SPEAKER: So you're probably familiar with the general appearance of DNA, as it's represented here in this computer model. Make it active. It's a double helix, two polymeric strands. Now, if we unwind those strands, we can see the stacked nucleotides that form this linked structure. And what I'm going to do is take five of those nucleotides out of the DNA helix in diagrammatic form and bring them out here. So that we can go through them in just a little bit more detail. So, as I mentioned, DNA is a polymer of linked nucleotides. And these nucleotides are little repeating units that go down one strand. On the other strand, they go the other way. So these are antiparallel strands We'll get into why that is in a minute. The major structural components of DNA is that they all share a deoxyribose sugar backbone and nitrogenous bases. The nitrogenous bases are the information carriers of the genetic material. The nucleotides themselves are linked together by phosphodiester bonds. And if we zoom in here, you can see why DNA has a negative charge. All right, now back to the nitrogenous bases. There are two kinds. There are purines, adenine and guanine. And there are pyrimidines, thymine and cytosine. And there are also rules governing which can bind to which by hydrogen bonding, the so-called base pairing rules. And in the base pairing rules, cytosine can bind with guanine, and adenine can bind with thymine. And this forms the basis of the genetic code. Now, put these back into the helix. I'll tuck them in. So here are the nucleotides. We'll put them back in there and twist it back up into this right-handed helical spiral of antiparallel strands where you can clearly see that the bases form sort of a semi-stacked ladder, like steps on a staircase.