DNA Fingerprinting

DNA fingerprinting is a means by which to identify a person based upon his or her genetic blueprint. One useful application of this technology is the determination of guilt or innocence among criminal suspects. A man is shown here breaking into a building and leaving some of his blood behind, depicted as the red droplet. The police find the blood at the crime scene and narrow the search for the criminal down to three suspects, shown here as the stick figures. A blood sample is then taken from each individual and is compared to the blood found at the scene of the crime by DNA fingerprinting. The first step in this process involves the extraction of DNA from each blood sample, including that found at the crime scene. Next, each sample is cut into fragments by enzymes known as restriction endonucleases, which cleave each person’s DNA in a unique pattern that can be thought of as the individual’s genetic fingerprint. The fragmented DNA of each suspect is then applied to separate lanes of a gel matrix, usually agarose, which is a jelly-like substance and is shown here as the grey rectangle. An electrical current is run through the gel to separate the different sized fragments from each another. The electricity causes the smaller pieces of DNA to move toward the bottom of the gel while the largest fragments remain near the top. At this stage, the DNA, which has been chemically stained, appears as white smears down each gel lane because it has been cut into a large number of tiny pieces that are too small to be seen individually. To visualize a pattern on the gel, the DNA fragments from each suspect and from the blood at the crime scene are first transferred to a thin membrane in a process known as a Southern blot. The membrane is then treated with single-stranded DNA probe molecules that are usually tagged with radioactivity or fluorescence. These probes bind, or base pair, to sequences that they match among the DNA samples on the membrane. Binding patterns of the probe to the DNA of each suspect can be visualized by several different methods, such as placing a photographic film over the membrane. The resulting image shows a series of bright bands where the tagged probe molecules were able to bind to the membrane, with the DNA found at the scene of the crime shown in the leftmost lane. If the probe reveals that the restriction enzyme cut pattern of the DNA from the crime scene exactly matches that of one of the suspects, the evidence against that suspect is as strong as a real fingerprint and can be used against him or her in a court of law.

Question 1: DNA fingerprinting is a method used to identify a person based upon his or her ___________.
  1. Fingerprint
  2. Hand-size
  3. DNA folds
  4. Genetic blueprint


Answer 1 Answer – (d) Genetic blueprint

Question 2: What is the first step in DNA fingerprinting?
  1. Measurement of DNA fingerprints.
  2. Extraction of RNA from blood samples and converting it to DNA.
  3. Extraction of DNA from each blood or other sample.
  4. Spreading the blood sample on a slide.


Answer 2 Answer – (b) Extraction of DNA from each blood or other sample.

Question 3: Which enzymes are in charge of cutting the DNA sample into fragments?
  1. DNA-cutters
  2. DNases
  3. Restriction exonucleases
  4. Restriction endonucleases


Answer 3 Answer – (d) restriction endonucleases

Question 4: The fragmented DNA of each suspect is applied to separate lanes of a gel matrix, called ______________.
  1. agarose
  2. agar
  3. argonase
  4. agargel


Answer 4 Answer – (a) agarose

Question 5: DNA is negatively charged, so when electricity is applied to the gel, with the negative pole being where the DNA is loaded, it causes the DNA fragments to separate by size with the __________ fragments moving faster and lower in the gel.
  1. larger
  2. tighter
  3. smaller
  4. equally charged


Answer 5 Answer – (c) smaller

Question 6: What is the main purpose of single-stranded DNA probe molecules?
  1. To make DNA single-stranded.
  2. To separate out the single strands of DNA.
  3. To sequence DNA.
  4. To bind, or base pair, to sequences that they match among the DNA samples on the membrane.


Answer 6 Answer – (d) To bind, or base pair, to sequences that they match among the DNA samples on the membrane.

Question 7: How can forensic scientists determine if the single-stranded DNA probe molecule is complementary to the DNA from the suspect blood samples?
  1. By placing a photographic film or using fluorescence detectors over the membrane to detect binding to the membrane.
  2. By looking at the gel.
  3. By measuring the distances between the bands.
  4. By counting the number of bands on a gel.


Answer 7 Answer – (a) By placing a photographic film or using fluorescence detectors over the membrane to detect binding to the membrane.