Chromatin

Date:
1971-1972
Reference:
PP/CRI/H/5/5
Catalogue details

Licence: In copyright

Credit: Chromatin. Source: Wellcome Collection.

    1/58
    base pairs. The same gaps are observed when extensive digests of chromatin with micrococcal nuclease are analyzed on denaturing or nondenaturing gels. However, the characteristic banding pattern of Fig. la cannot be obtained even after pro longed digestion with micrococcal nuclease (M. Noll, unpublished work). Even though single stranded fragments with lengths of each multiple of 10 bases are obtained by DNase I digestion of chromatin this does not prove that the cleavage sites on each DNA strand occur every 10 bases. Such an interpretation may gain support from the finding that the higher bands get converted to the lower bands with increasing digestion (not shown). Further, it is likely that the actual length of the DNA associated with the chromatin subunit is also a multiple of 10 base pairs since even after very limited digestions ( >50% of the fragments > 200 bases) no products are detected between the bands. The bands exhibit a characteristic intensity pattern - with a predominant band of 80 bases and relat ively weak bands at 60, 100 and 130 bases - which only changes during extensive digestion when the larger bands are degraded to the lower bands. This suggests that not all cleavage sites within the subunit are equivalent. The high accessibility of the DNA in chromatin clearly favours a model in which the DNA is on the outside of the chromatin subunit 2 . The regular rep eat of the DNase I cleavage sites within the subunit might then be visualized basically by two different models, (i) The DNA is wound around a his tone complex 2, 8 in a superhelical form without kinks. Only those sites of the sugar- phosphate backbone which are most exposed to the surroundings are attacked by the DNase I molecules. These sites are expected to occur every 10 bases on each strand reflecting the pitch of the DNA double helix, (ii) The DNA is not bent but kinked around the histone core with straight segments alternating with kink regions occurring every 10 base pairs. The fragments are produced by cuts either at the kink positions or in the straight regions depending on which sites are protected by the histone complex. While in the first model it is expected that the two strands are cut with a stagger of 5 bases in the second model no stagger occurs if the strands are cut at the kinks. A mixed model exhibiting both features has also to be considered.