A level Biology exam revision resources written by A level Examiners


Nucleic acids are involved in the storage and transfer of genetic information in all living organisms, including the simplest viruses. There are two types of nucleic acid in cells, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are so named because DNA was first isolated from nuclei, but both DNA and RNA also occur in other parts of the cell. For example DNA is also found in mitochondria and chloroplasts, whilst RNA is also found in the cytoplasm, particularly at the ribosomes.

Both DNA and RNA macromolecules and they are polymers. Their monomers are called nucleotides. DNA and RNA are therefore polynucleotides.

A single nucleotide is made from three components, these are:

  • Sugar
  • Organic base
  • Phosphate

Different nucleotides are formed according to the sugar and organic bases used. Cells continuously produce nucleotides and these form a "pool" from which nucleotides can be used up, as required for manufacturing DNA, RNA or a variety of other substances.

There are five different nitrogen-containing organic bases present in nucleic acids:

  • adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U).
  • A, T, C and G are found in DNA.
  • A, U, C and G are found in RNA (uracil replaces thymine here).

The organic bases all contain nitrogen but they belong to two different chemical families.

  • Adenine and guanine are purine bases and have a double-ringed structure.
  • Cytosine, thymine and uracil are pyrimidine bases and have a single-ringed structure.

The differences between the bases affects the way they can bind to each other. The significance of this will become clear when DNA replication and protein synthesis are examined.

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