DNA Fingerprinting—Electrophoresis at Work
Publication No. 10055
“DNA Fingerprinting—Electrophoresis at Work” is an introductory activity for students about to begin the study of biotechnology and electrophoresis. Students become medical forensic experts as they read a case in which a woman has been attacked or a case where an elderly man has been murdered. The entire case rests on their expert ability to interpret the results from DNA tests.
The world of forensic science was revolutionized recently with the discovery of a scientific technique for identifying humans and animals by using their DNA. This technology, DNA fingerprinting, can be used to identify the source of DNA in not only forensic medicine, but has been utilized in paternity cases, identification of disaster victims, and in the diagnosis of genetic diseases.
The arrows over the restriction sites show the precise location where the restriction enzyme cuts the DNA strand. To further illustrate, look at the following segment of double stranded DNA:
5 ′ - ATTAATCCGGCCAATGGAAGCCTTAAGGCCGTT- 3 ′
3 ′ -TAAT TAGGCCGGTTACCTTCGGAATTCCGGCAA-5 ′
Note: Restriction enzymes “read” DNA from the 5′ end to the 3′ end.
The segment is “digested” with the Hae111 restriction enzyme giving the following result:
5 ′ - ATTAATCCGG⇓CCAATGGAAGCCTTAAGG⇓CCGTT- 3 ′
3 ′ -TAATTAGGCC⇑GGTTACC T TCGGAAT TCC⇑GGCAA-5′
The DNA is cut in two places yielding three fragments of varying lengths. If you can imagine extrapolating this process over an individual’s entire genetic complement, you can also imagine the result—a large number of fragments of different lengths. The number of fragments and their relative sizes make up the DNA fingerprint for that individual.
Once the DNA sample has been fragmented, the investigator has to have some means of separating the fragments and of visualizing the fingerprint that the separation process produces. The separation technique used is electrophoresis.
Electrophoresis is a separation technique similar in principle to various forms of chromatography (i.e., particles moving through a medium at varying rates depending on properties of those particles). For DNA samples the medium generally used is an agarose gel (agarose is derived from agar, the gelling agent used in bacteriological culture media). In electrophoresis an electrical current is passed through the gel and this current serves to “drive” the migration of the DNA sample through the gel.
The forensic investigator introduces the fragmented DNA sample onto one end of the gel to initate the separation. Smaller sized fragments will migrate more quickly through the gel than the larger fragments and similar sized fragments move together in a “band.” Due to the size of the human genome, a very large number of bands is produced and each individual band is somewhat difficult to distinguish from those adjacent to it. Simply staining the gel at this point would result in a nearly continuous smear. The final step in the procedure is Southern Blot analysis—a technique by which a discreet number of the resulting bands are labeled and photographed.
In a Southern Blot the DNA bands are transferred (blotted) from the gel to a nitrocellulose membrane. The membrane is then treated with a radioactive DNA probe. The probe is a short DNA segment that recognizes and binds to a particular repetitive sequence present in several of the bands produced by the electrophoretic separation. A piece of X-ray film is applied to the membrane and the radioactively labelled bands expose the film to produce the final result—the DNA fingerprint. The banding patterns included with this exercise were produced in this precise fashion.
DNA fingerprinting, when properly performed, provides positive evidence of an individual’s identity and is a much more powerful tool than more familiar, classic forensic techniques such as fingerprinting and blood typing. These classic techniques are means of phenotypic (or exclusionary) testing. Phenotypic testing can only be used to prove that the crime scene evidence does not match the suspect.
DNA fingerprinting requires only a relatively small tissue sample and can identify any person from a crime scene who may have left only the tiniest trace of evidence. Potential sources of DNA include: blood samples (even though mature red blood cells carry no nuclear material, sufficient white blood cells are usually present), sperm samples, dried blood or semen from a fabric, a few skin cells from under a victim’s fingernails, or several hairs with roots attached.
A brief summary of the procedure follows:
In forensic testing, DNA samples from the victim, suspect, and the evidence from the crime scene are DNA fingerprinted and compared. Matches between the banding patterns provide evidence that can be used in a court of law. See the diagram of fingerprints below. Victim bands are included to demonstrate that the evidence must indeed have come from the suspect. An overhead transparency is provided so students can see and learn how to match the banding patterns.
Banding Patterns Overhead Transparency*
DNA Case Study #1, 15*
DNA Case Study #2, 15*
Suspect DNA Analysis Cards, 15*
Evidence DNA Analysis Cards (B, C, D), 15*
Evidence DNA Analysis Cards (A, E, F, G), 15*
*Materials included in kit.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping and using models
Analyzing and interpreting data
Engaging in argument from evidence
Disciplinary Core IdeasMS-LS1.A: Structure and Function
HS-LS1.A: Structure and Function
HS-LS3.A: Inheritance of Traits
HS-LS3.B: Variation of Traits
Systems and system models
Structure and function
HS-LS3-1. Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring.
Molecular Cell Biology. James Darnell, Harvey Lodish, David Baltimore. 2nd Edition. 1990 Scientific American Books, Inc., New York.