SARs are candidate DNA elements for defining the bases of chromatin loops and possibly for serving as cis elements of chromosome dynamics. SARs contain numerous A tracts, whose altered DNA structure is recognized by cooperatively interacting proteins such as topoisomerase II. We constructed multi-AT hook (MATH) proteins and demonstrate that they specifically bind the clustered A tracts of SARs in chromatin and chromosomes. They are also potent inhibitors of chromosome assembly in mitotic Xenopus extracts, demonstrating the importance of SARs in this process. Titration of SARs with MATH20 (20 hooks) blocks shape determination of chromatids but not chromatin condensation per se. SARs are also required for shape maintenance of chromosomes. If MATH20 is added after formation of chromatids, they collapse and are reshaped by an active, mitotic process into spherical chromatid balls.