Part of the problem is solved by supercoiling, a process mediated by enzymes, in which the DNA strand takes on an extra twist, coils within coils, like a telephone cord that's spent too much time with a garrulous teenager. But this supercoiling doesn't even begin to solve the problem of packing.

We’ve already seen that chromatin, the stuff of which chromosomes are made, is composed of DNA and protein. That protein, as you’re probably beginning to suspect, is what the cell uses to package the DNA. Most of it is in the form of histones, which are highly basic proteins—meaning they’ve got some positive charge hanging off them. It also means they’re attracted to negative charge--like the negative charge clustered around the sugar phosphate backbone of DNA. Sure enough, DNA loves to wrap itself around histones, like thread on a spool.

In fact, if you isolate DNA in a solution of low ionic strength (ie, less salty than what you’d find in a cell nucleus), you get a form of chromatin that looks like beads on a string. The beads are 10 nanometers (nm) in diameter, and they consist of just a little less than 2 full turns of DNA wrapped around a histone molecule—about 140 base pairs. This “bead” of DNA and protein is called a nucleosome, and is the fundamental packing unit of chromatin.

Figure 25. Packing of DNA on Histones. In the small panel at right, supercoiled DNA is shown wrapped around histone proteins. In the smaller panel at left, the "pearls on a string" appearance of the 10nm filament is represented, and above that we see how the filament is itself coiled to form the 30nm solenoid.