Poly(dA).poly(dT) is a B-type double helix with a distinctively narrow minor groove.

The structure of poly(dA).poly(dT) currently arouses great interest, mainly because dAn.dTn stretches are associated with considerable DNA bending. Until recently the heteronomous DNA described by Arnott et al., with the poly(dA) and poly(dT) chains in A and B conformations respectively, was the only detailed model of this structure. Following our earlier studies of the interaction of DNA and monovalent ions, we examined the X-ray diffraction of the bivalent Ca2+ salt of poly(dA).poly(dT) (Ca-poly(dA).poly(dT)) and found no sign of a heteronomous structure: Ca-poly(dA).poly(dT) in fibres shows fully equivalent B-type conformations of the opposite sugar-phosphate chains. A revision of the structure of the sodium salt, Na-poly(dA).poly(dT), based on this result, yields only a slightly heteronomous structure with each chain in a B-type conformation, which is in much better agreement with the experimental data underlying the original heteronomous model. Both structures, Ca- and Na-poly(dA).poly(dT), have a minor groove narrower than that of the B form: this peculiarity seems to be very important for the interaction of poly(dA).poly(dT) and biologically significant molecules (including proteins and antibiotics). The specific base-pair positions in poly(dA).poly(dT) may account for the DNA bending adjacent to dAn.dTn tracts.