We explored how sequence and structure of DNA controls the shape of polyelectrolyte complex micelles. By tuning DNA stiffness through purine content and partial hybridization, we found that more rigid nucleic acids favor elongated cylindrical and worm-like nanostructures instead of spherical particles. Using SAXS, TEM, DLS, and DENSS reconstructions, we developed structure-property relationships providing new design rules for nucleic acid polymer self-assembly. Our work was published in ACS Biomacromolecules!