Nucleotides & Phosphodiester Bonds (OCR A Level Biology): Revision Note

Nucleotides

• 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

Phosphodiester Bond

• 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)