Loops, Domains and Scaffolds in Chromatin
The 30 nm fibres in the form of a solenoid must be organized in structures at a higher level of organization. These higher levels of organizations have been examined using metaphase and interphase chromosomes. It has been shown by DNAase digestion that DNA in the form of nucleosomes is organized in loops, each with 85 kb of DNA and a length of 10-30 μm. These loops have actually been observed to emanate from a central scaffold in metaphase chromosomes.
In interphase nuclei, a nuclear matrix is found as a filamentous structure on the interior of nuclear membrane. Chromatin remains attached to this matrix through matrix attachment regions (MARs). Several MAR sequences have actually been identified. It is believed that the same sequences of DNA work as MAR in interphase nuclei and as SAR (scaffold attachment regions) in metaphase chromosomes. Both matrix and scaffold are proteinaceous in nature.