Although each mixture contains both the modified and unmodified forms of one nucleotide base, only one form will be incorporated into the synthesized strand at any point in the sequence. When one of the modified bases is incorporated, synthesis of the strand stops. This creates strands of various lengths for each base. The strands in each mixture are then separated. Gel electrophoresis separates the strands on the basis of their rate of movement under the influence of an electric field.
Smaller fragments move through a gel faster than larger fragments.
This forms a band pattern down the length of the gel. The position of a band across the width of a gel indicates the base nucleotide within that fragment. Thus, the sequence of the bands down the length of the gel is determined by the sequence of the bases within the DNA fragment. The sequence of the original DNA strand is then deduced from the complementary sequence of the new strand. Screening Pencil Preparation Duplicate Handout 6 to distribute to students.