In this video, we're going to talk a little bit more about the detailed DNA structure. First, it's helpful to recall the information that we covered about DNA in our previous lesson videos where we first introduced DNA. If you don't know anything about DNA structure, then please be sure to go back and check out those older videos on DNA before continuing here. Now that being said, recall from those older videos that DNA actually consists of two strands of nucleotide monomers or these nucleotide building blocks that are repetitively linked together.
If you take a look at our image down below, notice that we're showing you three different representations of the DNA molecule. We've got one representation of the DNA molecule over here, another representation of DNA here in the middle, and a third representation of the DNA molecule over here on the right. Previously, in our lesson videos, it was shown that DNA forms a double helix where there are two strands, one strand there and another strand here, that are wrapped around each other and twisted upon each other to create a double helix, this twisting ladder-type formation. But if you were to take this DNA double helix and you were to untwist the DNA double helix so that it's a straight formation, it would look something like what you see here. Then if you were to zoom into this structure, you would get this image right here, a more detailed view of the DNA molecule. Again, what you would notice is that the DNA molecule consists of these nucleotides that are repetitively linked together. Here is one nucleotide; this is one nucleotide here; this is another nucleotide here; here is another nucleotide. These nucleotides are just repetitively linked together to create a DNA strand. We have two DNA strands here. We have one DNA strand right here, and then we have a second DNA strand over here.
Notice that these two DNA strands are both made up of nucleotides, and they are connected to each other via hydrogen bonds that form between the nitrogenous bases. Here, we can label these dotted lines as hydrogen bonds that form between the two strands and that connect and keep the two strands held together.
Now, recall from our previous lesson videos that a single nucleotide consists of three components: a phosphate group, a five-carbon sugar, and a nitrogenous base, either adenine, guanine, thymine, or cytosine, abbreviated as A, G, T, or C. Also, again, recall that these two DNA strands are antiparallel with respect to each other, which means that they go in opposite directions in terms of their 5' and 3' ends. So, you can see that this strand over here on the left is going from 5' to 3', top to bottom. However, this other strand over here on the right is going from 5' to 3' in the opposite direction, from bottom to top. The DNA strands are going to be antiparallel. When you compare the 5' end to the 3' end, what you'll notice is that at the 5' end of each strand is a free phosphate group, and at the 3' end of each strand is a free hydroxyl group.
Here, comparing the same sides of the two strands, you'll notice that they are chemically different: one has a free phosphate group and one has a free hydroxyl group. That's why it's important to keep in mind the directionality of these DNA strands in terms of their 5' and 3' ends, and that will be very important as we move forward in our course and talk about DNA replication.
Notice here that the nitrogenous bases are towards the middle of the DNA molecule, and on the perimeter of the molecule is the sugar-phosphate backbone. They call it the sugar-phosphate backbone because it is a repetitive sequence of sugar-phosphate groups. DNA molecules have a sugar-phosphate backbone, and that's why we represent that sugar-phosphate backbone here, using blue lines. This concludes our brief introduction to some of the detailed DNA structure, and we'll be able to get some practice applying these concepts that we've learned as we move forward in our course. So, I'll see you all in our next video.