AQA A Level Biology复习笔记1.5.2 Nucleotide Structure & the Phosphodiester Bond

Nucleotide Structure & the Phosphodiester Bond


  • Both DNA and RNA are 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
    • A phosphate group



The basic structure of a nucleotide


DNA nucleotides

  • The components of a DNA nucleotide are:
    • A deoxyribose sugar with hydrogen at the 2' position
    • A phosphate group
    • One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)


RNA nucleotides

  • The components of an RNA nucleotide are:
    • A ribose sugar with a hydroxyl (OH) group at the 2' position
    • A phosphate group
    • One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or uracil (U)


  • The presence of the 2' hydroxyl group makes RNA more susceptible to hydrolysis
    • This is why DNA is the storage molecule and RNA is the transport molecule with a shorter molecular lifespan



An RNA nucleotide (top) compared with a DNA nucleotide (bottom)


Purines & Pyrimidines

  • The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines
  • The bases adenine and guanine are purines – they have a double ring structure
  • The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure



The molecular structures of purines and pyrimidines are slightly different


Nucleotide Structure Table


Phosphodiester bonds

  • DNA and RNA are polymers (polynucleotides), meaning that they are made up of many nucleotides joined together in long chains
  • Separate nucleotides are joined via condensation reactions
    • These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide


  • A condensation reaction between two nucleotides forms a phosphodiester bond
    • It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds (phosphate with double bond oxygen attached - oxygen - carbon)


  • The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the sugar-phosphate backbone (of the DNA or RNA molecule)


A section of a single polynucleotide strand showing a phosphodiester bond (and the positioning of the two ester bonds and the phosphate group that make up the phosphodiester bond)