Video Transcript
What is the role of DNA ligase in
forming recombinant DNA? (A) To form strands of DNA that are
complementary to the mRNA taken from an organism. (B) To cut sections of DNA, leaving
“sticky ends” to allow recombination. (C) To form a strand of DNA
complementary to a DNA template strand. (D) To join gaps in the
sugar–phosphate backbones of combined DNA.
This question is asking us about
DNA ligase and its role in forming recombinant DNA. Let’s review some terminology as it
relates to forming recombinant DNA. Recombinant DNA is the combination
of DNA from at least two different sources. For example, we can create
recombinant DNA by joining bacterial DNA and human DNA together. This is often done using bacterial
plasmids, which are small circular pieces of extrachromosomal DNA that exist in
bacteria to carry accessory genes not found on their chromosome. This can help them adapt to their
environments, for example, by carrying antibiotic resistance genes.
These two sources of DNA can be
combined to form recombinant DNA. In order for this DNA to be joined
together to form recombinant DNA, the DNA from the two sources needs to be cut. To do this, we use special enzymes
called the restriction enzymes to cut DNA. Restriction enzymes can recognize
and cut specific sequences in DNA called recognition sequences. For example, the restriction enzyme
EcoRI cuts at the sequence GAATTC, which we can see indicated here. When EcoRI cuts DNA, it cuts it in
the pattern that we see here. When DNA is cut by a restriction
enzyme, it cuts DNA by cleaving the phosphodiester bond in the sugar–phosphate
backbone. So this is how the bacterial
plasmid DNA, as well as the human DNA, can be cut with a restriction enzyme.
Now let’s discuss how these can be
joined together to form recombinant DNA. After the sequence is cut, you’ll
notice that there’s overhangs, or regions of unpaired DNA bases. These are called sticky ends
because they have a tendency to stick back together due to their complementary
bases. So, if we cut our bacterial plasmid
DNA and human DNA with the same restriction enzyme, we can bring those two pieces of
DNA back together because they’ll have complementary sticky ends. Here, we can see the bacterial
plasmid DNA and human DNA combined to form the recombinant DNA.
You’ll notice that there’s gaps in
the sugar–phosphate backbone of this recombinant molecule. This is from when the restriction
enzyme cleaved the phosphodiester bond earlier when it cut these molecules. To repair these gaps, an enzyme
called DNA ligase can be used to join the sugar–phosphate backbones. And we can see that here. Now the two molecules are joined
completely.
Getting back to our question, the
option that best describes the role of DNA ligase in forming recombinant DNA is
given by answer choice (D), to join gaps in the sugar–phosphate backbone of combined
DNA.
