Nucleotide Structure & the Phosphodiester Bond (AQA A Level Biology): Revision Note

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

    • The carbon atoms in the sugar are numbered as 1′, 2′, 3′, 4′, and 5′

  • A nitrogen-containing organic base

  • A phosphate group

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

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)