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In addition, the removal of nucleosomes usually corresponded to transcriptional activation and the replacement of nucleosomes usually corresponded to transcriptional repression, presumably because transcription factor binding sites became more or less accessible, respectively. In general, only one or two nucleosomes were repositioned at the ...
By doing this, the DNA is more accessible and leads to more transcription factors being able to reach the DNA. Thus, acetylation of histones is known to increase the expression of genes through transcription activation. Deacetylation performed by HDAC molecules has the opposite effect.
H3 is used to package DNA in eukaryotic cells (including human cells), and modifications to the histone alter the accessibility of genes for transcription. H3K4me3 is commonly associated with the activation of transcription of nearby genes. H3K4 trimethylation regulates gene expression through chromatin remodeling by the NURF complex. [2]
Methylation of histones can either increase or decrease transcription of genes, depending on which amino acids in the histones are methylated, and how many methyl groups are attached. Methylation events that weaken chemical attractions between histone tails and DNA increase transcription because they enable the DNA to uncoil from nucleosomes so ...
It has been found that the SWI/SNF complex (in yeast) is capable of altering the position of nucleosomes along DNA. [8] [9] These alterations are classified in three different ways, and they are seen as the processes of sliding nucleosomes, ejecting nucleosomes, and ejecting only certain components of the nucleosome. [5]
A fiber of nucleosomes is interrupted by regions of accessible DNA, which are 100-1000bp long regions devoid of nucleosomes. Transcription factors bind within accessible DNA to displace nucleosomes and form cis-regulatory elements. Sites of accessible DNA are typically probed by ATAC-seq or DNase-Seq experimental methods.
The level of nucleosomal packaging can have profound consequences on all DNA-mediated processes including gene regulation. Euchromatin (loose or open chromatin) structure is permissible for transcription whereas heterochromatin (tight or closed chromatin) is more compact and refractory to factors that need to gain access to the DNA template.
In addition to processes that regulate transcription at the stage of initiation, mRNA synthesis is also controlled by the rate of transcription elongation. [10] RNA polymerase pauses occur frequently and are regulated by transcription factors, such as NusG and NusA, transcription-translation coupling , and mRNA secondary structure.