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.
