Video Transcript
Which of the following statements
about DNA polymerase is correct? (A) DNA polymerase will not be able
to detect any errors made when a DNA strand is being synthesized. (B) DNA polymerase can only
synthesize strands of DNA that are less than 50 nucleotides long. (C) DNA polymerase can only
synthesize DNA strands in the five prime to three prime direction.
To answer this question, let’s
recall the function of DNA polymerase. DNA polymerase is an enzyme that
plays a crucial role in the process of DNA replication. In this process, the two strands of
DNA are unwound, after which DNA polymerase synthesizes two new strands using the
original strands as templates.
You might recall that each
nucleotide in DNA contains a nitrogenous base, which can be adenine, guanine,
thymine, or cytosine. In a molecule of DNA, the
nitrogenous bases on one strand form hydrogen bonds with the nitrogenous bases on
the other strand. This is what holds the two strands
together. These hydrogen bonds always form
according to the rules of complementary base pairing. Adenine always pairs with thymine,
and guanine always pairs with cytosine.
Now, let’s go over how DNA
replication works. DNA polymerase moves along each
strand of DNA synthesizing a new strand. As it encounters each nucleotide
along the original strand, it adds a complementary nucleotide to the new strand. In this way, two new
double-stranded molecules are synthesized. DNA polymerase is a very fast and
efficient enzyme. Usually, it synthesizes strands of
DNA without any errors. However, occasionally, DNA
polymerase adds a nucleotide to the new strand that isn’t complementary to the
corresponding nucleotide on the original strand.
The interesting thing about DNA
polymerase is that if it does make an error, it can correct it almost
immediately. This is called proofreading. Every time DNA polymerase adds a
nucleotide to the new strand, it checks to see whether this new nucleotide is
complementary to the corresponding one on the original strand. If not, it replaces it with the
correct one, before moving on.
Another important thing to remember
about DNA polymerase is that it can only synthesize a new strand of DNA in the five
prime to three prime direction. Here you can see the
sugar–phosphate backbone of the new strand of DNA. You might recall that every strand
of DNA has a five prime end and a three prime end. At the five prime end, there is a
free phosphate group on the end of the strand of DNA. At the three prime end, there is a
free hydroxy group. In order to add new nucleotides,
DNA polymerase needs a free hydroxy group that can react with the phosphate group of
a new nucleotide. Because of this, all synthesis must
happen in the five prime to three prime direction.
With this information in mind,
let’s take a look at our answers choices. As we have just learned, DNA
polymerase has a proofreading function, so we can eliminate option (A).
Option (B) states that DNA
polymerase can only synthesize strands of DNA that are less than 50 nucleotides
long. This isn’t true, so this option is
incorrect.
Therefore, the statement that is
correct about DNA polymerase is answer choice (C). DNA polymerase can only synthesize
DNA strands in the five prime to three prime direction.
