Cyclization of three satellite components of calf thymus DNA.

Cyclization of denatured and reannealed satellite components of calf thymus DNA was studied by electron microscopy. All three satellite DNA components studied (1.707g/cm-3, 1.714g/cm-3 and 1.721g/cm-3) form circular structures indicating that the sequences of the calf thymus satellite DNAs are arranged in a tandemly repetitious manner. Under appropriate annealing conditions the amount of circular structures is reproducible and practically no aggregates are formed. By comparison of cyclization experiments under defined conditions it is demonstrated that individual satellite components differ in the amount of circular structures formed during reassociation and in the distribution of linear and circular molecules. From the distribution of the contour lengths of circular molecules we conclude that the length of the repetitive sequence decreases with increasing buoyant density of the satellite components. The average lengths of the repetitive sequences calculated from electron microscopy measurements are in good agreement with those from renaturation kinetics.