Final answer:
DNA replication is started with an RNA primer, a short stretch of RNA nucleotides synthesized by primase. This primer is necessary for DNA polymerase to begin adding complementary deoxyribonucleotides to the template strand.
Step-by-step explanation:
DNA replication is initiated with a RNA primer, which is a short stretch of RNA nucleotides. In vivo, this process begins at sequences called origins of replication. The enzyme primase, part of the primosome, synthesizes the RNA primer. The RNA primer is necessary because DNA polymerase, which synthesizes the new DNA strand by adding nucleotides, can only add to an existing 3'-OH group, which the primer provides.
Once the RNA primer is in place, DNA polymerase III extends the primer by adding deoxyribonucleotides that are complementary to the template strand. In prokaryotes like E. coli, the DNA polymerase also requires the initial opening of the double helix by helicase and subsequent stabilization by single-strand binding proteins to prevent reannealing. DNA replication proceeds bidirectionally from the origin, creating two replication forks.
The leading strand is synthesized continuously in the direction of the replication fork, whereas the lagging strand is synthesized in short stretches called Okazaki fragments, which are later joined together by DNA ligase after the RNA primer is replaced by DNA nucleotides.
Eukaryotic cells similarly require a primer to initiate DNA replication at their multiple origins, and involve a similar cast of molecular characters and enzymes, including the notable addition of telomerase to deal with chromosome end replication challenges and various repair mechanisms to correct errors during and after replication.