Ii

NCC C CH

Cytosine

Nitrogenous Bases

The sugar and phosphate group are identical in all DNA nucleotides. However, the nitrogenous base may be any one of four different kinds—thymine (THIE-MEEN), cytosine (SIET-oh-SEEN), adenine (AD-uh-NEEN), or guanine (GWAH-NEEN). The nitrogenous bases and their chemical structures, called rings, are shown above in Figure 10-7. The nitrogenous bases are often represented by the first letter of their name—T (thymine), C (cytosine), A (adenine), and G (guanine).

Nitrogenous bases that have a double ring of carbon and nitrogen atoms, such as adenine and guanine, are called purines (PYUR-eenz). Nitrogenous bases that have a single ring of carbon and nitrogen atoms, such as cytosine and thymine, are called pyrimidines (pi-RIM-uh-DEENZ).

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