double ring structure
Figure 3.31 A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Carbon residues in the pentose are numbered 1 through 5 the prime distinguishes these residues from those in the base, which are numbered without using a prime notation . The base is attached to the 1 position of the ribose, and the phosphate is attached to the 5 position. When a polynucleotide is formed, the 5 phosphate of the incoming nucleotide attaches to the 3 hydroxyl group at the end of the growing chain. Two types of pentose are found in nucleotides, deoxyribose found in DNA and ribose found in RNA . Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2 position. Bases can be divided into two categories: purines and pyrimidines. Purines have a double ring structure, and pyrimidines have a single ring.