Ribonucleotide reductases (RNRs) are key metal containing enzymes which catalyze the synthesis of deoxyribonucleotides (DNA). There are three classes of RNRs I, II, III and depending on the class of RNR depends what "type" of reaction it catalyses although they all utilise similar mechanisms for the reduction reaction. **Class I** RNRs utilise an Iron center and the conversion of ferrous iron to ferric iron helps generate a tyrosyl free radical, with its action happeneing under aerobic conditions. there are two further subclasses of class I RNRs with IA being distributed amongst Eukaryotes, bacteriophages, eubacteria and viruses. The IB class is found amongst eubacteria which can also utilise a managanese. **Class II** RNRs utilise cobalamin AKA B12 which generates the free radical 5’-deoxyadenosyl radical and can occur in aerobic or anaerobic environments. **Class III** reductaes are glycine radical enzymes generated via S-adenosyl methionine and an iron-sulphur centre. they are found among eubacteria, bacteriophages and archaebacteria.