DNA Structure (WJEC GCSE Biology: Combined Science)

• DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for the growth and development of all organisms
• It is found in the nucleus of cells in structures called chromosomes

DNA and chromosomes diagram

DNA, chromosomes and the nucleus

• The individual units of DNA are called nucleotides

Nucleotide diagram

A nucleotide is made up of several components to form the overall structure of DNA

• The double helix shape is formed from two strands of DNA wound around each other, like a twisted ladder
• The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder

DNA double helix diagram

The DNA helix is made from two strands of DNA held together by hydrogen bonds

• It is the sequence of nucleotide bases that determines the order of the amino acids used to form different proteins
• Therefore the proteins produced by a cell are dependent on the DNA

• All nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached
• There are four different bases, A, C, T and G
• The bases on each strand pair up with each other, holding the two strands of DNA in the double helix
• The bases always pair up in the same way:
  • A always pairs with T 
  • C always pairs with G
• This rule of pairing is called complementary base pairing and is a key feature of DNA which allows it to be copied for protein synthesis or cell division

Complementary base pairing diagram

DNA base pairs

Higher Tier Only

• The names of the four different bases which form the structure of DNA are:
  • Adenine (A)
  • Cytosine (C)
  • Thymine (T)
  • Guanine (G)
• These bases pair up through complementary base pairing:
  
  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)