What is Primase?
Primase is an RNA polymerase enzyme. It synthesizes short RNA strands, known as primers, which are complementary to a DNA template. Unlike DNA polymerase, primase can initiate these RNA sequences without needing an existing strand. These primers serve as starting points for DNA synthesis.
Primase’s Role in DNA Duplication
Primase is essential for DNA replication because DNA polymerase cannot initiate new DNA strands. DNA polymerase requires an existing primer with a free 3′-hydroxyl group to add new nucleotides. Primase synthesizes these short RNA primers on the DNA template.
During DNA replication, the DNA double helix unwinds, creating a replication fork with two template strands. On one strand, known as the leading strand, DNA synthesis proceeds continuously in one direction. Primase synthesizes a single RNA primer at the origin of replication on this leading strand, allowing DNA polymerase to continuously extend the new DNA.
The other strand, called the lagging strand, is synthesized discontinuously in short segments. Because the lagging strand template runs opposite the replication fork movement, primase repeatedly synthesizes multiple RNA primers along this strand. Each primer provides a starting point for DNA polymerase to synthesize a short DNA segment, known as an Okazaki fragment. This repeated action ensures DNA replication proceeds efficiently on both strands, despite differing synthesis mechanisms.
The Primer’s Journey: From RNA to DNA
After primase synthesizes an RNA primer and DNA polymerase extends it, the temporary RNA primer must be removed. This removal ensures the newly synthesized DNA strand is entirely DNA. In prokaryotic organisms, DNA polymerase I removes these RNA primers due to its 5′ to 3′ exonuclease activity.
Following primer removal, DNA polymerase I fills the resulting gaps with DNA nucleotides. In eukaryotic cells, enzymes like RNase H and FEN1 (Flap Endonuclease 1) degrade and remove the RNA primers. RNase H specifically targets the RNA component of RNA-DNA hybrids, while FEN1 removes the remaining RNA and displaced DNA segments.
After the gaps are filled with DNA, small breaks or “nicks” remain between the newly synthesized DNA segments. The final step involves DNA ligase, which forms phosphodiester bonds to connect these segments, creating a continuous DNA strand. This coordinated action ensures temporary RNA primers are replaced with DNA, maintaining genetic material integrity.