Biochemistry Module 6 Practice Exam

The 3' end is defined by the free hydroxyl group attached to the 3' carbon of the terminal deoxyribose in DNA. This -OH is the attachment point for the next nucleotide during DNA synthesis, since nucleotides are added to the growing strand by forming a phosphodiester bond between the 3' OH of the last nucleotide and the 5' phosphate of the incoming one. That makes the strand extend in the 5' to 3' direction, with the 5' end carrying a phosphate and the 3' end carrying the free 3'-OH. The phosphate group marks the 5' end, not the 3' end, and amino or methyl groups do not define DNA strand ends.