What is essential for DNA polymerase to begin adding nucleotides?

DNA polymerase is an enzyme that synthesizes new DNA strands by adding nucleotides to a growing chain. For this process to initiate, it requires a primer - specifically a short strand of RNA. This RNA primer provides a free 3' hydroxyl (3' OH) group that serves as the starting point for the addition of DNA nucleotides. Without this primer, DNA polymerase would not have a starting point to begin synthesis, as it cannot initiate the formation of a new strand de novo, meaning it cannot create a new strand from scratch without an existing strand to extend.

In regard to the other choices, having fully formed DNA strands does not help DNA polymerase begin its action; it must attach to a template strand where the primer can attach. While a double-stranded DNA template is indeed required for the replication process, it is not the initial requirement for DNA polymerase to start synthesis, as the crucial element is the RNA primer. The removal of Okazaki fragments pertains to the later stages of DNA replication; it is related to the process of stitching together fragments on the lagging strand rather than the initiation of DNA synthesis. Therefore, the presence of a pre-existing RNA primer is essential for DNA polymerase to begin its function