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The lagging strand is synthesized in short, separated segments. On the lagging strand template, a primase "reads" the template DNA and initiates synthesis of a short complementary RNA primer. A DNA polymerase extends the primed segments, forming Okazaki fragments.
Starting from the free 3’-OH of the primer, known as the primer terminus, a DNA polymerase can extend a newly synthesized strand. The leading strand in DNA replication is synthesized in one continuous piece moving with the replication fork , requiring only an initial RNA primer to begin synthesis.
While the leading strand can use a single RNA primer to extend the 5' terminus of the replicating DNA strand, multiple RNA primers are responsible for lagging strand synthesis, creating Okazaki fragments. This leads to an issue due to the fact that DNA polymerase is only able to add to the 3' end of the DNA strand.
However, primase creates RNA primers at a much lower rate than that at which DNA polymerase synthesizes DNA on the leading strand. DNA polymerase on the lagging strand also has to be continually recycled to construct Okazaki fragments following RNA primers. This makes the speed of lagging strand synthesis much lower than that of the leading strand.
During lagging strand synthesis, the replicative polymerase sends the lagging strand back toward the replication fork. The replicative polymerase disassociates when it reaches an RNA primer. Helicase continues to unwind the parental duplex, the priming enzyme affixes another primer, and the replicative polymerase reassociates with the clamp and ...
One of the new strands, the leading strand, moves in the 5' to 3' direction until it reaches the replication fork, allowing DNA polymerase to take the RNA primer and make a new complementary DNA strand to the template strand. The lagging strand moves away from the replication fork in the 3' to 5' direction and consists of small fragments called ...
DNA Pol III uses one set of its core subunits to synthesize the leading strand continuously, while the other set of core subunits cycles from one Okazaki fragment to the next on the looped lagging strand. Leading strand synthesis begins with the synthesis of a short RNA primer at the replication origin by the enzyme Primase (DnaG protein).
The initiator protein remains bound to the 5' phosphate end of the nicked strand, and the free 3' hydroxyl end is released to serve as a primer for DNA synthesis by DNA polymerase III. Using the unnicked strand as a template, replication proceeds around the circular DNA molecule, displacing the nicked strand as single-stranded DNA.