DNA polymerase extends from the primer's end. Which precise statement is correct?

DNA polymerase extends from the primer's end. Which precise statement is correct?

• A. The 3' end of the primer is extended by DNA polymerase
• B. The 5' end of the primer is extended by DNA polymerase
• C. The 3' end of the primer serves as the starting point for DNA synthesis
• D. DNA polymerase cannot extend from the primer

Answer: (C)

DNA polymerase can only add nucleotides to a pre-existing strand that has a free 3' hydroxyl group; it cannot start synthesis from scratch. The primer provides that essential 3' end, so DNA synthesis begins at the primer's 3' end and proceeds by extending from there in the 5' to 3' direction. Saying the 3' end of the primer serves as the starting point precisely captures where synthesis begins, which is why this statement is correct. The 5' end cannot serve as the starting point because there’s no free 3' hydroxyl there for the polymerase to extend from, and while the 3' end is indeed the site of extension, identifying it as the starting point best conveys the initiation role of the primer.

DNA polymerase can only add nucleotides to a pre-existing strand that has a free 3' hydroxyl group; it cannot start synthesis from scratch. The primer provides that essential 3' end, so DNA synthesis begins at the primer's 3' end and proceeds by extending from there in the 5' to 3' direction. Saying the 3' end of the primer serves as the starting point precisely captures where synthesis begins, which is why this statement is correct. The 5' end cannot serve as the starting point because there’s no free 3' hydroxyl there for the polymerase to extend from, and while the 3' end is indeed the site of extension, identifying it as the starting point best conveys the initiation role of the primer.