In this video, we're going to talk about the formation and the breakdown of nucleic acids. And so recall from our previous lesson videos that if we want to build a polymer, then we're going to need a dehydration synthesis reaction. Dehydration synthesis reactions are going to link individual and separate nucleotides together so that they can begin to build the nucleic acid polymer. Now the covalent bonds that link these nucleotides together are specifically referred to as phosphodiester bonds. And notice that we have this yellow background behind the phosphodiester bonds in the text, and that's because it links to the yellow color that we have down below in our image, which we'll be able to see here shortly.
Now the formation of phosphodiester bonds between nucleotides is going to result in the sugar-phosphate backbone of the nucleic acid. And the nucleic acid backbone is going to have directionality as we already indicated in our previous lesson video. So we know that there's going to be a 5 prime end and a 3 prime end. We're specifically indicating that the 5 prime end is going to be the phosphate group end, the free phosphate group, which we'll be able to see down below. And the 3 prime end is going to be the free hydroxyl group, the hydroxyl end. Let's take a look at our image down below to start to clear some of this up.
We're looking at phosphodiester bond formation. Notice on the far left-hand side, we're showing you 2 separate nucleotide monomers. Here we're showing you a cytosine, and here we're showing you a thymine. And what you'll also notice is that these are specifically deoxyribonucleotides or DNA nucleotides because this position here is not containing a hydroxyl group. It has one less oxygen, deoxy.
These are DNA nucleotides, and notice that they're separate over here. If we want to join them together to start to build a DNA polymer, we're going to need the dehydration synthesis reaction, which we know is used to build up polymers. The dehydration synthesis dehydrates the molecule, releasing a water molecule, and it synthesizes a larger molecule in the process. Notice that these two nucleotides are joined together via this bond that we have highlighted in yellow, a phosphodiester bond.
What you'll note is that there is a sugar phosphate backbone that has been formed here where we have alternating sugar and then phosphate, then sugar and then phosphate, and so this is what we call the sugar-phosphate backbone. And branching off of the sugar phosphate backbone, we have the nitrogenous bases. Once again, this sugar phosphate backbone has directionality. It has a 5 prime end and a 3 prime end. The 5 prime end is going to be the phosphate group end, the end that has the free phosphate group.
When we take a look at our image down below, notice that the free phosphate group is over here on this end, and so this will be the 5 prime end for that reason. And then, of course, the 3 prime end is going to be the end that has the free hydroxyl group. Notice that there's a hydroxyl group at this end, the opposite end, and so this is going to be the 3 prime hydroxyl end. This here really concludes our introduction to the formation and the breakdown of nucleic acid polymers.
We'll be able to get some practice applying the concepts that we've learned here as we move forward in our course. So I'll see you all in our next video.