Which group is free at the 3' end of the sugar-phosphate backbone?

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Multiple Choice

Which group is free at the 3' end of the sugar-phosphate backbone?

Explanation:
The main idea is that nucleic acid chains grow from the 3' end because the terminal sugar carries a free hydroxyl on its 3' carbon. During polymerization, a new nucleotide is added by forming a phosphodiester bond between the 3'-OH of the existing strand and the 5'-phosphate of the incoming nucleotide, extending the chain toward the 3' direction. That leaves the 3'-OH as the free group at the end. The 5' end bears a phosphate attached to the 5' carbon, not a free hydroxyl, so it isn’t the free end for elongation. The sugar backbone itself doesn’t include an amino group, so that option isn’t applicable.

The main idea is that nucleic acid chains grow from the 3' end because the terminal sugar carries a free hydroxyl on its 3' carbon. During polymerization, a new nucleotide is added by forming a phosphodiester bond between the 3'-OH of the existing strand and the 5'-phosphate of the incoming nucleotide, extending the chain toward the 3' direction. That leaves the 3'-OH as the free group at the end. The 5' end bears a phosphate attached to the 5' carbon, not a free hydroxyl, so it isn’t the free end for elongation. The sugar backbone itself doesn’t include an amino group, so that option isn’t applicable.

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