Now, lets make our solution of DNA a little more salty. When we add sodium chloride or magnesium chloride,  our string of pearls (which is called a 10-nm filament) becomes a more compact helical structure, coiled up into a 30-nm solenoid.

30-nm solenoids are, in turn packed onto a scaffold of protein that looks curiously similar to one of those archetypal stickman figures everybody thinks of when they hear the word “chromosome.” One of the proteins on this scaffold is called topoisomerase II. It’s not just a structural protein, but also an enzyme that can wind and unwind DNA, and it has the unenviable task of keeping the DNA in a chromosome from coiling into tangles and knots.

Figure 26. Packing of 30 nm Solenoids into Metaphase Chromosomes. This is the most compact form of chromatin.