A primer is required to initiate the DNA synthesis by DNA polymerase. DNA polymerases require a pre-existing RNA or DNA strand, the primer, to initiate new DNA synthesis. These polymerases add deoxyribonucleotides only to the 3' end of a growing strand. Taking a close look at the nucleotide polymerization reaction, the 3' end of the primer contains a free 3'-hydroxyl group. The 3'-hydroxyl reacts with the 5' end of the next free nucleotide to be added.
Free nucleotides continue to be added to the growing DNA strand by the same type of reaction.
At the origin, the DNA duplex is melted by specialized proteins, resulting in a replication bubble. DNA replication begins at specific chromosomal sites, called replication origins. DNA synthesis starts at the origin and proceeds bidirectionally. Two replication forks move outward in opposite directions. At each growing replication fork, two new DNA strands are made.