Okazaki fragment

Okazaki fragments are short stretches of new DNA that form on the lagging strand during replication. The two strands of a double helix run in opposite directions, but DNA polymerase can only add nucleotides in the 5' to 3' direction. One new strand, the leading strand, can therefore grow smoothly toward the replication fork. The other must be built in the opposite direction, away from the fork, one short piece at a time as more template is unwound.

Each fragment starts with its own RNA primer, laid down by the enzyme primase. DNA polymerase extends the primer until it runs into the previous fragment. The RNA is then removed and replaced with DNA, a job done by DNA polymerase I in bacteria, and DNA ligase seals the remaining nick so the pieces become one unbroken strand. In bacteria the fragments are roughly 1,000 to 2,000 nucleotides long; in eukaryotes they are much shorter, around 100 to 200.

The fragments are named after Reiji and Tsuneko Okazaki, who reported in 1968, from work at Nagoya University, that the lagging strand is copied in pieces. They are a favorite exam question on why replication is discontinuous.