Hi, this is a continuation of a video lecture on chapter three, cell biology and DNA. I think it's one of the most difficult chapters in our textbook. So, we're going to take a little bit more time with this one. If you remember, I said that DNA is a very long molecule that holds all the genetic information for an organism. We can kind of think of it as a cookbook or a recipe book that holds all the instructions for creating a new organism. We're going to take a look at the structure of DNA and its functions. DNA, deoxyribonucleic acid, is a bunch of atoms stuck together to form a very, very, long shape of a spirally ladder. What we refer to as a double helix. And if we were to take that double helix and untwist it it will look very much like a ladder and we can use the analogy of a ladder to better understand the structure of DNA. The side rails of DNA or the backbone of the DNA molecule are made up of a sugar and phosphate molecules. To be more precise- is made up of sugar and phosphate molecules in an alternating sequence. It also consists of four nitrogen bases: adenine, thymine, cytosine, and guanine. They form, like the rungs of a ladder. The four nitrogen bases adenine, thymine, cytosine, and guanine are called complementary base pairs. So, what does that mean? It means that adenine always pairs with T or thymine, cytosine represented here by the C always pairs with G ,guanine, no matter what the sequence is, A will always pair with T and C will always pair with G. Adenine pairs with thymine, cytosine pairs with guanine.