In eukaryotes, which polymerases synthesize the leading and lagging strands, and what role does Pol alpha-primase play?

• A. DNA polymerase epsilon synthesizes the leading strand, DNA polymerase delta synthesizes the lagging strand, with DNA polymerase alpha-primase supplying RNA-DNA primers.
• B. DNA polymerase delta synthesizes the leading strand, DNA polymerase epsilon synthesizes the lagging strand, with Pol alpha-primase proofreading.
• C. DNA polymerase alpha handles both strands, while epsilon acts as primase.
• D. DNA polymerase delta synthesizes both strands; Pol alpha-primase has no role.

Answer: (A)

In eukaryotic replication, two specialized polymerases handle synthesis on the two strands, while a priming complex starts each new strand. The leading strand is synthesized continuously by a highly processive polymerase, while the lagging strand is made in short fragments by another polymerase that works well with frequent priming. The RNA-DNA primers needed to start synthesis are provided by a dedicated priming enzyme complex, which is DNA polymerase alpha-primase. It lays down a short RNA primer and then extends a bit with DNA to create an RNA-DNA primer, which is later extended by the main polymerases.

So, the leading strand is built by a polymerase with high processivity and proofreading ability, the lagging strand by a different polymerase that handles discontinuous synthesis of Okazaki fragments, and the RNA-DNA primers required to start each fragment come from the primase–polymerase alpha complex. This arrangement explains why the described combination—leading strand by one polymerase, lagging strand by another, with Pol alpha-primase supplying primers—best fits what happens during eukaryotic DNA replication.