DNA bending at A-tracts. Bending in DNA segments containing A-tracts of six adenosines in a row can be detected by using enzymes to cut the DNA at specific sites. (a) The DNA sequence used in this experiment contains two A-tracts. Arrows point to sites that can be cleaved by various restriction enzymes (numbered 1 to 5) to generate DNA fragments of equal length that contain an A-tract at various positions in the fragment. (b) DNA fragments are analyzed by gel electrophoresis. Even though all DNA fragments are the same size (~215 base pairs), their rate of migration through the gel depends on the relative location of the A-tract. When the A-tract is located in the middle, the DNA fragment is more bent and migrates slowly; when the A-tract is near the end, the fragment is less bent and migrates faster. The lane marked MM contains molecular markers that provide a reference for DNA fragment size in base pairs (bp).