Question

DNA replication occurs from

• A. 5'-3' direction
• B. 3'-5' direction
• C. 3'-5' at a time
• D. 5'- direction only

Hint:

All nucleic acid polymerases (DNA polymerase, RNA polymerase, Reverse Transcriptase) synthesize strands in the \( 5' \rightarrow 3' \) direction.
The template is always read in the opposite direction: \( 3' \rightarrow 5' \).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the directionality of the synthesis of a new DNA strand during the process of replication.
Detailed Explanation:

• Role of DNA Polymerase:
The enzyme responsible for DNA synthesis is DNA polymerase.
This enzyme can only add new nucleotides to the free \( 3'\text{-OH} \) (hydroxyl) group of a pre-existing polynucleotide chain.
Therefore, the new strand is always synthesized starting from its \( 5' \) end towards its \( 3' \) end.

• Biochemical Basis:
The incoming nucleotide (deoxynucleoside triphosphate) attaches its \( 5' \) phosphate group to the \( 3'\text{-OH} \) of the last nucleotide in the growing chain.
Pyrophosphate is released, providing the energy for the phosphodiester bond formation.

• Leading and Lagging Strands:
Because the DNA template is antiparallel, and synthesis only occurs \( 5' \rightarrow 3' \):
The Leading strand is synthesized continuously toward the replication fork.
The Lagging strand is synthesized discontinuously in short segments called Okazaki fragments, but each fragment is still synthesized in the \( 5' \rightarrow 3' \) direction.

Step 3: Final Answer:
DNA replication (synthesis) always proceeds in the \( 5' \) to \( 3' \) direction.