Okazaki fragment

From The School of Biomedical Sciences Wiki
Jump to: navigation, search

The term Okazaki fragment relates to short oligonucleotide sequences that are synthesised during DNA replication as part of the lagging strand of DNA running in the 5' 3' direction. They were named after Reiji Okazaki who, using radioactive labels in bacterial DNA, observed an increase of short oligonucleotides directly after the replication process had begun, but a greater amount of larger polynucleotides after a longer time period had elapsed. This helped explain the mechanisms involved in the replication of the 5' 3' strand of parental DNA by DNA polymerase [1].

Synthesis of Okazaki fragments

Due to DNA polymerase only being able to replicate DNA in the 5' 3' direction, only the 3' 5' strand of DNA can be replicated continuously. The antiparallel strand is termed the "lagging strand", as its replication is discontinuous. DNA polymerase III replicates short sections of the DNA strand, each around 1000 nucleotides long[2], by polymerising deoxynucleoside triphosphates (dATP, dGTP, dCTP or dTTP[3]. In order for DNA polymerase III to bind, a short RNA primer is synthesised by the enzyme DNA primase at regular intervals along the lagging strand as the replication fork moves along the DNA molecule. The lagging strand forms a loop in order for the DNA polymerase III holoenzyme to synthesise each fragment in the 5' 3' direction. This is also referred to as the "Trombone Model[4]

Joining of Okazaki fragments

The DNA polymerase III holoenzyme leaves gaps in the base sequence between individual Okazaki fragments where the RNA primer is still bound. DNA polymerase I now removes the RNA primer and closes the gap between the two fragments by synthesising the complementary strand in the 5' 3' direction. The remaining nick in the deoxyribose-phosphate backbone is sealed by the action of DNA ligase, which joins the 5' phosphate on the growing lagging strand to the 3' hydroxyl-group on the newly synthesised Okazaki fragment[5].

References

  1. Becker W, Kleinsmith L, Hardin J and Bertoni G (2009) The World of the Cell, 7th edition,San Francisco: Pearson Benjamin Cummings, p. 560
  2. Becker W, Kleinsmith L, Hardin J and Bertoni G (2009) The World of the Cell, 7th edition,San Francisco: Pearson Benjamin Cummings, p. 560
  3. Berg J, Tymoczko J, Stryer L (2007) Biochemistry, 6th edition, New York: W.H. Freeman, p. 793
  4. Berg J, Tymoczko J, Stryer L (2007) Biochemistry, 6th edition, New York: W.H. Freeman, p. 800
  5. Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walter P (2008) Molecular Biology of the Cell, 5th edition, New York: Garland Science, p. 272
Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox