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Okazaki Fragments
 

Okazaki Fragments

The duplication of the DNA strand is brought about by the movement of the replication forks. Replication of the two parental strands takes place simultaneously as the fork moves along. All DNA polymerases can extend the two DNA strands only in the 5' -->3' direction.

The two strands are, however, polarized in opposite directions. The question therefore arises: how can DNA polymerase bring about simultaneous replication of the two strands? How does the enzyme move forward on one DNA strand and backward on the other?Okazaki et al (1968) have suggested that only one strand, the 3'à5' or continuous strand, is continuously replicated.

The other strand (5'-->3') replicates in a discontinuous manner, synthesizing short fragments which have been called Okazaki fragments. This direction of synthesis is opposite to that of the movement of the replicating fork. The fragments are then connected together by a joining enzyme, polynucleotide ligase. The segments produced are about 100 nucleotides long in animal cells and 1,000 -7,000 nucleotides long in prokaryotes.

Okazaki has also suggested that both strands may replicate discontinuously forming short segments which are later joined up. If both new strands are synthesized discontinuously, the fragments formed in the same direction as the movement of the fork would be larger than the fragments on the opposite side, because the segments would be joined to each other before completion of the synthesis of the youngest fragment.

In vitro synthesis from E. coli has shown the existence of the classes of short fragments, 108 and 358. These are synthesized from opposite strands, the shorter fragments from the discontinuous strand and longer fragments from the continuous strand. This suggests that in E. coli both strands synthesize discontinuously.

Many other replicating systems show discontinuous replication in both strands. Replication appears to be discontinuous in most cases. In mitochondrial DNA, however, replication is apparently largely continuous.

There is evidence that Okazaki fragments are initiated with RNA primers. The RNA is later degraded and the gap replaced by newly made DNA before the Okazaki fragments are joined by the ligase enzyme.
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