In eukaryotes , DNA packaging is accomplished in three steps , or rather , three levels of folding and condensing .
1 . DNA wraps itself around eight proteins called histones , forming a bead-like structure called a nucleosome . Nucleosomes link together to form chromatosomes
( which have a beads-on-a-string appearance under the microscope ). Step 1 can compact DNA to a size 7x smaller .
2 . The chromatosomes then fold themselves into a 30-nanometer fiber coil that is circumferenced by six nucleosomes , one ( solenoid model ) to two ( zig-zag model ) layers thick . Step 2 can further compact DNA to a size 40x smaller .
3 . DNA is then further compressed through supercoiling - a process by which scaffold proteins progressively fold and twist the fiber into the chromatin typically seen in interphase and then into the characteristic shape of the metaphase chromosome . By step 3 , DNA has been compacted down to 10,000x its original length . This is akin to taking a rope as long as a football field and compacting it down to less than half an inch !
Most prokaryotes lack histones , but they do have supercoiled forms of their DNA . Most prokaryotes have a single circular chromosome attached to the cell membrane , whereas most eukaryotes have one or more linear chromosomes within a nucleus .