The researchers used optical tweezers to probe the barrier encountered by enzymes like RNA polymerase that prompt DNA in nucleosomes to uncoil from histones. With one end of a nucleosomal DNA strand fastened to a microscope slide and the other end attached to a synthetic microsphere and optically trapped in a laser beam, the researchers could accurately gauge dynamic changes in bond strength and organization as DNA was forced to uncoil from histones. Such precise measurements and observations of individual molecules are not available by classical biochemical techniques, which are more indirect and rely on averaged results from huge populations of molecules.
Under the magnification of an electron microscope, nucleosomes appear to be beads on strings of chromatin. Closer scrutiny shows the beads to be the fundamental organizational units of the genome, occurring, on average, every 200 base pairs along the DNA strands, with 147 base pairs of DNA wrapped 1.65 times around the eight-member clusters of histone proteins. One chromosome's double-stranded fiber of DNA, if stretched into a straight line, would be about two inches long. Were it not for compact storage -- in part due to the coiling of DNA in nucleosomes, which are, in turn, condensed into higher-order structures -- two inches of DNA would be too long to fit inside cells. However, DNA must be decondensed for its genes to be read or copied.