Understanding the structure of chromosomes is important for recognizing the behavior of chromosomes during cell division. To our current knowledge, chromosomes consist of DNA and various (i.e., histone and nonhistone) proteins, which are assembled and compacted in a complex manner, resulting in the 10,000-fold packing of the DNA molecule in each chromosome. The arrangement of DNA and proteins in the chromosome has been studied using electron microscopy, since the resolution needed for visualizing these structures is far

beyond that of light microscopy. These electron microscopic studies revealed the presence of fibrous structures in various orders of the packing of DNA molecules into chromosomes.