Nucleotides & Phosphodiester Bonds (Edexcel A (SNAB) A Level Biology)

• 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 mononucleotide

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

RNA nucleotide compared with an DNA nucleotide

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 structure of each base is different, depending on whether they are a purine or pyrimidine

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