 The structure of this molecule consists of a purine base ( adenine ) attached by the 9 ′ nitrogen atom to the 1 ′ carbon atom of a pentose sugar ( ribose ) . Three phosphate groups are attached at the 5 ′ carbon atom of the pentose sugar . It is the addition and removal of these phosphate groups that inter @-@ convert ATP , ADP and AMP . When ATP is used in DNA synthesis , the ribose sugar is first converted to deoxyribose by ribonucleotide reductase . 
