So in our last lesson video, we mentioned that dideoxynucleotides are special nucleotides that are commonly used in DNA sequencing techniques. And so here in this video, we're going to talk more about these dideoxynucleotides. Now first, it's helpful to recall from way back in our previous lesson videos when we first introduced DNA that phosphodiester bonds are the types of bonds that form between the 3′ hydroxyl group (OH group) and the 5′ phosphate group of 2 nucleotides during DNA synthesis. Now, these dideoxynucleotides are sometimes referred to as ddNTPs. And dideoxynucleotides, again, are very special nucleotides that differ from the usual normal DNA nucleotides that a cell might use. And so these dideoxynucleotides, they actually contain a 3′ hydrogen atom instead of containing the 3′ hydroxyl group like what the normal DNA nucleotides have. And so by replacing this 3′ hydroxyl group with the 3′ hydrogen atom, this actually blocks the covalent formation of the phosphodiester bond. And so dideoxynucleotides will block the formation of a phosphodiester bond. And so during the elongation step of DNA synthesis, if a ddNTP is incorporated, if a dideoxynucleotide is incorporated, then the elongation during DNA synthesis will be terminated, and elongation will come to a stop if a ddNTP is included. And this is going to be very important once we get to DNA specifics of the DNA sequencing technique as we move forward. And so, of course, the elongation is going to be terminated by the incorporation of the ddNTP at the 3′ end of the new DNA strand. And so we'll be able to see this down below in our image. And so in this example here, we're looking at how dideoxyribonucleotides can terminate elongation during DNA synthesis. And so in this image at the top here, in the greenish color, what we have is the regular DNA nucleotide, which is the deoxyribonucleotide. And this is the normal nucleotide as we covered it when we first introduced DNA way back in our previous lesson videos. And what we have at the bottom here is the special DNA nucleotide that we're calling the dideoxynucleotide ribonucleotide. And so, notice that it has the hydrogen atom at the 3′ position instead of having the hydroxyl group at the 3′ position. And so, once again, over here on the left-hand side, we have the normal deoxyribonucleotides. And, what we can see is that the normal ones can form phosphodiester bonds. And so you can see the normal one is able to form a phosphodiester⏞bond2,differentnucleotides. So this would be the phosphodiester bond. And so the normal nucleotides can form phosphodiester bonds. But what's important to note is that these dideoxyribonucleotides, the special ones over here, they cannot form phosphodiester bonds. And so you can see here we've got the red 'X' to represent that the phosphodiester bond cannot form. And so this is going to have important implications during the elongation phase of DNA synthesis because, of course, with the normal nucleotides that we have over here, DNA synthesis will be able to proceed as normal. However, as you can see over here on the right-hand side, with these dideoxyribonucleotides when there's a hydrogen atom at the 3′ end instead of a hydroxyl group, then DNA replication is going to come to a stop. It will be terminated, as we mentioned, up above here. And so these dideoxynucleotides are going to help terminate DNA synthesis, the elongation of DNA synthesis. And again, this is going to be important as we move forward and talk about dideoxy sequencing. So this here concludes our brief introduction to dideoxynucleotides. And as we move forward, we'll be able to talk about dideoxy sequencing. So I'll see you all in our next video.