Video Transcript
DNA from different sources can be
combined, or hybridized, in a series of steps. Firstly, the double-stranded DNA is
broken into single strands. After that, how are the single
strands of DNA from different organisms annealed to each other? (A) The enzyme DNA ligase is used
to catalyze the formation of peptide bonds. (B) The enzyme DNase is used to
repair the broken covalent bonds between bases. (C) The temperature is rapidly
increased to provide the energy required for hydrogen bonds between bases to
form. (D) The strands are physically
forced together until they bind. Or (E) the temperature is cooled so
hydrogen bonds between complementary bases can form.
This question is asking us about
the steps involved in DNA hybridization. Let’s clear these answer choices so
we can have more room to work with. DNA hybridization is the process of
combining two complementary single-stranded DNA or RNA molecules and has many useful
applications, for instance, in determining evolutionary relationships between
organisms.
Suppose a new species of ape was
discovered, we’ll call it big foot because of its impressively large feet, and we
want to see how closely related we are. We happen to have a bit of their
DNA, a portion of which is shown here, and we want to compare the two sequences. One way we can do this is with DNA
hybridization. You’ll notice that both strands are
indicated, one being the five prime the three prime strand and the other being the
three prime the five prime strand. You’ll also notice that these two
strands are being held together by hydrogen bonds shown here as these black
lines. These hydrogen bonds are holding
the two strands together with a certain amount of energy.
By increasing the temperature, we
can break these bonds. This then allows a double-stranded
DNA molecule to separate into two single-stranded DNA molecules. This process can be reversed by
lowering the temperature. This allows the hydrogen bonds to
reform between the two strands, and they can anneal or come back together to form
the double-stranded molecule. In this case, both of the original
strands came back together in the human and big foot DNA. This does happen, but sometimes the
big foot DNA will come together with the human DNA too. Because the two sequences aren’t
identical, not all of the bases will pair.
Now, if we were to isolate this
human-big foot hybrid and increased temperature, because of these mismatches, the
energy or temperature required to break these strands apart can be used to estimate
how similar the sequences are, which can be used to describe an evolutionary
relationship. During DNA hybridization, no
enzymes are used, only changes in temperature. And by lowering the temperature,
the two single strands of DNA can anneal to each other to form hydrogen bonds.
