DAT Bootcamp Molecular Genetics Practice Test

The end with the terminal hydroxyl is the 3' end. In DNA, the sugar at the end carries a free hydroxyl on the 3' carbon, which is the site where new nucleotides are added during synthesis. Nucleotides are joined by phosphodiester bonds formed between the 3' hydroxyl of the growing strand and the phosphate of the incoming nucleotide, so extension occurs at the 3' end. The 5' end, in contrast, bears a phosphate on the 5' carbon and does not have a free hydroxyl for chain elongation. A 3' phosphate end would lack the necessary 3'-OH altogether, and a 5' phosphate end is simply another way to describe the end with a terminal phosphate, not a hydroxyl.