Parallel electrophoresis of the four ddNTP termination reactions produces a "ladder" in which each "rung" is a fragment one nucleotide longer than the one below it, either in the same or a different lane.  By labeling the lanes of the gel with the letter of the base complementary to the ddNTP reaction loaded, the sequence of the original DNA strand can be read directly in the 5' 3' direction, by moving from bottom to top of the autoradogram.

HOMEWORK: In a typical experiment, both strands of a dsDNA molecule are sequenced and compared as cross-checks. As both strands are read 5' 3' direction, direct alignment of the two sequences is difficult. Can you suggest a simple means to read the second autoradiogram with the base complementary sequence, so that the reverse strand sequence reads the same as the forward strand sequence, in the same 5' 3' direction?