1.Nitrogenous BasePurines and pyrimidines are the two categories of nitrogenous bases. Adenine and guanine are purines. Cytosine, thymine, and uracil are pyrimidines. In DNA, the bases are adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, the bases are adenine, thymine, uracil, and cytosine,

2.Pentose SugarIn DNA, the sugar is 2'-deoxyribose. In RNA, the sugar is ribose. Both ribose and deoxyribose are 5-csrbon sugars. The carbons are numbered sequentially, to help keep track of where groups are attached. The only difference between them is that 2'-deoxyribose has one less oxygen atom attached to the second carbon.

3.Phosphate GroupA single phosphate group is PO43-. Thephosphorus atomis the central atom. Oneatom of oxygenis connected to the 5-carbon in the sugar and to the phosphorus atom. When phosphate groups link together to form chains, as in ATP (adenosine triphosphate), the link looks like O-P-O-P-O-P-O, with two additional oxygen atom attached to each phosphorus, one on either side of the atom.

The building blocks ofDNAarenucleotides, which aremadeup ofthree parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure 9.3). ... Adenine (A) and guanine (G) are double-ringed purines, and cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines.