This means that there is lots of specialization of tissues, meaning that while every cell still contains all the DNA, the information in only some of it is used. Complexity of how this is accomplished is only starting to be unravelled, mostly in Drosophila . Here the controls primarily involve proteins called transcription factors that bind upstream of the promoters and in turn control the binding of RNA polymerase. Show sequence of pictures from recent Science. But changes also occur in the gross organization of DNA, so briefly consider how DNA is organized in eukaryotes.
Histones are the fundamental proteins involved in binding DNA into nucleosomes .
These are then packed into at least two more complex stages, and another for mitosis. The level of packing crudely controls whether genes in a particular region of a chromosome will be accessible for transcription. A truly major form of regulation at the chromosome level is the inactivation of the X chromosome in female mammals. This occurs in a mosaic pattern, seen clearly in calico cats, which are females with one copy of a gene on the X chromosome for orange fur, and one for black fur. They are a patchwork of orange and black splotches according to which X chromosome is inactivated.