What Are Okazaki Fragments?

okazaki-fragments Credit: Science Photo Library/Science Photo Library/Getty Images

Okazaki fragments are short segments of DNA that were newly synthesized using the lagging strand of the original piece of DNA during DNA replication. Conversely, a new DNA strand is synthesized continuously, rather than in fragments, along the leading strand that runs antiparallel to the lagging strand.

DNA is double-stranded, consisting of two polymer strands that run antiparallel to each other. One strand runs in the five-prime to three-prime direction, while its complementary strand runs three-prime to five-prime. During DNA replication, the two strands separate a little at a time, and two new strands are synthesized using each original strand as a template. The moving area where the original DNA unravels is called a replication fork.

Biologically and chemically, new DNA can be created only in the five-prime to three-prime direction, which complicates the replication process since the two new strands must be replicated simultaneously but run in opposite directions. One of the new strands is formed in the direction pointing away from the replication fork, while the other is formed in the same direction that the replication fork moves along the DNA.

The strand that forms facing the replication fork is able to synthesize in a continuous manner. Its template strand runs in the three-prime to five-prime direction, so the new strand is able to form in the complementary five-prime to three-prime direction. The other strand is formed using a template that already runs in the five-prime to three-prime direction, so an RNA primer must attach to the template to make it possible for DNA to synthesize in the necessary five-prime to three-prime direction. A new RNA primer is added each time enough of the original DNA is unraveled to use it as a template to create a fragment of new DNA. Additional fragments form until the replication process is complete.