Video Transcript
What is the function of restriction
enzymes? (A) To break the hydrogen bonds in
double-stranded DNA. (B) To cut sequences of DNA at
specific recognition sites. (C) To form a sequence of mRNA
complementary to a strand of DNA. (D) To join gaps in the
sugar–phosphate backbone of DNA.
To answer this question correctly,
let’s discuss what restriction enzymes are by reviewing the process of DNA
recombination. DNA recombination is a very useful
process that combines genetic material from two different sources. One of the very first steps in this
process involves a restriction enzyme that is derived from bacteria. In fact, restriction enzymes are
produced by certain bacteria to destroy foreign invading DNA, for example, during a
viral infection.
How can an enzyme destroy DNA? By chopping it up, of course. So, restriction enzymes must cut
the DNA backbone, which is held together by phosphodiester bonds. They do this at very specific sites
called palindromic sequences. A palindrome refers to something
that is read the same backward or forward, like the words “madam” or “racecar.” Remember that DNA is always read in
the 5 prime to 3 prime direction. Therefore, a palindrome will have
the same sequence in the 5 prime to 3 prime direction on both strands. An example of a palindromic
sequence could be CCATGG.
If a restriction enzyme cuts its
specific sequence in a staggered fashion, the resulting DNA will have sticky
ends. These are essential for DNA
recombination. If two strands of DNA are cut with
the same restriction enzyme, they will leave overhangs that are complementary. These overhangs can then base pair
with one another and recombine. The enzyme DNA ligase will restore
the DNA backbone, and a DNA molecule containing genetic information from two
different sources is formed. DNA recombination is a very useful
process, and it is all made possible thanks to restriction enzymes.
Now that we have discussed their
function, let’s take a look at our answer choices. We know that restriction enzymes
cut the DNA backbone at specific locations called palindromes. Because the DNA backbone is held
together by phosphodiester bonds, we can eliminate answer (A), to break the hydrogen
bonds in double-stranded DNA.
Answer (D), to join the gaps in the
sugar–phosphate backbone of DNA, describes the function of the enzyme DNA
ligase.
Answer (C) is also incorrect as RNA
polymerase is the enzyme that synthesizes a sequence of mRNA from a DNA
template.
We are looking for the function of
restriction enzymes, which is given in answer choice (B): to cut sequences of DNA at
specific recognition sites.
