H

Uracil

Figure 2.23 Formulas of Purines and Pyrimidines DNA contains the pyrimidine thymine. RNA contains uracil in place of thymine.

Figure 2.22 A Nucleotide This is one subunit of DNA.This subunit is called adenylic acid or deoxyadenosine-5'-phosphate because the base is adenine. If the base is thymine, the nucleotide is thymidylic acid; if guanine, guanylic acid; and if cytosine, cytidylic acid. If the nucleotide lacks the phosphate molecule, it is called a nucleoside, in this case, deoxyadenosine.

Figure 2.24 Joining Nucleotide Subunits (a) Formation of covalent bond between nucleotides by dehydration synthesis. (b) Chain of nucleotides showing the differences between 5' end and 3' end.The chain always is extended at the 3' end, which has the unbonded —OH hydroxyl group.

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