DNA is a flexible molecule, but the degree of its flexibility is subject to debate. The commonly-accepted
persistence length of lp ≈ 500Å is inconsistent with recent studies on short-chain DNA that show much greater
flexibility but do not probe its origin. We have performed x-ray and neutron small-angle scattering on a short
DNA sequence containing a strong nucleosome positioning element and analyzed the results using a modified
Kratky-Porod model to determine possible conformations. Our results support a hypothesis from Crick and Klug
in 1975 that some DNA sequences in solution can have sharp kinks, potentially resolving the discrepancy. Our
conclusions are supported by measurements on a radiation-damaged sample, where single-strand breaks lead to
increased flexibility and by an analysis of data from another sequence, which does not have kinks, but where our
method can detect a locally enhanced flexibility due to an AT domain.
