the nucleotide and its three parts
/ NEW-klee-oh-tide /
Imagine the long thread of DNA inside you as a chain made of identical links, like a bicycle chain. The single link is called a nucleotide. Learn the link and you understand the chain, because DNA and RNA are nothing more than thousands or millions of these links joined end to end.
Every nucleotide is built from exactly three parts clicked together: a nitrogenous base (the 'letter' that carries information — A, G, C, T or U), a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and one to three phosphate groups (clusters of phosphorus and oxygen carrying negative charge). The base hangs off the sugar; the phosphate is attached to the sugar at the carbon called 5'. When nucleotides link into a strand, the phosphate of one joins the sugar of the next, leaving the bases sticking out sideways to spell a message.
This matters because the nucleotide is where chemistry meets information. The sugar and phosphate are the same in every link, so they form a monotonous backbone; only the base changes from link to link, and that changing letter is the genetic message. The very same molecule, when it carries three phosphates instead of being locked in a chain, is also the cell's fuel and signal — ATP is just the nucleotide adenosine with three phosphates.
The building block the cell actually adds to a growing DNA strand is a nucleotide carrying three phosphates (a dNTP); two of the three phosphates are snapped off and the energy released drives the link into place.
Base + sugar + phosphate — three parts, one link.
A common slip is to call any base 'a nucleotide'. The base alone is just one of the three parts; you need base plus sugar plus at least one phosphate before the unit is truly a nucleotide.