The negatively charged DNA is wrapped around the positively charged histones. This helps to strengthen the interaction of the DNA and histones as unlike charges attract. In this form, the major groove of the DNA(used for binding to transcription factors) is blocked by the histones, and therefore gene expression is not possible.
Histones have to be removed for gene expression to take place.
Change in chromatin structure due to binding and loosening of DNA from the histones is known as chromatin remodeling.
The chemical modification of histones is based on the histone code; a hypothesis that the transcription of genetic information encoded in DNA is in part regulated by chemical modification to histone proteins.
Histone modifications involve