Which of the following statements correctly describes a structural property of dideoxynucleoside triphosphates ddNTPs used in DNA sequencing?

Introduction / Context:
Dideoxynucleoside triphosphates ddNTPs are modified nucleotides used in classical chain termination DNA sequencing methods. Their special structure causes DNA synthesis to stop when they are incorporated. This question tests whether you understand the key structural difference between ddNTPs and normal deoxynucleoside triphosphates dNTPs that leads to chain termination.

Given Data / Assumptions:

• The focus is on ddNTPs and their sugar component.

• Several statements describe possible features of these molecules.

• DNA polymerase usually extends a growing chain by adding nucleotides to a free 3 prime hydroxyl group.

• Only one statement correctly captures why ddNTPs stop further elongation.

Concept / Approach:
Normal dNTPs have a deoxyribose sugar with a hydrogen at the 2 prime carbon and a hydroxyl group at the 3 prime carbon. DNA polymerase adds the incoming nucleotide by forming a phosphodiester bond between the 3 prime hydroxyl of the growing strand and the 5 prime phosphate of the new nucleotide. In ddNTPs, the sugar lacks both the 2 prime and 3 prime hydroxyl groups. In place of the 3 prime hydroxyl, there is a hydrogen. Because there is no 3 prime hydroxyl group, no further nucleotide can be added once a ddNTP is incorporated, causing chain termination.

Step-by-Step Solution:
Step 1: Recall that chain elongation by DNA polymerase requires a free 3 prime hydroxyl group at the end of the growing DNA chain. Step 2: Understand that normal dNTPs provide a deoxyribose sugar with this 3 prime hydroxyl, allowing continued addition of nucleotides. Step 3: In ddNTPs, the sugar is modified so that the 3 prime position has a hydrogen instead of a hydroxyl group. Step 4: Without a 3 prime hydroxyl group, DNA polymerase cannot form the next phosphodiester bond, so no further nucleotides can be added. Step 5: Evaluate option A, which states that they have a hydrogen at the 3 prime carbon instead of a hydroxyl group. This correctly describes ddNTPs. Step 6: Recognise that statements claiming the presence of a free 3 prime hydroxyl or continued polymerase addition contradict the mechanism of chain termination.

Verification / Alternative check:
Descriptions of Sanger DNA sequencing in molecular biology sources explain that ddNTPs lack the 3 prime hydroxyl and therefore terminate chain elongation when incorporated. Structural diagrams show that the sugar in ddNTPs has no hydroxyl groups on both the 2 prime and 3 prime carbons. These explanations directly support the statement that a hydrogen at the 3 prime position is responsible for chain termination.

Why Other Options Are Wrong:

DNA polymerase can add a new nucleotide to a chain ending in a 3 prime ddNTP: This is incorrect, because the lack of a 3 prime hydroxyl prevents any further addition.

They have a free 3 prime hydroxyl group on the sugar just like normal dNTPs: This would make them behave like normal nucleotides and not terminate chains, which contradicts their actual function.

They have an extra oxygen at the 2 prime carbon of the sugar compared to ribose: Ribose already has a hydroxyl at the 2 prime carbon; ddNTPs lack both 2 prime and 3 prime hydroxyl groups.

They contain no phosphate groups and cannot be incorporated into DNA: ddNTPs are triphosphates and can be incorporated by polymerase; their lack of 3 prime hydroxyl causes termination, not inability to enter the chain.

Common Pitfalls:
Learners sometimes confuse ribose, deoxyribose, and dideoxyribose sugars. Another common mistake is to think that any modification in the sugar will terminate the chain, without focusing on the key importance of the 3 prime hydroxyl group. To avoid these errors, remember that polymerases always extend from a free 3 prime hydroxyl, and removing that specific group, as in ddNTPs, is what stops further elongation.

Final Answer:
The correct statement is that ddNTPs have a hydrogen at the 3 prime carbon of the sugar instead of a hydroxyl group.