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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 ''Escherichia coli'' and are approximately 150 nucleotides long in eukaryotes. They are separated by ~10-nucleotide RNA primers and are unligated until RNA primers are removed, followed by enzyme ligase connecting (ligating) the two Okazaki fragments into one 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. 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 replication fork which is travelling in the opposite direction) synthesis occurs 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 lead 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''.〔An American scientist, by the last name Shandel, discovered this mechanism prior to Okazaki, however he was never accredited with the discovery since the head of his research team decided the discovery was an erroneous interpretation of test results.〕 ==Experiments== The work of Kiwako Sakabe and Reiji Okazaki provided experimental evidence supporting the hypothesis that DNA replication is a discontinuous process. Previously, it was commonly accepted that replication was continuous in both the 3’ to 5’ and 5’ to 3’ directions. 3’ and 5’ are specifically numbered carbons on the deoxyribose ring in nucleic acids, and refer to the orientation or directionality of a strand. In 1967, the Okazakis and their colleagues suggested that there is no found mechanism that showed continuous replication in the 3’ to 5’ direction, only 5’ to 3’ using DNA polymerase, a replication enzyme. The team hypothesized that if discontinuous replication was used, short strands of DNA, synthesized at the replicating point, could be attached in the 5’ to 3’ direction to the older strand.〔 To distinguish the method of replication used by DNA experimentally, the team pulse-labeled newly replicated areas of ''Escherichia coli'' chromosomes, denatured, and extracted the DNA. A large amount of radioactive short units meant that the replication method was likely discontinuous. The hypothesis was further supported by the discovery of polynucleotide ligase, an enzyme that links short DNA strands together. In 1968, Reiji and Tsuneko Okazaki gathered additional evidence of nascent DNA strands. They hypothesized that if discontinuous replication, involving short DNA chains linked together by polynucleotide ligase, is the mechanism used in DNA synthesis, then “newly synthesized short DNA chains would accumulate in the cell under conditions where the function of ligase is temporarily impaired.” ''E. coli'' were infected with Bacteriophage T4 that produce temperature-sensitive polynucleotide ligase. The cells infected with the T4 Phages accumulated a large amount of short, newly synthesized DNA chains, as predicted in the hypothesis, when exposed to high temperatures. This experiment further supported the Okazakis’ hypothesis of discontinuous replication and linkage by polynucleotide ligase. It disproved the notion that short chains were produced during the extraction process as well. The Okazakis’ experiments provided extensive information on the replication process of DNA and the existence of short, newly synthesized DNA chains that later became known as Okazaki fragments. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Okazaki fragments」の詳細全文を読む スポンサード リンク
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