Structure & replication of DNA

DNA is the molecule that stores the instructions for every living thing, yet this vast, information-packed molecule is built from just one small repeating unit — the nucleotide. In this lesson we meet the nucleotide (a pentose sugar, a phosphate group and a nitrogenous base) and the special phosphorylated nucleotide ATP. We sort the five bases into two families — the double-ring purines adenine and guanine, and the single-ring pyrimidines cytosine, thymine and uracil — then build the famous double helix: two antiparallel sugar-phosphate backbones joined by complementary base pairs, A with T by two hydrogen bonds and C with G by three. We compare DNA with single-stranded RNA, and finally walk slowly through semi-conservative replication in S phase — helicase unzipping the helix and DNA polymerase building a new strand on each old template, so every copy keeps one old strand and one new one. We finish by designing a model of DNA that shows base pairing and semi-conservative replication with your own hands.