P P P

Broken bond

HO H

HO H

HO H

HO H

above works for both RNA and DNA since they both have hydroxyl groups at the 3' position on their sugars. However, dideoxyribose is a sugar that lacks the oxygen of both the 2' and the 3' hydroxyl groups (Fig. 24.05). Nucleotides containing dideoxyri-bose can be incorporated into a growing nucleic acid chain, but that is literally the end of the chain. Incoming nucleotides must be added to the 3'-hydroxyl group of the previous nucleotide. But since dideoxyribose has no 3'-hydroxyl group, no further nucleotides can be added, and the chain is terminated.

Just as we use dG to refer to a normal deoxynucleotide with the base guanine, we use ddG to refer to the dideoxynucleotide with the base guanine. Consider what happens if ddGTP, dideoxyguanosine triphosphate, the dideoxyribose analog of dGTP, is added to a growing DNA chain. When the polymerase reaches the next G, it puts in ddG instead of dG. Then the chain is terminated (Fig. 24.06). During the sequencing dideoxynucleotide Nucleotide whose sugar is dideoxyribose instead of ribose or deoxyribose dideoxyribose Derivative of ribose that lacks the oxygen of both the 2' and the 3' hydroxyl groups

A) Structures of ribose, deoxyribose, and dideoxyribose

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