Control of Gene Expression PDF

CONTROL OF GENE EXPRESSION In Eukaryotic Organisms Gene Expression is the process by which the cells make use of the information included in the gene to synthesize a protein through the following steps: 1. formation of mRNA (transcription) 2. formation of protein (translation)  Anatomy o...

CONTROL OF GENE EXPRESSION In Eukaryotic Organisms Gene Expression is the process by which the cells make use of the information included in the gene to synthesize a protein through the following steps: 1. formation of mRNA (transcription) 2. formation of protein (translation)  Anatomy of a Gene To understand the process of transcription we will have a look at the structure of a gene. A gene is formed of 1. RNA Coding region (translated region): the region or sequence of DNA used as template by RNA polymerase to form mRNA. This RNA coding region has two ends: transcription start site (5’ end) & transcription termination site (3’ end) 2. In the 5’ end there is a region of DNA called 5’ untranslated region (5’UTR): It is formed of a promotor and enhancer regions. Gene Anatomy illustration  Enhancers and Promoters A promoter is a sequence of DNA to which RNA polymerase and transcription factors (TF) bind to start transcription An enhancer is a sequence of DNA to which enhancer or silencer proteins can bind and either activate or deactivate the transcription process  Different types of cells can express different sets of genes According to its functions and requirements, a cell can express certain types of genes. Example: Stem cells in the bone marrow from which RBCs are formed can express genes for production of α and β chains of hemoglobin But these genes can not be expressed by nervous cells This is what we call differential gene expression Chromatin Remodeling Is an Important aspect of Eukaryotic Gene Expression control The development of specialized organs, tissues, and cells and their function in the intact organism depend upon the differential expression of genes Some of this differential expression is achieved by having different regions of chromatin available for transcription in cells from various tissues. For example, the DNA containing the -globin gene cluster is in "active" chromatin region in the reticulocyte but in "inactive" chromatin region in muscle cells. Disruption of nucleosmes (Chromatin Remodelling) as a mechanism of regulation of gene expression Chromatin can be changed from compact into loose structure or the reverse by: 1. Histone acetylation deacetylation: Acetylation is known to occur on lysine residues in the amino terminal tails of histone molecules. This modification reduces the positive charge of these tails and decreases the binding affinity of histone for the negatively charged DNA. :  2. Methylation of deoxycytidine residues in DNA: Acute methylation of deoxycytidine residues in a specific region of the tyrosine aminotransferase (gluconeogenic enzyme) gene—in response to glucocorticoid hormones—has been associated with an increased rate of transcription of the gene  Regulation of Eukaryotic Gene Expression There Are Many Levels of Regulation 1. Transcriptional Control: Regulation of RNA synthesis 2. Post-transcriptional: HnRNA processing in the nucleus to form mRNA 3. mRNA transport: Movement of RNA from nucleus to cytoplasm 4. Translational regulation: - mRNA and protein turnover - Post-translational protein modification. Euokaryotic (Multicellular) transcriptional unit