Nucleic Acids (College Board AP® Biology): Study Guide

DNA & RNA

• DNA and RNA are polynucleotides; they are made up of many nucleotide monomers linked together in a chain

Nucleotide structure

• DNA & RNA nucleotides contain three components:

  1. a five-carbon, or pentose, sugar

     • DNA = deoxyribose sugar

     • RNA = ribose sugar

  2. a phosphate group

  3. a nitrogenous base

     • DNA = adenine, guanine, cytosine and thymine

     • RNA = adenine, guanine, cytosine and uracil

3' and 5' ends

• In a polynucleotide the nucleotide monomers are arranged in a linear sequence which has two distinct ends:

  • the 3′ (three prime) end, which has a terminal hydroxyl (–OH) group on carbon 3 of a sugar

  • the 5′ (five prime) end, which has a terminal phosphate group on carbon 5 of a sugar

• During nucleic acid synthesis, new nucleotides are always added to the 3′ end of the growing strand; this process forms strong covalent bonds between the nucleotides

DNA structure

• DNA is structured as an antiparallel double helix, with the two strands running in opposite 5′ → 3′ orientations

• The nitrogenous bases within the double helix are joined by hydrogen bonds:

  • adenine (A) pairs with thymine (T) → A–T

  • cytosine (C) pairs with guanine (G) → C–G

• While RNA is usually single-stranded, it can form folds which are held together by base pairing; when this occurs:

  • adenine (A) pairs with uracil (U) instead of thymine

  • cytosine (C) pairs with guanine (G) as in DNA

Comparing DNA and RNA

• Structural differences between DNA and RNA include:

  • DNA contains the sugar deoxyribose while RNA contains the sugar ribose

  • DNA contains the nitrogenous base thymine (T) while RNA contains uracil (U)

  • DNA is typically double-stranded while RNA is typically single-stranded