Video Transcript
True or False: The more complementary two single-stranded DNA molecules are, the more
stable the double-stranded DNA molecule that they form will be.
In order to answer this question, let’s review some key information regarding the
structure of DNA. Here, you can see a section of a DNA molecule with its two strands indicated. Here, we can see the subunit of a nucleic acid, the nucleotide. Each nucleotide in DNA is made up of three parts: a phosphate group, a deoxyribose
sugar, and a nitrogenous base. In DNA, there are four nitrogenous bases: guanine, represented in orange; cytosine,
represented in blue; adenine, represented in green; and thymine, represented in
pink.
These nitrogenous bases can form hydrogen bonds with each other. Guanine pairs with cytosine using three hydrogen bonds, and adenine pairs with
thymine using two hydrogen bonds. Although a single hydrogen bond is weak, collectively, over a long sequence of DNA,
these bonds are strong and are what keeps the two strands together. So, if we have a highly complementary DNA sequence, we can see that the two strands
are being held together by a large number of hydrogen bonds.
In fact, if we count up the hydrogen bonds, we’ll see that there’s 20 of them. There is one pair here that is mismatched and doesn’t form hydrogen bonds. So these bases won’t pair with each other, and so they stick out a bit. With two DNA strands that are only slightly complementary, there will be fewer
hydrogen bonds between the two strands and more mismatches.
Here, we see only eight hydrogen bonds. Each hydrogen bond has a certain amount of energy that is needed to break it. The 20 hydrogen bonds from the first example contain a higher amount of energy
compared to the eight hydrogen bonds that we see in the second example. It would take more energy to break apart these 20 bonds compared to only breaking
apart the eight bonds. So the highly complementary DNA molecule that we see here is more stable than the
slightly complementary one.
Therefore, looking back at the statement “True or False: The more complementary two
single-stranded DNA molecules are, the more stable the double-stranded DNA molecule
that they form will be,” the correct answer is true.
