DNA synthesis polarity — All known DNA polymerases add dNTPs in which direction relative to the growing strand, and only to what type of primer end?

Introduction:
Directionality is a fundamental rule of DNA synthesis. This question checks whether you know the universal polarity of DNA polymerases and the primer requirement that underlies replication and repair mechanisms.

Given Data / Assumptions:

• DNA polymerases cannot start synthesis de novo.
• They extend from a primer with a free 3'-OH group.
• Nucleotide addition chemistry is conserved across life.

Concept / Approach:
Nucleotidyl transfer adds the alpha phosphate of an incoming dNTP to the 3'-OH of the primer, releasing pyrophosphate. This reaction can only proceed 5' to 3' along the newly synthesized strand, explaining leading and lagging strand asymmetry and the need for RNA primers in replication.

Step-by-Step Solution:

Verification / Alternative check:
Okazaki fragment formation and replication fork architecture directly reflect this polarity: primase provides RNA primers; DNA polymerase extends 5' to 3' only.

Why Other Options Are Wrong:

• 3' to 5' synthesis: not supported by known polymerase chemistry.
• Any end or no primer: contradicts the requirement for a 3'-OH primer.
• Organism dependent polarity: not observed; polarity is universal.

Common Pitfalls:
Confusing the reading of the template (3' to 5') with the direction of synthesis (5' to 3').

Final Answer:
5' to 3' direction, onto a preformed 3'-OH primer end.