 In all known organisms , the <unk> that make up DNA are synthesized by the action of ribonucleotide reductase ( RNR ) enzymes on their corresponding ribonucleotides . These enzymes reduce the sugar residue from ribose to deoxyribose by removing oxygen from the 2 ′ hydroxyl group ; the substrates are ribonucleoside diphosphates and the products <unk> diphosphates ( the latter are denoted <unk> , <unk> , <unk> , and <unk> respectively . ) All ribonucleotide reductase enzymes use a common sulfhydryl radical mechanism reliant on reactive cysteine residues that oxidize to form disulfide bonds in the course of the reaction . RNR enzymes are recycled by reaction with thioredoxin or <unk> . 
