What is a primer in DNA replication?

In the context of DNA replication, a primer is a critical component that provides a free 3' hydroxyl group needed for the addition of nucleotides during DNA synthesis. Specifically, it is a short nucleic acid sequence, usually made up of RNA in cells, that binds to the single-stranded DNA template. This binding creates a double-stranded section, allowing DNA polymerase, the enzyme responsible for synthesizing new DNA strands, to add complementary nucleotides to the growing DNA chain.

DNA replication cannot start from scratch; it requires pre-existing strands to which new nucleotides can be added. The primer thus serves as this necessary starting point, getting the replication process underway. Once DNA synthesis initiates, the RNA primer can later be removed and replaced with DNA, but its role in initiating the process is indispensable. Understanding the function of the primer is essential for grasping the nuances of DNA replication and the enzymatic processes involved.