The packaging of tremendous amounts of genetic information into the small space within a cell has been called the ultimate storage problem. Consider the chromosome of the bacterium E. coli, a single molecule of DNA with approximately 4.6 million base pairs. Stretched out straight, this DNA would be about 1000 times as long as the cell within which it resides (Figure 8.15). Human cells contain more than 6 billion base pairs of DNA, which would measure more than 2 meters (more than 6 feet) stretched end to end. Even the DNA in the smallest human chromosome would stretch 14,000 times the length of the cell nucleus. Clearly, DNA molecules must be tightly packed to fit into such small spaces.

As mentioned earlier, the structure of DNA can be considered at three hierarchical levels: the primary structure of DNA is its nucleotide sequence; the secondary structure is the double-stranded helix; and the tertiary structure is the higher-order folding of DNA. Here, we consider the tertiary structure, which allows DNA to be packed into the confined space of a cell.