Which enzyme adds a short segment of RNA to initiate DNA replication?

The enzyme that adds a short segment of RNA to initiate DNA replication is primase. During the process of DNA replication, primase synthesizes a short RNA primer that provides a starting point for DNA polymerases, which are responsible for elongating the new DNA strand. The RNA primer is necessary because DNA polymerases can only add nucleotides to an existing strand of nucleic acid; they cannot start a new strand from scratch.

Primase thus plays a crucial role in ensuring that DNA replication can commence, allowing the subsequent synthesis of the new DNA strands to proceed efficiently. Once the RNA primer is laid down by primase, DNA polymerase III can then extend the primer, synthesizing the new DNA strand by adding DNA nucleotides complementary to the template strand.

Other enzymes mentioned have different roles; for instance, DNA ligase is responsible for joining Okazaki fragments on the lagging strand, DNA polymerase III is key for elongating the DNA strand but requires the primer created by primase, and topoisomerase helps relieve the strain of the double helix during unwinding, but none are involved in the initial primer synthesis.