Okazaki fragment

Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are complementary to the lagging template strand, together forming short double-stranded DNA sections. Okazaki fragments are between 1000 and 2000 nucleotides long in prokaryotes (e.g. Escherichia coli) and are roughly 100 to 200 nucleotides long in eukaryotes. They are separated by ~120-nucleotide RNA primers and are unligated until RNA primers are removed, followed by enzyme ligase connecting (ligating) an Okazaki fragment onto the (now continuous) newly synthesized complementary strand.

On the leading strand DNA replication proceeds continuously along the DNA molecule as the parent double-stranded DNA is unwound, but on the lagging strand the new DNA is made in installments, which are later joined together by a DNA ligase enzyme. This is because the enzymes that synthesise the new DNA can only work in one direction along the parent DNA molecule and the two strands are anti-parallel . On the leading strand this route is continuous, but on the lagging strand it is discontinuous.

DNA is synthesised from 5' to 3', so when copying the 3' to 5' strand, replication is continuous. Phosphodiester links form between the 3' to 5' and nucleotides can be added with the aid of the enzyme DNA polymerase for the continuous leading strand. However, in order to synthesise the lagging strand (the 5' to 3' strand) synthesis must occur in small sections (100-200 nucleotides at a time in eukaryotes). These new stretches of DNA are" called Okazaki fragments and each one requires its own RNA primer.

A series of experiments eventually led to the discovery of Okazaki fragments. The experiments (see below) were conducted during the 1960s with Reiji Okazaki, Tsuneko Okazaki, Kiwako Sakabe and their colleagues during their research on DNA replication of Escherichia coli. In 1966, Kiwako Sakabe and Reiji Okazaki first showed that DNA replication was a discontinuous process involving fragments. The fragments were further investigated by the researchers and their colleagues through their research including the study on bacteriophage DNA replication in Escherichia coli.

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