Final answer:
The 30-nm fiber needs to be folded into higher-order structures called loop domains to fit into a cell nucleus during compacting of DNA into chromosomes for cell division.
Step-by-step explanation:
To fit into a cell nucleus, the 30-nm fiber needs to be folded into higher-order structures, which are called loop domains. The 30-nm fiber, formed from nucleosomes, undergoes further compaction through the formation of these loop domains, each of which consists of DNA wrapped around histone proteins. The nucleosome itself resembles "beads on a string", and when these nucleosomes coil tightly together, they form a structure with a diameter of about 30 nm.
This fibrous structure is then looped and folded with the aid of scaffold proteins to fit inside the compact space of the cell nucleus. During cell division, particularly at the metaphase stage of mitosis, this structure is condensed further, forming chromosomes that are even more compacted, around 700 nm in width, and organized for cell division.