DNAs that are prepared in this way may then be analysed by a method called gel electrophoresis. This involves the use of electric currents to facilitate the separation of linear DNA molecules through a gel support, usually consisting of the polymer agarose or polyacrylamide. These polymers form a molecular sieve that permit the DNA to pass through at a rate that is approximately inversely proportional to the log of the molecular weight as measured in kilobase pairs (Kb). The current initiates the movement of DNA from the site of application to the positively charged cathode as the negatively charged phosphate groups in the DNA molecule are drawn to the cathode by electrostatic attraction. If DNA fragments of Known molecular weight are electrophoresed simultaneously thre molecular weight of the DNA fragments generated by restriction enzyme digestion may be ascertained by comparing their rate of mobility with that of the standards of known molecular weight. This is usually calculated by preparing a graph representing the log of the molecular weight of DNA standards versus the measured distance traveled beach band in millimeters (mm). The distances of the unknown fragments is measured and their molecular weights are determined by locating the position these measured distances are located on the graph. Restriction enzyme digestion of DNA followed by gel electrophoresis is a commonly used method for preparing DNA maps and determining the molecular weights of unknown DNA samples.
The DNA used in this experimental protocol was obtained by culturing bacteria (E.coli) that contain plasmid DNA. Two types of plasmids were prepared from E.coli, designated plasmid X and Y. After the plasmid DNA was extracted from the bacterial cells, it was then digested with restriction enzymes, which are capable of making double stranded cuts in DNA molecules at specific