How is sequencing terminated when a modified nucleotide is used to form a complementary DNA strand?

• A. Modified nucleotide does not form bonds with other nucleotides
• B. It blocks all polymerase activity
• C. It causes the polymerase to skip bases
• D. It promotes rapid chain elongation

Answer: (A)

The idea being tested is how chain termination works in sequencing reactions that use a modified nucleotide as a terminator. In this approach, the terminator nucleotide lacks a 3' hydroxyl group, which prevents the formation of the next phosphodiester bond. When the polymerase incorporates this terminator, elongation stops at that point, so you get DNA fragments of varying lengths that end wherever a terminator was incorporated. By separating these fragments by size, you can read the sequence of the template strand.

So the best answer is that the modified nucleotide does not form the next bond with another nucleotide because the missing 3' hydroxyl prevents elongation. The other options don’t fit: the polymerase isn’t completely blocked—elongation just terminates at the point of incorporation; it doesn’t cause skipping bases; and it certainly doesn’t promote faster chain growth.