Which enzyme lays down RNA primers to initiate DNA synthesis?

Which enzyme lays down RNA primers to initiate DNA synthesis?

• A. DNA Polymerase
• B. RNA Polymerase
• C. Ligase
• D. Primase

Answer: (D)

Primase is the enzyme that lays down RNA primers to start DNA synthesis. DNA polymerases can’t begin a new strand from scratch; they need a free 3' hydroxyl to extend from. Primase provides that starting piece by synthesizing a short RNA primer on both the leading and lagging strands. Once the primer is in place, DNA polymerase can add DNA nucleotides and continue synthesis. In bacteria, this priming enzyme is a specialized RNA polymerase (often referred to simply as primase), and in eukaryotes it’s part of the DNA polymerase alpha-primase complex. After synthesis gets going, the RNA primer is removed and replaced with DNA, and ligase seals the remaining nicks. The other enzymes don’t perform this priming function: DNA polymerase builds DNA but cannot initiate it, ligase seals nicks, and RNA polymerase generally transcribes RNA for gene expression rather than priming DNA replication.

Primase is the enzyme that lays down RNA primers to start DNA synthesis. DNA polymerases can’t begin a new strand from scratch; they need a free 3' hydroxyl to extend from. Primase provides that starting piece by synthesizing a short RNA primer on both the leading and lagging strands. Once the primer is in place, DNA polymerase can add DNA nucleotides and continue synthesis. In bacteria, this priming enzyme is a specialized RNA polymerase (often referred to simply as primase), and in eukaryotes it’s part of the DNA polymerase alpha-primase complex. After synthesis gets going, the RNA primer is removed and replaced with DNA, and ligase seals the remaining nicks. The other enzymes don’t perform this priming function: DNA polymerase builds DNA but cannot initiate it, ligase seals nicks, and RNA polymerase generally transcribes RNA for gene expression rather than priming DNA replication.