Video Transcript
DNA hybridization can be used to
help determine the evolutionary relationship between two species, the process of
which has been simplified and outlined in the diagram provided. When DNA hybridizes, it will form
hydrogen bonds between any base pairs that are complementary. An X on the diagram indicates a
hydrogen bond has not formed. Which of the following is an
assumption scientists make when using this technique? (A) DNA taken from two different
species will form more hydrogen bonds if those two species are closely related. (B) DNA taken from two different
species will form fewer hydrogen bonds if those two species are closely related. Or (C) the number of hydrogen bonds
formed between hybridized DNA does not indicate how closely related those species
are.
To answer this, let’s first quickly
review how DNA hybridization works and how it can be used to help determine
evolutionary relationships between species.
You might remember learning that
the two strands that make up a DNA molecule are complementary to one another. The nucleotides on one strand form
hydrogen bonds with the nucleotides on the other strand. These hydrogen bonds are what keep
the two strands together. With that in mind, let’s see how we
would apply DNA hybridization to compare the DNA of humans to that of
chimpanzees.
Let’s assume here that species A is
human and species B is chimpanzee. As we can see here, both sections
of DNA have two strands that are held together by hydrogen bonds. To apply DNA hybridization, the two
types of DNA would first be mixed together in a solution. Now, using heat, the hydrogen bonds
between the strands can be separated. This causes the two strands to
separate, as can be seen in the diagram. Then, the solution would be cooled
down, which allows the hydrogen bonds to reform. This is called annealing.
When annealing happens, some
strands of human DNA might bind to strands of chimpanzee DNA. However, in this case, not all
nucleotides would be able to form bonds with each other. Since humans and chimpanzees have
some differences in their DNA, at some points on this hybridized strand, the
opposite nucleotides would not be complementary and therefore would not form
hydrogen bonds.
In our diagram, the lack of
hydrogen bond is represented as X. As you can see here, out of the six
nucleotides represented, four have formed hydrogen bonds, while two have not. If, instead of chimpanzee, we had
used mouse DNA with the human DNA, fewer hydrogen bonds would have formed. We can see from the fact that the
human and mouse DNA strands don’t match as well that the DNA in these two species is
less similar, and therefore they must be less closely related than the human and
chimpanzee.
With this information, we can now
answer our question. When using DNA hybridization,
scientists make the assumption stated in option (A). DNA taken from two different
species will form more hydrogen bonds if those two species are closely related.
