FIGURE 21.13 Chemical Synthesis of DNA—Nucleotide Addition
During chemical synthesis of DNA, the DNA is added in a 3' to 5' direction. In order to add the successive bases correctly, the 3'-OH of incoming bases must be activated by phosphoramidite (purple) but the 5'-OH must be blocked with a dimethoxytrityl (DMT) group. After the first nucleotide is anchored to the glass bead its DMT blocking group is removed by acid. The next nucleotide can then link to the exposed 5'-OH by a phosphate linkage. Notice that the second nucleotide still has its 5'-OH blocked with DMT. The process continues by using acid to remove DMT from the second nucleotide, adding the third nucleotide and so on (not shown here).
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