Which enzyme allows DNA polymerase III to begin replicating the 3' strand?

Primase is the enzyme that synthesizes short RNA primers during DNA replication. These primers are essential because DNA polymerases, including DNA polymerase III, cannot initiate synthesis of a new strand of DNA on their own; they can only add nucleotides to an existing strand of nucleic acid.

When DNA replication begins, primase lays down an RNA primer on the single-stranded DNA template. This primer provides a free 3' hydroxyl (OH) group for DNA polymerase III to extend the new DNA strand. DNA polymerase III then adds DNA nucleotides, synthesizing the new strand in the 5' to 3' direction. Thus, without primase, DNA polymerase III would not have the necessary starting point to begin replication of the 3' strand.

In contrast, ligase is involved in joining Okazaki fragments on the lagging strand, helicase unwinds the DNA double helix, and polymerase I plays a role in replacing RNA primers with DNA after replication is complete. Each of these other enzymes has specific functions that are crucial for DNA replication, but they do not initiate the process by providing the necessary primer for DNA polymerase III.