1.6 Nucleic acids

Summary

Nucleic acids — DNA and RNA — are polymers of nucleotide monomers, each composed of a five-carbon (pentose) sugar, a phosphate group, and a nitrogenous base. Biological information is encoded in the specific linear sequence of these nucleotides.

Every nucleic acid strand has directionality defined by its 5' phosphate and 3' hydroxyl ends, and new nucleotides are always added to the 3' end during synthesis through the formation of a covalent phosphodiester bond.

DNA exists as an antiparallel double helix in which the two strands run in opposite orientations and are held together by hydrogen bonds between complementary base pairs: adenine with thymine, and cytosine with guanine.

DNA

• Contains the sugar deoxyribose
• Contains the base thymine
• Is typically double-stranded
• Serves as the stable, long-term repository of genetic information

RNA

• Contains the sugar ribose
• Contains the base uracil
• Is typically single-stranded
• Adenine pairs with uracil rather than thymine
• Its single-stranded flexibility allows it to participate in diverse functions, including carrying genetic messages, forming ribosomal structures, and transporting amino acids

At a Glance

Key Definition Complementary base pairing is the central principle connecting structure to function in both molecules, ensuring that genetic information can be accurately stored, copied, and transmitted.