Nucleic acids

The essentials

Nucleic acids are long-chain polymers of nucleotides found in the nuclei of cells. They carry hereditary information and direct the synthesis of proteins. There are two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Complete hydrolysis of a nucleic acid gives a pentose sugar, phosphoric acid and nitrogen-containing heterocyclic bases.

Key points

• Nucleoside = base + sugar. Nucleotide = base + sugar + phosphate (attached at C-5′ of the sugar).
• Nucleotides are joined by phosphodiester linkages between C-5′ and C-3′ of the sugars.
• DNA sugar: β-D-2-deoxyribose. RNA sugar: β-D-ribose.
• DNA bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA bases: A, G, C and uracil (U) instead of thymine.
• Watson and Crick proposed that DNA is a double helix: two strands coiled around each other and held by hydrogen bonds between specific base pairs. A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). The two strands are complementary.
• Because of base pairing, in double-stranded DNA the amount of A equals T and the amount of G equals C.
• RNA is usually a single strand that may fold on itself.
• Types of RNA: messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA); all take part in protein synthesis.
• Functions: DNA is the chemical basis of heredity; it can replicate itself, passing on genetic information to daughter cells. Proteins are synthesised by RNA molecules according to the message coded in DNA.

Exam tips

• For base composition numericals: %A = %T and %G = %C, and the four add up to 100%.

Common mistakes

• Saying RNA contains thymine or that DNA contains ribose.