Fractal growth in uv-irradiated DNA: Evidence of nonuniversal diffusion limited aggregation

Branched structures and fractals occur widely in nature (trees, corals, sand dunes, etc.) showing its preference for nonintegral dimensionality characteristic of nonequilibrium growth phenomena [B. B. Mandelbrot, The Fractal Geometry of Nature (Freeman, New York, 1982)]. Neural networks, blood platelets, some cancerous growth patterns, etc. are also known examples of fractals in physiological systems. Many theoretical models have been invoked to explain different fractal and ramified structures [T. Vicsek, Fractal Growth Phenomena (World Scientific, Singapore, 1989)]. The diffusion limited aggregation (DLA) of particles in a random walk that was computer simulated by Witten and Sander [T. A. Witten and L. M. Sander, Phys. Rev. Lett. 47, 1400 (1981)] is a more universal mechanism. Recently, a nonuniversal DLA has been theoretically proposed [P. Ossadnik, C. Lam, and L. M. Sander, Phys. Rev. E 49, 1788 (1994)] where particles in a random walk are not of the same size. This paper reports the significant observation that uv-photolyzed DNA in an alkaline solution aggregates, on drying, in a fractal-like structure. Furthermore, this observation provides experimental confirmation of nonuniversal diffusion limited aggregation [P. Ossadnik, C. Lam, and L. M. Sander, Phys. Rev. E 49, 1788 (1994)].