Molecular Genetics


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Structures consisting of DNA wrapped around an octamer of histones are called nucleosomes and are a fundamental unit of chromatin structure.


  • By mass, nuclei contain equal amounts of protein and DNA.
  • About 1/2 of the nuclear protein consists of highly basic proteins called histones. There are 5 principal kinds: H1, H2A,H2B, H3 and H4.
  • Olins and Olins (ref) observed 10 nm beads on strings in chromatin spread on electron microscope grids. nu bodies
  • Hewish and Burgoyne (ref) treated chromatin with nucleases, extracted DNA, and observed its size distribution. The pattern found is shown at left (left lane short digestion times; right lane, long digestion times).
  • The spacing between DNA bands in the ladder shown can vary from about 0.17 to 0.25 kbp among organisms and among tissues.
  • The nuclease-treated chromatin (before DNA extraction) was fractionated by size. One fraction contained the 146 bp DNA. It also contained two each of H2a, H2b, H3, and H4 histones per DNA fragment.

  • The circular double-stranded DNA of some viruses exists in nuclei as minichromosomes. The tracing length of the circle is about 7.5 fold less than that of the DNA extracted from these minichromosomes. (image from


Facts | Interpretations | Further Info. | Other Pages


Facts | Interpretations | Further Info. | Other Pages

Further information

  • DNA double-helices when constrained by being a covalently closed circle or by being immobilized by proteins can have superhelical twists. Nucleosomal and supernucleosomal DNA, on removal of proteins is superhelical.
  • The original Hewish-Burgoyne experiment (re f) used an endogenous nuclease that was activatable in the presence of calcium ions. It is this nuclease that is responsible for DNA breakdown during apoptotic cell death.
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    This is page 11226 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999, 2003, 2009

    E-mail inquiries to U. Melcher------------Last Updated: 14 August, 2009