Biotechnology Practice Test

A lab uses Sanger sequencing to determine a DNA segment. During DNA synthesis, normal dNTPs and small amounts of fluorescently labeled ddNTPs are present. Incorporation of a ddNTP prevents addition of further nucleotides to that strand. The resulting fragments are separated by capillary electrophoresis, and a detector records fluorescence to infer the terminal base of each fragment. Which statement best explains why many fragment lengths are produced in a single reaction?

A student separates DNA fragments using agarose gel electrophoresis. A DNA ladder is loaded in lane 1, and an unknown sample is loaded in lane 2. After running the gel, the unknown sample shows a strong band that migrated farther from the wells than the 500-bp ladder band and slightly less far than the 300-bp ladder band. The gel conditions and voltage are typical for DNA separation. Which estimate best predicts the size of the unknown DNA fragment?