DNA molecules carry genetic instructions for our cells. Most of the time that DNA is tightly coiled around proteins. A new study shows that the coiled DNA acts much like the string on a yo-yo. And that’s good, because by being rolled up, each cell can store a lot of instructions.
If each piece of DNA from a human cell were laid end to end, the collection of strands would stretch about two meters (6.6 feet) long. Yet these long genetic molecules must fit into a cell nucleus just 10 micrometers (0.0004 inch) in diameter. How can the body shoehorn so much DNA in? It wraps each strand of DNA around a series of proteins called histones (HISS-toanz).
Eight histones clump together, and a section of DNA wraps roughly twice around the package, forming a nucleosome (NU-clee-oh-zoam). DNA loops into one nucleosome after another along its entire length — hundreds of thousands of nucleosomes in all. This gives DNA the appearance of a beaded necklace, explains Jaya Yodh. A biophysicist, she works at the University of Illinois at Urbana-Champaign. (A biophysicist studies the physical forces in biological systems.) Those beads pack together, cramming the entire DNA strand into a very tiny space.
Such cramped conditions are great for storing DNA. But for cells to use the genes on each DNA strand, the coils have to unwind. Yodh and her team wondered whether the flexibility of DNA played a role in that unwinding.
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