Okazaki fragments

Asymmetry in the synthesis of leading and lagging strands

Okazaki fragments are short sequences of DNA nucleotides (approximately 150 to 200 base pairs long in eukaryotes) which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication.[1] They were discovered in the 1960s by the Japanese molecular biologists Reiji and Tsuneko Okazaki, along with the help of some of their colleagues.

During DNA replication, the double helix is unwound and the complementary strands are separated by the enzyme DNA helicase, creating what is known as the DNA replication fork. Following this fork, DNA primase and DNA polymerase begin to act in order to create a new complementary strand. Because these enzymes can only work in the 5’ to 3’ direction, the two unwound template strands are replicated in different ways.[2] One strand, the leading strand, undergoes a continuous replication process since its template strand has 3’ to 5’ directionality, allowing the polymerase assembling the leading strand to follow the replication fork without interruption. The lagging strand, however, cannot be created in a continuous fashion because its template strand has 5’ to 3’ directionality, which means the polymerase must work backwards from the replication fork. This causes periodic breaks in the process of creating the lagging strand. The primase and polymerase move in the opposite direction of the fork, so the enzymes must repeatedly stop and start again while the DNA helicase breaks the strands apart. Once the fragments are made, DNA ligase connects them into a single, continuous strand.[3] The entire replication process is considered "semi-discontinuous" since one of the new strands is formed continuously and the other is not.[4]

[2] During the 1960s, Reiji and Tsuneko Okazaki conducted experiments involving DNA replication in the bacterium Escherichia coli. Before this time, it was commonly thought that replication was a continuous process for both strands, but the discoveries involving E. coli led to a new model of replication. The scientists found there was a discontinuous replication process by pulse-labeling DNA and observing changes that pointed to non-contiguous replication.

  1. ^ Balakrishnan L, Bambara RA (February 2013). "Okazaki fragment metabolism". Cold Spring Harbor Perspectives in Biology. 5 (2): a010173. doi:10.1101/cshperspect.a010173. PMC 3552508. PMID 23378587.
  2. ^ a b Okazaki T (2017-05-11). "Days weaving the lagging strand synthesis of DNA - A personal recollection of the discovery of Okazaki fragments and studies on discontinuous replication mechanism". Proceedings of the Japan Academy. Series B, Physical and Biological Sciences. 93 (5): 322–338. Bibcode:2017PJAB...93..322O. doi:10.2183/pjab.93.020. PMC 5489436. PMID 28496054.
  3. ^ Cooper GM (2000). "DNA Replication". The Cell: A Molecular Approach (2nd ed.). Sunderland (MA): Sinauer Associates.
  4. ^ MacNeill SA (October 2001). "DNA replication: partners in the Okazaki two-step". Current Biology. 11 (20): R842–R844. doi:10.1016/s0960-9822(01)00500-0. PMID 11676941. S2CID 15853820.