Video Transcript
The diagram provided shows the
basic outline of how the enzyme reverse transcriptase can be used to clone a section
of genetic material. What is the role of DNA polymerase
in this process? (A) To join gaps in the
sugar–phosphate backbone of the mRNA and the cDNA molecules. (B) To form a strand of DNA that is
complementary to the cDNA. (C) To form a strand of cDNA that
is complementary to the mRNA. (D) To provide the site for the
synthesis of the cDNA strand. Or (E) to break hydrogen bonds
between complementary base pairs.
To understand this question, let’s
first remind ourselves about the techniques used to clone a section of genetic
material.
DNA cloning consists of three main
steps. First, we need to isolate a DNA
sequence of interest, for example, a gene that codes for a protein that has a
therapeutic effect. Second, we need to insert this
sequence in a vector, which is often a bacterial plasmid. Third, we need to make many copies
or clones of this genetic material, for example, by transferring this plasmid into
bacteria by transformation.
The diagram in the question shows
us one way to isolate a DNA sequence of interest. Let’s bring this diagram back.
Sometimes, it’s possible to isolate
the mRNA of the desired gene that codes the protein of interest. Then, the enzyme reverse
transcriptase can use this mRNA as a template to synthesize a single-stranded
complementary DNA that is called cDNA. The next step consists of
converting this cDNA into a double-stranded DNA molecule. The enzyme that can form a strand
of DNA that is complementary to cDNA is a DNA polymerase. The correct answer to this question
is thus (B). In this process, the role of DNA
polymerase is to form a strand of DNA that is complementary to the cDNA.
