 Then the enzyme simultaneously acts as a 5′→3′ exonuclease, removing primer ribonucleotides in front and adding deoxyribonucleotides behind until the region has been replaced by DNA, leaving a small gap in the DNA backbone between Okazaki fragments which is sealed by DNA ligase.In eukaryotic primer removal, DNA polymerase δ extends the Okazaki fragment in 5′→3′ direction, and upon encountering the RNA primer from the previous Okazaki fragment, it displaces the 5′ end of the primer into a single-stranded RNA flap, which is removed by nuclease cleavage.Cleavage of the RNA flaps involves either flap structure-specific endonuclease 1 (FEN1) cleavage of short flaps, or coating of long flaps by the single-stranded DNA binding protein replication protein A (RPA) and sequential cleavage by Dna2 nuclease and FEN1.