Just like the complex structure of one functional protein, a chromosome also has a wired, compacted structure that is flexible during the process of life. Chromatin is a highly compact and organized assembly of DNA and proteins. From a naked single molecular to a visible chromosome, DNA in mammalian is condensed approximately 10,000 to 20,000-fold. We will introduce the experimental tools from two category: “higher-order” and “primary-order” structure of chromosomal DNA according to the folding complexity.
The primary-order chromatin refers to the unpacked chromatin fiber where 11-nm coils of nucleosomes are exposed. The nucleosome is the fundamental unit of chromatin and is represented as a beads-on-the-string model.
The higher-order genome structure is most clearly visible during the interphase and mitosis when chromatin fibers extensively fold into chromosomes. An interphase chromosome is formed by a tightly coiled 250 nm chromatid. Microscopic imaging has demonstrated that each chromosome may be confined to genomic compartments. Within these compartments, intra-chromosomal interactions are most frequent within regions known as megabase-sized topologically associ- ating domains (TADs). The active TADs are rich in genes, open chroma- tin marks, transcription factors and DNase I-hypersensitive sites (DHSs) and show early replication. In contrast, the inactive TADs harbor few genes and DHSs and show late replication 1.