What is the significance of the 3' hydroxyl (-OH) group in DNA synthesis?

What is the significance of the 3' hydroxyl (-OH) group in DNA synthesis?

• A. DNA Polymerase adds nucleotides to the 5' end.
• B. DNA Polymerase adds nucleotides to the 3' hydroxyl (-OH) group end.
• C. 3' OH terminates synthesis.
• D. 3' OH is not involved in synthesis.

Answer: (B)

The key idea is that DNA synthesis extends the strand by using a free 3' hydroxyl group to form the next phosphodiester bond. DNA polymerase catalyzes the bond between the 3' OH of the existing strand and the α-phosphate of the incoming deoxynucleoside triphosphate, releasing pyrophosphate in the process. This setup makes synthesis go in the 5' to 3' direction, because the 3' end of the growing strand provides the nucleophile needed for bond formation. A primer with a free 3' OH is required to start elongation; without it, polymerase cannot add nucleotides. If the 3' OH is blocked or absent, elongation cannot occur. So the significance is that the 3' hydroxyl is the essential site for nucleotide addition, not the 5' end, and not something that terminates synthesis.

The key idea is that DNA synthesis extends the strand by using a free 3' hydroxyl group to form the next phosphodiester bond. DNA polymerase catalyzes the bond between the 3' OH of the existing strand and the α-phosphate of the incoming deoxynucleoside triphosphate, releasing pyrophosphate in the process. This setup makes synthesis go in the 5' to 3' direction, because the 3' end of the growing strand provides the nucleophile needed for bond formation. A primer with a free 3' OH is required to start elongation; without it, polymerase cannot add nucleotides. If the 3' OH is blocked or absent, elongation cannot occur. So the significance is that the 3' hydroxyl is the essential site for nucleotide addition, not the 5' end, and not something that terminates synthesis.