Nucleotides & Phosphodiester Bonds (OCR A Level Biology)

• Nucleotides are the monomers from which DNA and RNA polymers are built
• Nucleotide structure includes:
  • a pentose sugar
  • a nitrogen-containing organic base
  • a phosphate group

Nucleotide structure diagram

All nucleotides contain a pentose sugar, a phosphate and a nitrogenous base

DNA vs RNA nucleotides

• The components of a DNA nucleotide are:
  • a deoxyribose sugar
  • a phosphate group
  • one of four nitrogenous bases:
    • adenine (A)
    • cytosine (C)
    • guanine (G)
    • thymine (T)
• The components of an RNA nucleotide are:
  • a ribose sugar
  • a phosphate group
  • one of four nitrogenous bases:
    • adenine (A)
    • cytosine (C)
    • guanine (G)
    • uracil (U)

DNA nucleotides contain deoxyribose sugar and thymine, while RNA nucleotides contain ribose sugar and uracil

Purines and pyrimidines

• The nitrogenous base molecules in DNA and RNA occur in two structural forms:
  • purines
  • pyrimidines
• Adenine and guanine are purines: they have a double ring structure
• Cytosine, thymine and uracil are pyrimidines: they have a single ring structure

Deoxyribose and ribose sugars

• The sugars in nucleotides are pentose sugars, meaning that they contain 5 carbon atoms
• Deoxyribose sugar is found in DNA and ribose sugar in RNA
• Deoxyribose contains one fewer oxygen atom that ribose
  • In ribose carbon 2 has an OH group, while in deoxyribose carbon 2 has an H group

Nucleotide structure table

• DNA and RNA are polymers (polynucleotides), meaning that they are made up of many nucleotides joined together in long chains
• Separate nucleotides are joined together 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

• 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)
• As the synthesis of polynucleotides requires the formation of phosphodiester bonds, the same is true for the reverse process: the breakdown of polynucleotides requires the breakage of phosphodiester bonds

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)