OCR AS Biology

Revision Notes

2.3.1 Nucleotides & Phosphodiester Bonds

Nucleotides

  • DNA and RNA are nucleic acids: polymers that are made up of many repeating units (monomers) called nucleotides
  • Each nucleotide is formed from:
    • A pentose sugar (a sugar with 5 carbon atoms)
    • A nitrogen-containing organic base
    • A phosphate group

Basic structure of a nucleotide, downloadable AS & A Level Biology revision notes

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

Comparison between RNA nucleotide and DNA nucleotide, downloadable AS & A Level Biology revision notes

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

Purines and pyrimidines (1), downloadable AS & A Level Biology revision notesPurines and pyrimidines (2), downloadable AS & A Level Biology revision notes

The molecular structures of purines and pyrimidines are slightly different

Nucleotide Structure Table

Table 25 Nucleotide structure, downloadable AS & A Level Biology revision notes

Exam Tip

Although DNA and RNA nucleotides are very similar, make sure you know the key differences between them: unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (in place of this they contain the nitrogenous base uracil) and unlike DNA, RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose).

You don’t need to know the structural formulae of the bases, just which are purines and which are pyrimidines.

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

Phosphodiester bond in a polynucleotide strand, downloadable AS & A Level Biology revision notes

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)

Exam Tip

In condensation reactions, a molecule of water is released. In hydrolysis reactions, a molecule of water is added.

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