Concept 3 Practice: Determining DNA Lengths
A marker DNA sample often is run alongside experimental samples to help determine the length of DNA. The marker DNA sample (here, the Hind III Λ digest) contains a number of DNA molecules, each with a known molecular size.

To determine the length of a fragment in the band DNA II, select the orange band that represents the fragment. A line will be drawn from the distance the fragment traveled (on the x axis) to the calibration curve. At the point that it intersects the calibration curve, another line will be drawn to the value for the length in base pairs of the fragment.

To determine the length of a DNA fragment in the band DNA I, click once on the ruler and drag it directly above the DNA band, DNA I. Select the orange band that represents the DNA fragment to measure the length in base pairs of the fragment.

As you measure the DNA fragments, note that the shortest fragments travel the farthest distance.