Question

What are the changes that happen to chromatin as stem cells are restricted in their final state?

Answer 1

Stem cells can be differentiated to any type of cell in the body. So, the plasticity of genome is very high in comparison to differentiated cells. Differentiated cells have fixed fate and as a result they cannot be very plastic in nature. The organ or tissue specific localization of differentiated cells induce expression of such genes which are needed for the functioning of that organ or tissue only. But at the same time the other genes which are not required in a particular organ remain turned off.

Chromatin in differentiated or final state cells are more compact in some part and less compact (heterochromatin) in some other parts (Euchromatin). Euchromatin parts are generally expressed in a particular cell while heterochromatin parts remain turned off. Different epigenetic mechanism operates to control regulated gene expression.  Histone proteins remain more compact in heterochromatin region in comparison to euchromatin regions of the chromatin. Methylation causes silencing of gene, so rate of methylation increases in final state cells in comparison to stem cells. DNA methylation regulates gene expression by recruiting proteins involved in gene repression or by inhibiting the binding of transcription factor(s) to DNA. Histone acetylation also play an important role in gene expression. This is mediated by Histone acetyltransferase enzyme and deacetylation is caused by Histone deacetylase enzyme. Acetylation causes enhanced gene expression as they remove positive charge on histones and DNA Histone interaction become week and therefore DNA become accessable to transcription factors and gene are expressed.

These are some of the modification s which occur in chromatin of final state cell or differentiated cells in comparison to stem cell.