Structural studies of chromatin by using proteases.

Chromatin structure The repeating unit of chromatin, the nucleosome, contains typically about 200 bp of DNA of which 146bp are externally wound in I .75 superhelical left-handed superturns around an octamer of the core histones. This octamer consists of two copies each of H2A, H2B, H3 and H4 histones and together with its DNA is termed the core particle. It has a diameter of 1l.Onm and a thickness of 5.7nm. An additional 2 x lObp of DNA are associated with a single copy of the fifth histone, HI , and the complex of 166bp DNA with all the histones is termed the chromatosome. One hundred and sixty-six base pairs of DNA represents about two superhelical turns. The remaining DNA is termed ‘linker DNA’. When extended at low ionic strength, this ‘beads-on-a-string’ structure can be re-folded to a higher order structure by salt addition, provided H 1 is