Summary

• Living things carry genetic information in large molecules called DNA (deoxyribonucleic acid ), which is packaged into chromosomes in the nuclei of cells.

• DNA molecules are made up of subunits called nucleotides, which consist of a sugar, a base and a phosphate.

• There are four different nucleotides in DNA, each with a different base – G, C, A or T.

• Nucleotides can join together in two ways.

• Firstly, nucleotides can join by phosphodiester bonds (covalent bonds) between sugars and phosphates, resulting in single-stranded DNA.

• Secondly, nucleotides can join by hydrogen bonds (weaker bonds) between bases, resulting in double-stranded DNA.

• The hydrogen bonding between bases on different strands of DNA occurs according to the base-pairing rules – only G-C and A-T pairs can form.

• Double-stranded DNA molecules have a twisted structure known as a double helix.

• The order of the four different nucleotides in DNA molecules determines the genetic make-up of an organism.

(Image: brian0918, Wikimedia Commons)