Chapter 19 · Biochemistry
19.7Nucleic Acids: Molecular Blueprints
9 min · two checks
Predict
What three pieces make a nucleotide, and which bases pair?
The idea
- Identify the parts of a nucleotide.
- State the base-pairing rules.
A nucleotide is phosphate plus sugar plus base. DNA uses deoxyribose and the bases adenine, thymine, guanine, and cytosine. RNA uses ribose, and uracil stands in for thymine. Nucleotides join by phosphodiester links between the sugar of one and the phosphate of the next, so a strand has a backbone of sugar and phosphate and a sequence of bases sticking out. The sequence is the information. The backbone is the same all the way down.
Base pairing is hydrogen bonding with a shape match. A pairs with T (or U in RNA) using two hydrogen bonds. G pairs with C using three, so G–C pairs are harder to pull apart. The pairs are the same width, which lets a long double helix stay regular. A strand has direction, 5′ to 3′, and the two strands of DNA run opposite ways. That antiparallel geometry is what makes copying possible.
Keep these
- Nucleotide = phosphate + sugar + base.
- DNA: A–T and G–C. RNA: U replaces T.
- The sequence of bases is the information. The backbone carries it.