Nucleotides (OCR Gateway GCSE Biology)

• 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