LIFESCI 7A Chapter 3: LS7A week 7 Thursday
Document Summary
Underwinding is caused by an enzyme, topoisomerase ii, that breaks the double helix, rotates the ends to unwind the helix, and then seals the break. Underwinding creates strain on the dna molecule, which is relieved by the formation of supercoils, in which the dna molecule coils on itself. nucleoid in prokaryotes, a cell structure with multiple loops formed from supercoils of dna. First, eukaryotic dna is wrapped twice around a group of histone proteins called a nucleosome. nucleosome is made up of eight histone proteins: two each of histones h2a, h2b, h3, and h4. This first level of packaging of the dna is sometimes referred to as beads on a string, with the nucleosomes the beads and the dna the string. It is also called a 10-nm fiber in reference to its diameter, which is about five times the diameter of the dna double helix.
