who DNA folding in nuclus

Because of its ability to coil and supercoil. The dna coils because it is a double helix. The backbone actually runs in different directions making it coil. The dna coils around itself, and then it coils around bead-like proteins called histones, and those histones (with the dna coiled around it, which makes it a nucleosome) wrap and coil to form chromatin fiber. That chromatin fiber SUPERcoils into a chromosome. DNA coils tightly in order to condense into a chromosome so that it'll be easier to copy, transport, and it takes up less space.

The folding of a biomolecule is an inherently complex process involving an energy landscape that describes the  possible conformations and nuclear/segment motions (1–7). Different time scales (8–10) are involved and span the slow, largeamplitude motions and fast, local motions. The former is the rate-limiting step in the folding process and involves the passage through basins and barriers of the energy surface, ultimately reaching the final native  structure. In contrast, elementary local motions are those of structures that may or may not be transient intermediates; their existence is critical to the description of the pathways and rates of folding and melting. Such elementary motions are often hidden from detection when the time resolution used is relatively long. Much work has been done on protein folding and their energy landscapes. For DNA and RNA, the melting problem (11) is usually identified in textbooks by two states:
folded state (F) º unfolded state (U)