Final answer:
Primase is the enzyme that synthesizes the RNA primer required for DNA polymerase to initiate DNA synthesis on the 5' side of DNA.
Step-by-step explanation:
The enzyme that lays down the RNA primer for DNA polymerase to begin synthesizing DNA on the 5' side is called primase. Primase is part of a larger complex known as the 'primosome' and is responsible for creating a short RNA segment that is complementary to the DNA template. The RNA primer is typically about five to ten nucleotides long. DNA polymerase III then uses this RNA primer to start adding DNA nucleotides in a 5' to 3' direction, extending the new DNA strand.
During DNA replication, the RNA primers are eventually removed, and DNA polymerase I replaces the RNA nucleotides with DNA nucleotides to complete the synthesis. The action of DNA ligase follows, which serves to seal any gaps between the Okazaki fragments, thus ensuring the continuous integrity of the newly synthesized DNA strand.