Guide To Learn
At the Level of Post-transcriptional Modification In eukaryotes, much of the gene regulation takes place during the mRNA processing. Approximately 75 per cent of pre-mRNAs are degraded within the nucleus. This selective degradation acts as the control point of gene regulation. Alternative splicing of pre-mRNA permits the synthesis of different versions of a protein. Alternative […]
Regulatory proteins contain domains not only for DNA binding but also for protein–protein interactions with RNA polymerase, other regulatory proteins or with other subunits of the same regulatory proteins. For examples, they include many eukaryotic transcription factors that function as gene activators. Some important examples of protein–protein interaction domains include the leucine zipper and the […]
Regulatory proteins generally bind to specific DNA sequences. Their affinity for these target sequences is approximately 104–106 times higher than their affinity for many other DNA sequences. Most regulatory proteins have discrete DNA-binding domains containing substructures that interact closely and specifically with the DNA. Figure 7.13 Gene regulation by miRNAs Within the regulatory proteins, the […]
Various molecular mechanisms operate to control gene expression at the level of transcription. cis-Acting Regulatory Sequences: Promoters and Enhancers These are the sequences that control the transcription of adjacent genes. Genes transcribed by RNA polymerase II have core promoter elements including the TATA box and Inr sequences. These cis-acting sequences serve as the binding sites of […]
Chromosomal regions that are activated for transcription are marked by a variety of structural changes. The packaging of eukaryotic DNA into chromatin limits its availability as a template for transcription. Thus, modifying chromatin structure plays a key role in controlling the gene expression in eukaryotes. The transcription of eukaryotic gene is strongly repressed when it is […]
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 […]