The Okazaki fragments are DNA segments are synthesized in the chain behind during the process of DNA replication. They are named after their discoverers, Reiji Okazaki and Tsuneko Okazaki, who in 1968 studied DNA replication in a virus that infects the bacterium Escherichia coli.
DNA is made up of two strands that form a double helix, which looks a lot like a spiral staircase. When a cell is going to divide it must make a copy of its genetic material. This process of copying genetic information is known as DNA replication.
During DNA replication, the two chains that make up the double helix are copied, the only difference is the direction in which these chains are oriented. One of the strings is in the 5 '→ 3' direction and the other is in the opposite direction, in the 3 '→ 5' direction.
Most of the information on DNA replication comes from studies done with the E. coli bacteria and some of its viruses.
However, there is enough evidence to conclude that much of the aspects of DNA replication are similar in both prokaryotes and eukaryotes, including humans.
Okazaki Fragments and DNA Replication
At the beginning of DNA replication, the double helix is separated by an enzyme called helicase. DNA helicase is a protein that breaks the hydrogen bonds that hold DNA in the double helix structure, thus leaving the two strands loose.
Each strand in the DNA double helix is oriented in the opposite direction. Thus, a chain has the direction 5 '→ 3', which is the natural direction of replication and that is why it is called a conducting strand. The other chain has direction 3 '→ 5', which is the reverse direction and is called a lagging strand.
DNA polymerase is the enzyme in charge of synthesizing new DNA strands, taking as a template the two previously separated strands. This enzyme only works in the 5 '→ 3' direction. Consequently, only in one of the template strands (the leader strand) can the continuous synthesis of a new DNA strand take place.
On the contrary, since the strand is in the opposite orientation (3 '→ 5' direction), the synthesis of its complementary chain is carried out discontinuously. This implies the synthesis of these segments of genetic material called Okazaki fragments.
Okazaki fragments are shorter in eukaryotes than in prokaryotes. However, the conducting and lagging strands replicate by continuous and discontinuous mechanisms, respectively, in all organisms.
Training
Okazaki fragments are made from a short piece of RNA called a primer, which is synthesized by an enzyme called primase. The primer is synthesized on the lagged template strand.
The enzyme DNA polymerase adds nucleotides to the previously synthesized RNA primer, thus forming an Okazaki fragment. The RNA segment is subsequently removed by another enzyme and then replaced by DNA.
Finally, the Okazaki fragments are attached to the growing DNA strand through the activity of an enzyme called ligase. Thus, the synthesis of the lagged chain occurs discontinuously because of its opposite orientation.
References
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- Okazaki, R., Okazaki, T., Sakabe, K., Sugimoto, K., & Sugino, A. (1968). Mechanism of DNA chain growth. I. Possible discontinuity and unusual secondary structure of newly synthesized chains. Proceedings of the National Academy of Sciences of the United States of America, 59 (2), 598–605.
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