Passage II

A restriction enzyme is a protein that cuts DNA at a specific sequence of base pairs. Because almost all humans have small differences in DNA sequence, a restriction enzyme tends to cut in different places in each person's DNA. This creates a distinctive pattern of DNA fragments or bands, sometimes called a DNA fingerprint, that is generally unique for each person. A DNA fingerprint can be analyzed using gel electrophoresis (a technique that uses an electric field to separate DNA fragments into bands based on size) along with various DNA visualization methods.

A painting was stolen from a museum. No fingerprints were located; however, a sample of skin cells and blood was found on a window broken by the thief, Investigators decided to use blood typing and DNA fingerprinting to determine which of 4 suspects (Suspects 1-4) matched the skin and blood from the museum.

Investigation

Table 1 shows the blood type of the museum sample (Sample M) and of Suspects 1-4 (Samples 1-4, respectively).

DNA from Sample M and from Samples 1-4 was placed in separate tubes so that each tube contained DNA from 1 of the suspects or the museum sample. Restriction Enzyme X was added to each tube, and the samples were incubated for 1 hr at 37°C. Investigators used gel electrophoresis to separate the DNA fragments in each sample into bands. The fragments in a band were similar in length. Figure 1 shows the DNA fingerprints that resulted when the samples were cut by Restriction Enzyme X and visualized with an appropriate method.