This video, we're going to talk more about step 1b of the steps of DNA cloning, which is using ligation enzymes to help finalize the creation of the recombinant DNA molecule. And so what's important to note is that DNA ligase is an enzyme, and it is an enzyme that ligates or covalently joins the 2 sticky ends together that were created in step 1a. And that is going to create the final recombinant DNA molecule that contains DNA from 2 different sources. Now, it is important to note that only DNA fragments that have been cut by the same restriction enzyme are capable of being ligated back together. And that's because the sticky ends that are generated by a restriction enzyme are going to be quite unique, and only the correct sticky ends can be ligated together.

And so if we take a look at our example down below, we can see that a restriction enzyme and a DNA ligase are both needed, and used to clone a recombinant DNA plasmid. And so down below here, we're looking at creating recombinant DNA. And what's important to note is that over here on the left-hand side, we're showing you a bacterial plasmid. And over here on the right-hand side, we're showing you a eukaryotic cell, such as, for example, a human cell. And say there is a gene of interest highlighted here in orange that is within the human cell, and, this orange region here represents the gene of interest. Now what's important to note is that, of course, in step 1a, we know that restriction enzymes are going to be used, and the restriction enzymes are going to recognize restriction sites. And so over here, zooming into the plasmid DNA, you can see that there's one restriction site as you can see. And over here in this gene of interest, notice that it is being flanked by 2 restriction sites. And, the actual gene of interest is just this small little region that you see right here in the middle.

And so you use restriction enzymes to cut the plasmid DNA and restriction enzyme to cut the to cut these DNA molecules is going to generate sticky ends, these single-stranded DNA overhangs. And so notice that, we have these sticky ends that are color-coded here in these colors. And what can happen is, these sticky ends, they can match and pair with each other where this sticky end comes and matches with this region, and this sticky end over here comes and matches with the other region. And so when that happens, this overlapping of these sticky ends, across different molecules, you can get what we have down below, which is the gene of interest right here in the middle, now, is being pieced back together with the right in-between the DNA plasmid where the DNA plasmid was cut. And so, of course, in order to covalently join and seal the gene of interest in the middle with the plasmid, what we'll need is these ligase enzymes. And the ligase enzymes are being represented by these little glue bottles because the DNA ligase is going to connect the DNA fragments just like glue is used to connect separate things together.

And so, down below here, what we're showing you is really just the recombinant DNA molecule because it now has DNA from 2 different sources. It has the gene of interest, which was isolated from the human cell, and of course, it has the plasmid DNA, which was from the bacteria. And so, the molecule ends up looking like what we see over here, where you have the bacterial plasmid and the gene of interest within it. And so we've created our recombinant DNA molecule. And so now that we've created this recombinant DNA molecule, we know just in general how this a bacterial host cell. But for now, this here concludes our introduction here to step 1b, using ligation enzymes, and we'll be able to get some practice applying these concepts as we move forward in our course.