Chapter 5: Nucleotides & the DNA Backbone
Proteins are built from a message written in DNA. This short chapter covers the molecular building block of that message: the nucleotide.
Three parts of a nucleotide
A nucleotide is built from exactly three pieces:
- A nitrogenous base — either a purine (adenine, guanine — two fused rings) or a pyrimidine (cytosine, thymine, uracil — one ring).
- A five-carbon sugar — ribose (in RNA) or deoxyribose (in DNA, missing the 2'-hydroxyl group ribose has).
- One or more phosphate groups, which give the nucleotide its negative charge.
Nucleoside vs. nucleotide
A nucleoside is just base + sugar, with no phosphate. Add one or more phosphate groups and it becomes a nucleotide — AMP, ADP, and ATP are the mono-, di-, and triphosphate forms of the adenosine nucleotide, and this same molecule doubles as the cell's universal energy currency introduced in Chapter 1.
Building the DNA backbone
Single nucleotides are joined end to end by phosphodiester bonds: a phosphate group bridges the 3' carbon of one sugar to the 5' carbon of the next sugar. Repeating this link over and over builds the sugar-phosphate backbone of a DNA (or RNA) strand, with the nitrogenous bases projecting off to the side — the bases that ultimately pair up (A–T, G–C) to hold the double helix together.
Key teaching point
Three simple building blocks — a base, a sugar, and a phosphate — are enough to build the entire genetic code. The same base-pairing logic that makes DNA replication possible is what lets a gene be transcribed into mRNA (Chapter 4), and that mRNA translated into the very proteins this book has spent four chapters describing. Nucleotide chemistry is the root of the whole story.