Nucleotides (Cambridge International A-Level Biology)

Nucleotide Structure

Spec point — describe the structure of nucleotides, including the phosphorylated nucleotide ATP

• Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules

  • Like proteins (polypeptides) and carbohydrates (polysaccharides), these nucleic acids are polymers
  • This means they are made up of many similar, smaller molecules (known as subunits or monomers) joined into a long chain
  • The subunits that make up DNA and RNA are known as nucleotides
  • Therefore DNA and RNA can also be known as polynucleotides

Nucleotides

• Nucleotides are made up of three components:

  • A nitrogen-containing base (also known as a nitrogenous base)
  • A pentose sugar (containing 5 carbon atoms)
  • A phosphate group

The basic structure of a nucleotide

ATP

• Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
• ATP is another type of nucleotide and hence it is structurally very similar to the nucleotides that make up DNA and RNA
• It is a phosphorylated nucleotide

Exam Hint You need to learn the different groups that nucleotides are made up of (phosphate groups, pentose sugars and nitrogenous bases).

Purines & Pyrimidines

Spec point — state that the bases adenine and guanine are purines with a double ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single ring structure

• 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

  • This means that they have a double-ring structure

• The bases cytosine, thymine, and uracil are pyrimidines

  • These bases have a single-ring structure

The molecular structures of purines and pyrimidines are slightly different

Exam Hint You don’t need to know the structural formulae of these bases (as shown in the diagram above)—you just need to know which are purines and which are pyrimidines.