Directionality of DNA replication During chromosomal DNA synthesis by DNA polymerases, nucleotide addition and strand elongation proceed in which direction?
-
A3' to 5'
-
B5' to 3'
-
CRandom without a set direction
-
DVaries from organism to organism with no constraint
-
EBoth 5' to 3' and 3' to 5' simultaneously by the same active site
Answer
Correct Answer: 5' to 3'
Explanation
Introduction:DNA polymerases have strict chemical polarity. Understanding this polarity explains the existence of leading and lagging strands, Okazaki fragments, and specific nuclease activities involved in replication fidelity and primer removal.
Given Data / Assumptions:
- Nucleotides are added to a free 3 hydroxyl group on the growing strand.
- Incoming deoxynucleoside triphosphates provide the energy for bond formation.
- Replication forks manage antiparallel templates using leading and lagging strategies.
Concept / Approach:
The chemistry of phosphodiester bond formation requires nucleophilic attack by the 3 hydroxyl on the alpha phosphate of the incoming nucleotide. This enforces 5 to 3 strand growth regardless of species, although different polymerases participate.
Step-by-Step Solution:
1) Define polymerase action: extend only from a 3 hydroxyl end.2) Conclude that elongation direction is 5 to 3 relative to the newly synthesized strand.3) Connect this to replication architecture, where one strand must be discontinuous.4) Recognize that proofreading often uses a separate 3 to 5 exonuclease, not reverse synthesis.Verification / Alternative check:
Biochemical assays uniformly show incorporation in the 5 to 3 direction, and no known cellular DNA polymerase synthesizes 3 to 5.
Why Other Options Are Wrong:
3 to 5 synthesis is not observed. Random or organism specific reversal is not supported. A single active site cannot perform simultaneous opposite direction synthesis.
Common Pitfalls:
Confusing proofreading direction with synthesis direction. Proofreading removes nucleotides 3 to 5, but synthesis still proceeds 5 to 3.
Final Answer:
5' to 3'