Video Transcript
DNA from different sources can be
combined, or hybridized, in a series of steps, as outlined in the diagram
provided. Which of the following best
explains what is happening in step 1? (A) The hydrogen bonds between
double-stranded DNA are broken as the strands are physically separated. (B) The hydrogen bonds between
double-stranded DNA are broken using heat. (C) The peptide bonds between
double-stranded DNA are broken by DNA polymerase. (D) The covalent bonds between
double-stranded DNA are broken as the temperature is rapidly decreased. Or (E) the phosphodiester bonds
between double-stranded DNA are broken using restriction enzymes.
To answer this question correctly,
we need to discuss the process of DNA hybridization and the steps required for it to
occur. To give us more space to do this,
let’s remove our answer choices for now.
Hybridization is the combination of
two complementary, single-stranded DNA or RNA molecules, often from two different
sources, to form a double-stranded hybrid molecule. Note that DNA is generally found in
its double-stranded form and not in its single-stranded form.
In order for hybridization to
happen, the two strands of the double-stranded helix have to be separated. You might recall that single
strands of DNA are held together via hydrogen bonding between base pairs. Two hydrogen bonds join adenine and
thymine, while cytosine and guanine are held together by three hydrogen bonds.
With this in mind, note that these
hydrogen bonds must be broken in order to separate the two strands of DNA into
single strands. This is achieved by heating the DNA
strand. To break hydrogen bonds, a
temperature of 95 to 100 degrees Celsius is required. You might recall that 100 degrees
Celsius is the boiling temperature of water. So lots of heat is required to
separate these DNA strands.
Thankfully, the phosphodiester
bonds that join each nucleotide in the DNA backbone are much stronger and do not
separate when the molecule is heated to 95 to 100 degrees Celsius. The result of heating DNA to these
temperatures in the first step of DNA hybridization is single-stranded DNA.
Now that we have discussed DNA
hybridization and the steps that are involved in the process , let’s take another
look at our diagram and answer choices. Our question refers to the first
step in our diagram, which is showing the hydrogen bonds between the double-stranded
DNA being broken and the two strands separating to form single-stranded DNA. We can eliminate answer choices
(C), (D), and (E) as the bonds referred to in these choices are incorrect.
We have just discussed how the
first step of hybridization, resulting in the separation of hydrogen bonds between
double-stranded DNA, is achieved by heating. When the DNA cools down in the
second step of DNA hybridization, double-stranded DNA helices form.
Therefore, the correct answer to
our question “Which of the following best explains what is happening in step 1?” is
answer choice (B). The hydrogen bonds between
double-stranded DNA are broken using heat.
