The role of proteins in cellular function is crucial because they produce the building blocks for cellular structure and form enzymes that catalyze all of the cell's chemical reactions, and regulate gene expression(Alberts, 2002).The transcription of each gene is controlled by regulatory region of DNA approximately near the site where transcription begins. Regulatory regions in animals are complex and act as tiny microprocessors, responding to different kind of signals that they translate and combine to switch the neighboring gene on or off. These switching consists of two types of fundamental components: 1) Short stretch of DNA of defining sequence and 2) Gene regulatory proteins that recognize and bind to them. Moreover, different collections of gene regulatory proteins are existing in different cell types and thereby direct the patterns of gene expression that produce each cell type its special characteristics. In this essay we will focus on some epigenetic mechanisms that are responsible in regulating gene expression in the development of an organism from an undifferential cell, resulting in the successive formation and development of organs and parts that did not pre–exist in the fertilized egg . Firstly, the core histone proteins and linker histones have been described genetically and biochemically as likely repressive to transcriptional initiation. These proteins collect DNA into nucleosomal arrays that in turn compressed into higher-order chromatin structures that can also provide to the repression of transcription initiation and elongation. Each nucleosome core includes more than 146bp of DNA folded in two superhelical turns around an octamer containing two molecules each of the four core histones (H2A, H2B, H3 and H4). DNA is forced to the surface of a positively charged ramp made up of the C-terminal histone-fold domains of the core histones. This wrapping of the double helix is stabilized by the N-terminals of the core histone that lie on the outside of the nucleosome DNA. Linker histones bind with the core histones and the linker DNA between nucleosome cores to stabilize the folding of the nucleosomal array into the chromatin fibers. This is an uneven and loosely packed solenoid with approximately six to seven nucleosomes per turn. Each turn includes at least 1000bpDNA. The dynamic properties of higher order structure and nucleosomes are seen in the movement of linker histones between different segments of chromatin and in the mobility of histones octamers between adjacent sequences. Linker histones are not essential for the assembly of higher-order chromatin structures. On the other hand, the elimination of linker histones and the associated increase in mobility of core histone DNA interactions ease transcription. Histones are the target for different types of post-translational modifications that change the structural properties of chromatin. These involve acetylation and phosphorylation of the basic N-terminal tail domains of the core histones and phosphorylation of the basic N-terminal domain of the linker histones. These modifications might be expected to make the interaction of these domains with DNA in the nucleosome less strong than usual. The real physical consequences of acetylating all of the core histone tails within the nucleosome in the absence of other proteins are relatively minor. There is a modest decrease in the wrapping of DNA around the histone octamer and nucleosome pack together less successful in array. Nevertheless, histone acetylation dose
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The molecular mechanisms underlying differential gene expression controls during Animals development Name Tutor Course Date Introduction The molecular mechanisms that control gene expression are crucial for cell differentiation and development of an animal…
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