Nucleic Acids (College Board AP® Biology)

Molecular Structure of Nucleic Acids

Nucleotide Bases & Nucleic Acids

• DNA and RNA are nucleic acid polymers that are made up of many repeating units called nucleotides

• Each nucleotide is formed from:

  • A pentose sugar (a sugar with 5 carbon atoms)

  • A nitrogen-containing organic base (with either 1 or 2 rings of atoms)

    • There are four bases in DNA: adenine (A), cytosine (C), guanine (G) and thymine (T)

    • In RNA, a fifth base, uracil (U), appears in place of thymine

  • A phosphate group (this is acidic and negatively charged)

• The base and phosphate group are both covalently bonded to the sugar

Diagram of the General Structure of a DNA Nucleotide

The basic structure of a nucleotide

Components of Nucleic Acid Structure

• Nucleotide bases join together in chains to form DNA or RNA strands

• The phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one

  • This forms a "sugar phosphate backbone" with a base linked to each sugar

• The polymer of nucleotides is known as a strand

• Nucleotide bases can be joined in any combination/sequence

• The sequence of nucleotide bases acts as a store of genetic information for an organism

RNA & DNA

• RNA & DNA different in structure and function

  • DNA is double-stranded, RNA is usually single-stranded

  • DNA contains the bases A, T, C, and G, whereas RNA contains the bases A, U, C and G

  • DNA has a deoxyribose sugar whereas RNA has a ribose sugar

  • The similarities and difference between DNA and RNA are dealt with in Topic 1.6 Nucleic Acids