DNA Structure (Cambridge (CIE) IGCSE Biology)

Structure of a DNA Molecule: Extended

Extended Tier Only

• DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for the growth and development of all organisms

• It consists of two strands of DNA wound around each other in what is called a double helix

DNA, chromosomes and the nucleus

• The individual units of DNA are called nucleotides

A nucleotide

• All nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached

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

  • Adenine always pairs with Thymine (A-T)

  • Cytosine always pairs with Guanine (C-G)

DNA base pairs

• 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

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

• It is this sequence of bases that holds the code for the formation of proteins