolecules including mRNA, tRNAs and ribosomal subunits out of the nucleus and the transport of all nuclear proteins […]
Besides being bound by two membranes, both mitochondria and chloroplasts also contain similar type of electrotransport proteins and use an F-class ATPase to synthesize ATP. The growth and division of mitochondria and chloroplasts are not coupled to nuclear division. Proteins encoded by the mitochondrial and chloroplast DNA are synthesized on the ribosomes within the organelles […]