40. The long DNA strand depicted below is serving as a template for lagging strand DNA synthesis.

The short lines represent the newly synthesized okazaki fragments.
At which position among A, B, C and D would DNA primase act next?
(1) A              (2) B
(3) C              (4) D

Background: Role of DNA Primase in Lagging Strand Synthesis

During DNA replication, the lagging strand is synthesized discontinuously in short segments called Okazaki fragments. Each Okazaki fragment begins with an RNA primer synthesized by DNA primase, which provides a free 3′-OH group for DNA polymerase to extend.

• Primase synthesizes short RNA primers (~9–10 nucleotides).

• DNA polymerase then extends these primers to form Okazaki fragments (~100–200 nucleotides in bacteria).

• As the replication fork progresses and exposes new single-stranded DNA (ssDNA), primase must synthesize new primers at appropriate intervals to initiate the next Okazaki fragment.

How Primase Determines Where to Act Next

• Primase acts on single-stranded DNA regions exposed by helicase ahead of the replication fork.

• It synthesizes primers at intervals ensuring Okazaki fragments are regularly spaced.

• After an Okazaki fragment is completed, the lagging strand template continues to unwind, exposing a new ssDNA region where primase can act next.

Applying This to Positions A, B, C, and D

• The short lines represent existing Okazaki fragments synthesized downstream.

• Primase will act at the next available ssDNA region upstream of the last synthesized Okazaki fragment, closest to the replication fork where the lagging strand template is newly exposed.

• Among positions A, B, C, and D, primase acts at the position closest to the replication fork that is not yet primed.

Based on Replication Dynamics and the Figure

• Positions A, B, C, and D represent potential sites on the lagging strand template.

• Since primase synthesizes primers sequentially as the fork progresses, and Okazaki fragments are synthesized from the fork backward, the next primase action site is the position closest to the fork without a primer.

• Typically, this corresponds to the position immediately upstream of the last Okazaki fragment synthesized.

Conclusion: Next Primase Action Site

Primase will act next at position A.

Explanation

• Position A is the closest unprimed ssDNA region near the replication fork.

• Positions B, C, and D are farther away or already primed (occupied by Okazaki fragments).

• This follows the known mechanism where primase lays down primers progressively as the replication fork unwinds DNA.

Correct Answer

(1) A