THE ORIGIN OF REPLICATION
DNA synthesis begins at a specific base sequence, termed the origin of replication. DNA replication is bidirectional, starting at the origin of replication and proceeding in both directions from that point. A eukaryotic chromosome may have multiple origins of replication and may replicate at many points along its length.

Individual nucleotides within DNA strands are covalently linked; the 3' carbon of one sugar is linked to the 5' phosphate of the adjacent sugar to form a 3'-5' phosphodiester linkage—a sugar-phosphate backbone. Therefore, each strand of DNA has a 5' carbon attached to a phosphate on one end (the 5' end) and a 3' carbon attached to a hydroxyl group on the other end (the 3' end). Though DNA replication is bidirectional with respect to the DNA helix, replication on a single strand always proceeds in a 5' to 3' direction.