Nucleotides (OCR GCSE Biology A (Gateway))

DNA is Made of Four Nucleotides

• The individual monomer subunits of DNA are called nucleotides

• There are four distinct nucleotides

• The four nucleotides contain the same sugar and phosphate group but differ from each other in the type of base attached

  • The base on each nucleotide is the only part that varies

  • There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)

  • This means there are four different types of nucleotides

A monomer nucleotide

Base pairing

• The bases of one DNA strand form strong cross-links with bases of the other strand

  • Cross-linking helps keep the two DNA strands tightly wound in a double helix

• The bases always pair up in the same way:

  • Adenine always pairs with Thymine (A-T)

  • Cytosine always pairs with Guanine (C-G)

• This is known as ‘complementary base pairing’

DNA complementary base pairs