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Chromatin

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The complex of DNA and DNA-binding protein that makes up a eukaryotic chromosome.  The DNA is wrapped around histone proteins, each of which is composed of four core histones, H2A, H2B, H3 and H4.  The wrapping of the DNA around histones effectively packages over 2 meters of human DNA into compacted nucleosomes. The chromatin can exist in two forms:  tightly coiled chromatin called heterochromatin that is not actively transcribed or loosely coiled chromatin called euchromatin that is actively transcribed.

In mature, fully differentiated somatic cells, several of the genes that encode for the iPS reprogramming factors are located within the heterchromatin and are therefore considered transcriptionally silent.  Because the reprogramming factors are not actively expressed, scientists must induce the expression of these genes artificially using transgenic strategies.  International research efforts are currently exploring alternative methods to help unlock necessary reprograming genes that are located within the tightly packed heterochromatin.
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