Deoxyribonucleic acid, or DNA, serves as the fundamental instruction manual for all known living organisms. Its accurate duplication, called DNA replication, occurs before cell division. DNA primase is an enzyme that plays a specific role in this intricate process.
The Role of DNA Primase
DNA primase is an enzyme that synthesizes short RNA sequences known as RNA primers. These primers are typically 5 to 10 nucleotides long and are complementary to the DNA template strand. Primase, classified as an RNA polymerase, lays down these primers at specific initiation sites called origins of replication on the DNA strand.
On the leading strand, which is synthesized continuously, only one RNA primer is generally needed to begin the replication process. However, on the lagging strand, DNA synthesis occurs in short, discontinuous Okazaki fragments, each requiring its own RNA primer to initiate synthesis. Once the RNA primer is in place, it provides a starting point for another enzyme, DNA polymerase, to begin adding new DNA nucleotides.
Why Primase is Indispensable
DNA primase is necessary because DNA polymerase, the enzyme responsible for synthesizing the new DNA strand, cannot initiate DNA synthesis from scratch. DNA polymerase requires an existing 3′-hydroxyl group to which it can add new nucleotides. Without this pre-existing attachment point, DNA polymerase is unable to begin forming a new DNA chain.
Primase solves this problem by synthesizing a short RNA primer de novo, meaning it can start a new strand from nothing. The RNA primer provides the necessary 3′-hydroxyl group, allowing DNA polymerase to bind and elongate the new DNA strand. This capability makes it a fundamental component of the DNA replication machinery.
The RNA Primer’s Journey
Once the RNA primer has initiated DNA synthesis, it is temporary and must be removed to ensure the final DNA molecule consists entirely of DNA nucleotides. In prokaryotic cells, enzymes like DNA polymerase I and RNase H remove these RNA primers. RNase H specifically degrades the RNA component of RNA-DNA hybrids.
After RNA primer removal, a gap remains in the newly synthesized DNA strand. DNA polymerase fills this gap with the appropriate DNA nucleotides. Finally, an enzyme called DNA ligase seals the remaining nicks or breaks in the DNA backbone, connecting the newly synthesized DNA fragments into a continuous strand. This ensures the integrity and continuity of the replicated DNA molecule.