<P> This effect results in unusual ease in circularising small DNA molecules and a higher probability of finding highly bent sections of DNA . </P> <P> DNA molecules often have a preferred direction to bend, i.e., anisotropic bending . This is, again, due to the properties of the bases which make up the DNA sequence - a random sequence will have no preferred bend direction, i.e., isotropic bending . </P> <P> Preferred DNA bend direction is determined by the stability of stacking each base on top of the next . If unstable base stacking steps are always found on one side of the DNA helix then the DNA will preferentially bend away from that direction . As bend angle increases then steric hindrances and ability to roll the residues relative to each other also play a role, especially in the minor groove . A and T residues will be preferentially be found in the minor grooves on the inside of bends . This effect is particularly seen in DNA - protein binding where tight DNA bending is induced, such as in nucleosome particles . See base step distortions above . </P> <P> DNA molecules with exceptional bending preference can become intrinsically bent . This was first observed in trypanosomatid kinetoplast DNA . Typical sequences which cause this contain stretches of 4 - 6 T and A residues separated by G and C rich sections which keep the A and T residues in phase with the minor groove on one side of the molecule . For example: </P>

The width of a dna double helix is constant