Question Video: Comparing DNA Fingerprints Biology

The diagram shows simple DNA fingerprints of two individuals. Which of the following statements is true about these fingerprints? [A] Individual B has two fragments that are both smaller than the fragments of individual A. [B] Individual A has more fragments than individual B. [C] Individual A has a larger genome than individual B. [D] In these samples, there are no fragments of DNA that individuals A and B have in common.

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Video Transcript

The diagram shows simple DNA fingerprints of two individuals. Which of the following statements is true about these fingerprints? (A) Individual B has two fragments that are both smaller than the fragments of individual A. (B) Individual A has more fragments than individual B. (C) Individual A has a larger genome than individual B. (D) In these samples, there are no fragments of DNA that individuals A and B have in common.

Before answering the question, let’s briefly recall ourselves about the principle of DNA fingerprinting. DNA fingerprinting is a method used to reveal the unique DNA pattern that identifies each individual. However, the DNA between two humans is 99.9 percent similar on average. How is it then possible to detect our differences?

Actually, the remaining 0.1 percent DNA is highly variable, so much that the likelihood of two persons having the same DNA is close to zero, unless these persons are twins. So DNA fingerprinting relies entirely on this highly variable portion of our DNA. How is it done?

To produce a DNA fingerprint, the DNA of each individual is first extracted from a biological sample, which can be, for example, from their blood or saliva. Then, in each tube, the DNA of each individual is cut by enzymes called restrictions enzymes at certain sites, shown here in pink. As we can see here in yellow, the highly variable regions of the DNA varies in size and in position across the DNA of each individual. As a consequence, the cut of the enzyme produces fragments that vary in size and sometimes in number.

Here, in our example, the cut results in two fragments of different sizes. The DNA on the left has been cut in two medium-size fragments, one smaller than the other. And the DNA on the right has been cut in one small fragment and one large fragment. Now, don’t forget that these fragments are still in a tube. We need a way to separate them to visualize their different sizes.

To visualize the different fragments obtained by cutting the DNA with restriction enzymes, a process called gel electrophoresis is used. It’s a method that separates a mixture of DNA fragments based on their charge and size. A special gel is prepared and plunged in a saline solution that conducts electricity. This gel contains tiny wells where we add the mix of DNA fragments from each individual. Then, electricity is applied so the end of the gel closest to the wells is negatively charged and the other end is positively charged.

Since DNA molecules are negatively charged, the fragments contained in each well migrate through the gel towards the positive end. After some time, we can stop the current and expose the gel to a special light to see where the fragments have migrated. The results look like the diagram provided in the question, a series of bands in front of each well corresponding to each individual. These are the DNA fingerprints of individuals A and B.

These different bands correspond to DNA fragments of different sizes. Indeed, small fragments can migrate quickly because they can squeeze through the net of the gel more easily. The large fragments, however, travel slowly and remain closer to the top of the gel. So now let’s review our choices to answer the question.

Choice (A) is correct by stating that individual B has two fragments but only one is smaller than the fragments of individual A. So we can eliminate this choice.

Now, we can see that both individuals have two bands of the same thickness. This suggests that both individuals have the same number of fragments. We can thus eliminate this choice.

Choice (C) states that individual A has a larger genome than individual B. This doesn’t seem very likely because both genomes seem to have the same number of cuts and same number of fragments. So we can eliminate this choice.

Choice (D) states that there are no fragments of DNA that individuals A and B have in common. Indeed, if individuals A and B had bands aligned on the same level of the gel, this would mean that the cut produced fragments of similar sizes. We can see that the fragments produced by the cut in the DNA of individual A and B have very different sizes.

Thus, choice (D), stating that “In these samples, there are no fragments of DNA that individuals A and B have in common,” is correct.

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