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So I was just reading that whether heterochromatin or euchromatin is more abundant in a particular human cell depends on how active that cell is. But considering that most of the 25,000 or so genes in the human genome are transcriptionally inactive in any given cell, how is it possible for cellular activity to affect the chromatin structure to a degree that significantly alters how a nucleus looks on microscopic images? Since, after all, whether the cell is active or not will only affect little of the chromatin.

And since most of the genome is inactive, shouldn't it be in a heterochromatic form, and thus most of the chromatin in any given cell will be heterochromatin?

(I know this is not true, but I was wondering why)

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    $\begingroup$ I think whether DNA is in the heterochromatic or euchromatic stage depends on the stages of a cell's life cycle. There is a page on wikipedia with a good comparative image: en.wikipedia.org/wiki/Chromatin#/media/… I think to answer your question though, it varies? $\endgroup$ – Ro Siv Sep 13 '15 at 23:40
  • $\begingroup$ I dont think the forms of chromatin are stages but are rather states that depend on the transcriptional activity of the cell. And yes it does indeed vary but my question was how come such variation affect the overall structural conformation of the chromatin since most of it is inactive anyway. $\endgroup$ – Remoun Sep 14 '15 at 5:48
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Percentage of euchromatin varies between cell types and organisms. It has been shown that upto 88% of the human genome is transcribed; a phenomenon called pervasive transcription [1]. Highly specialized cells may have a lower percentage of transcribed regions and in females one entire X-chromosome is silenced (some regions in the silenced-X do show transcriptional activity).

Euchromatic regions not only constitute the geneic regions but also regulatory intergenic regions such as enhancers, locus control regions (LCR), silencers etc. It has been found that ~97% of these elements fall in the DNAse hypersensitve (DHS) region of the chromatin [2].

DNAse hypersensitivity is directly proportional to the chromatin state such that open/accessible regions are more prone to DNAse-I digestion. You may call these regions as euchromatin.

Though the total DHS density is not reported in the paper [2] or supplementary data, the ENCODE project that the paper concerns, will have all the detailed information. You can can find these datasets here, from which you can calculate DHS density distribution for different cell types. However, most of these cells are actually cell lines and not from primary tissues.

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