Primase

Primase is an enzyme that synthesizes short RNA sequences called primers. These primers serve as a starting point for DNA synthesis during DNA replication. Primase is essential because DNA polymerases, the enzymes that synthesize new DNA strands, cannot initiate DNA synthesis de novo. They require a pre-existing 3'-OH group to add nucleotides to.

Primase is a type of RNA polymerase, specifically an RNA-dependent RNA polymerase. This means it uses a DNA template to synthesize RNA. In eukaryotes and bacteria, primase is often found in a complex with other proteins to form a primosome. The primosome helps to localize primase to the replication fork and coordinate its activity with other replication enzymes.

During DNA replication, primase synthesizes short RNA primers of about 10-12 nucleotides in length on both the leading and lagging strands. On the leading strand, only one primer is required to initiate replication at the origin. However, on the lagging strand, multiple primers are needed to initiate the synthesis of short DNA fragments called Okazaki fragments.

Once a primer is synthesized, DNA polymerase extends it, adding DNA nucleotides to the 3' end. After DNA polymerase has completed its task, the RNA primers are removed by another enzyme (in eukaryotes, typically RNase H and flap endonuclease 1 (FEN1)). The resulting gaps are then filled in by DNA polymerase and sealed by DNA ligase.

In summary, primase is a crucial enzyme in DNA replication that provides the necessary starting point for DNA polymerase by synthesizing short RNA primers. Without primase, DNA replication could not occur.