1.6 Nucleic Acids

Nucleotides: the monomers

Nucleic acids — DNA and RNA — store and transmit genetic information. Their monomer is the nucleotide, made of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. Nucleotides link through phosphodiester bonds into a sugar-phosphate backbone.

DNA structure

DNA is double-stranded, uses the sugar deoxyribose, and has four bases: adenine (A), thymine (T), guanine (G), cytosine (C). The strands are antiparallel (one runs 5'→3', the other 3'→5') and held together by complementary base pairing: A pairs with T, and G pairs with C, via hydrogen bonds.

RNA is different

RNA is usually single-stranded, uses the sugar ribose, and replaces thymine with the base uracil (U). New nucleic-acid strands are always built in the 5'→3' direction. These differences let RNA act as a working copy and messenger of DNA's instructions.

Key terms

Nucleotide

The monomer of nucleic acids: a five-carbon sugar + a phosphate + a nitrogenous base.

Complementary base pairing

A–T and G–C in DNA (A–U in RNA), held together by hydrogen bonds.

Antiparallel

The two DNA strands run in opposite directions (5'→3' and 3'→5').

Deoxyribose vs. ribose

DNA's sugar is deoxyribose (one fewer oxygen); RNA's sugar is ribose.

Phosphodiester bond

The covalent bond linking nucleotides into the sugar-phosphate backbone.

Try an AP-style question

What percentage of the bases are guanine?

1. 28%
2. 22%
3. 56%
4. 44%