DNA replication is the process of producing two identical copies of DNA, in which each template for the synthesis of a new complementary daughter strand. The central enzyme involved is DNA polymerase, which catalyzes the joining of deoyribonucleoside 5'-triphosphates (dNTPs) to form the growing DNA chain.
The points where the DNA first are opened are called replication origins.DNA replication begins at a single origin of replication, and the two replication forks assembled there proceed (at approximately 500-1000 nucleotides per second) in opposite directions until they meet up roughly halfway around the chromosome.
Polymerase needs to add nucleotides to an already existing nucleotide chain. Therefore at the beginning of replication on both the leading and the lagging strands, short stretches of RNA are synthesized to act as polymerization starters or primers.
On the leading strand, only one initial primer is needed because after the initial priming, continuous addition can use the growing DNA strand as the primer. However, on the lagging strand, every Okazaki fragment needs its own primer. The primers are synthesized by a set of proteins called a primosome, of which a central component is an enzyme primase, a type of RNA polymerase. Removal of the RNA primers and filling in of the gaps left by their removal with DNA is performed by a different DNA polymerase, pol I. After pol I has done its job, ligase joins the 3′ end of the gap-filling DNA to the 5′ end of the downstream Okazaki fragment.